For homeowners, property buyers, and anyone reading a roofing proposal

Roof flashing is where roofs actually leak.

Steep-slope roofs · single-family and small multifamily

The field of a roof is a redundant, self-draining surface. Every place that surface is interrupted, someone has to rebuild the redundancy out of sheet metal. That is flashing, and that is where the water gets in.

30-second answer

What is roof flashing, and why does it cause more leaks than the shingles do?

Flashing is the sheet metal and membrane that carries water across every place the roof's lapped surface is interrupted — walls, valleys, chimneys, skylights, pipes, and edges. The field of a roof is redundant: every course laps the one below it. An interruption is not. Flashing rebuilds that redundancy, and where it is missing, wrong, or reused past its life, water gets in.

Stop — immediate hazard

Water at a light fixture, a ceiling fan, or an electrical panel is an electrical hazard first and a roofing question second.

If water is running from or near a light, a fan, a smoke alarm, or an electrical panel, do not touch the fixture, the water, or the switch, and keep people out of the room. Turn off power to that circuit only if the panel itself is dry and you can reach it without standing in water. If you cannot do that, leave it alone.

A ceiling that is bulging or sagging is holding weight it was not built to hold. Stay out from under it. Nothing further down this page is more urgent than either of those two things.

Leave the area and call your electric utility or a licensed electrician. If there is smoke, arcing, a burning smell, or a ceiling that is sagging or bulging under held water, leave the building and call 911.

Learning paths and saved lessons

At a glance

The short versionSection link

The dimensions below are from the 2024 International Residential Code, which is a model provision — a document a state or local government may adopt, amend, or ignore. It is not the law where you live until your jurisdiction adopts it, and your jurisdiction may have adopted a different edition with local amendments. Confirm with the authority having jurisdiction before treating any number here as a requirement.

What flashing is: Sheet metal, membrane, or both, at every interruption in the roof surfaceNot a product you buy once. A family of details, each specific to one location.
Where the model code puts it: Wall and roof intersections, changes in roof slope or direction, and around roof openings2024 IRC R903.2.1. The same section requires a flashing that diverts water away from where an eave meets a vertical sidewall — the detail the trade calls a kickout, which the code never names.
Minimum metal thickness (model code): 0.019 in — No. 26 galvanized sheet2024 IRC R903.2.1: where flashing is of metal, the metal shall be corrosion resistant and not less than this thickness.
Sidewall base flashing (model code): Not less than 4 in high and 4 in wide2024 IRC R905.2.8.3, for asphalt shingles: continuous or step flashing, not less than 4 in in height and 4 in in width. The code does not dimension the length along the slope — the dimension that makes consecutive step pieces lap. R905.2.8 also requires compliance with the shingle manufacturer's approved installation instructions.
Drip edge (model code): ≥ 1/4 in below the sheathing, ≥ 2 in back onto the deck, laps ≥ 2 in2024 IRC R905.2.8.5, at both eaves and rakes of shingle roofs.
Open metal valley lining (model code): Not less than 24 in wide2024 IRC R905.2.8.2. Closed valleys take a lining not less than 36 in wide.
Cricket required at (model code): Any chimney or penetration more than 30 in wide2024 IRC R903.2.2, measured perpendicular to the slope, on the ridge side.
Service life: Not a single number, and not the same as the covering'sThe metal, the fasteners, the sealant, and the substrate behind it age at different rates. A planning range for one flashing detail is not a planning range for another, and neither is a warranty term.

Tradeoffs

This page's advice — treat flashing as scope, and expect it to be replaced — and where that advice is wrongSection link

The general position here is that flashing belongs in a written scope of work and that a re-roof is the cheapest moment in the building's life to replace it. That is not always the right call.

Best when

The roof is being torn off anyway, so the flashings are exposed and the labor to replace them is marginal rather than standalone.
The existing flashings are galvanized steel or aluminum of unknown age, in a climate with freeze-thaw, salt, or long wet seasons.
There is a roof-to-wall intersection ending above a wall, and no kickout is visible from the ground — the omission is the single most expensive one on this page.
The cladding above the roof-wall run is already coming off for other work, so the weather-resistive barrier can be re-lapped correctly at the same time.
The new covering is a different product from the old one, and its instructions call for a flashing size or profile the existing pieces do not have.

Think twice if

The counterflashing is sound copper or lead let into historic masonry. Cutting it out risks the masonry, and the replacement is a specialist trade, not a roofing crew's afternoon.
The flashings are two or three years old and were installed as part of a documented repair — replacing them buys nothing and reopens details that are currently working.
Removing sound step flashing means opening a cladding you cannot reinstate: some stucco, some stone veneer, some historic siding cannot be taken off and put back.
The building is likely to be re-clad within a year or two. Doing the wall-side work twice is worse than sequencing it once.
The roof predates 1990 and the flashings are bedded in asphalt roof cement or lapped with old felts. Those may contain asbestos, and the decision becomes a testing-and-abatement question before it is a roofing one.

What changes the answer

Metal and age: copper and lead outlive several asphalt roofs; thin galvanized steel in a wet or coastal climate may not outlive one.
What is above the roof: siding that lifts cleanly, masonry that needs a saw cut, or stucco that does not come off at all.
The adopted code and how your jurisdiction inspects re-roofs — some inspectors look at flashing, some look only at the permit.
Whether a kickout exists. If it does not, the wall below it has been taking concentrated water for as long as the roof has been there, and the question stops being about flashing and becomes about the wall.
The specific shingle's installation instructions, which the model code itself defers to for everything it does not dimension.
Who is paying: an insurance scope written for storm damage may fund the covering and not the flashings around it.

The mechanism

The field is redundant. The interruptions are not.Section link

This is the whole idea, and everything below is an application of it.

One roof carrying every flashing condition a steep-slope house can present. The numbers are the row numbers of the table in the next section, where each is described in full; the gutter is labeled rather than numbered because it is drainage, not flashing, and row 13 — a slope transition — needs a second roof plane this drawing does not have. The dormer deliberately carries one valley of each kind: the open valley on the left is drawn as a band because its lining stays exposed, the closed-cut valley on the right as a dashed line because its lining is concealed under the shingles. A full text description is available to screen readers. Schematic, not to scale, and deliberately not a construction detail.Original diagram, Understanding Roofing.

The field — the open expanse of shingle, tile, or shake between the interruptions — is not waterproof, and it was never designed to be. It is water-shedding. Each course of covering laps over the one below it, so a drop landing anywhere in the field meets a downhill-facing overlap, runs onto the next course, and leaves at the eave. Under it, the underlayment does the same thing again. Nothing in the field has to be sealed, because nothing in the field has to stop water; it only has to keep pointing it downhill.

That arrangement is quietly redundant. If one shingle in the middle of a roof cracks, the course below it is still lapped, and the underlayment below that is still lapped. Water has to defeat two or three independent overlaps in the same square foot before it reaches the deck. In the field it usually cannot, which is why the middle of a roof plane is the last place a leak starts.

An interruption ends the lap

Now cut a hole in that surface for a pipe, or run it into a wall, or fold two planes together into a valley. At every one of those places the lapped courses stop at a cut edge, and the redundancy stops with them. Somebody has to rebuild it out of a manufactured piece: flashing, a sheet of metal or membrane shaped for one location and installed so that it too is lapped (the glossary entry carries the short definition) — its top edge tucked under what is above, its bottom edge lying over what is below. The trade calls this being installed in shingle fashion, and everything else on this page is an application of it.

The National Roofing Contractors Association puts the same point in one sentence in its builders’ handbook: flashing “prevents water entry into a building at all junctions of the roofing to a wall, skylight, chimney, parapet, dormer, vent, or other roof penetration,” and “without flashing, most roofs would leak like a sieve.”

Base flashing and counterflashing are two halves of one detail

Wherever a roof meets something vertical, the detail comes in two parts, and confusing them is the most common way to misread a proposal.

Base flashing is the piece that belongs to the roof. It sits on the roof surface and turns up the vertical face. In the model code, base flashing against a vertical sidewall is either continuous or step flashing, and not less than 4 inches in height and 4 inches in width.
Counterflashing (also called cap flashing) belongs to the wall. It comes down over the top edge of the base flashing so that water running down the wall face is shed over the base flashing rather than behind it. The Insurance Institute for Business & Home Safety, quoting the NRCA glossary, defines it as “formed metal secured on or into a wall, curb, pipe, rooftop unit or other surface, to cover and protect the upper edge of a base flashing and its associated fasteners.”

Two pieces, two trades, one joint. When a repair consists of running a bead of sealant along the top of a base flashing, what has happened is that the counterflashing was never there, or was removed, and sealant is standing in for it. Sealant is a maintenance item: it has a service life and it will need renewing. Counterflashing does not.

Flashing is also a wall detail, not only a roof detail

Every roof-to-wall flashing has to be integrated with the wall’s drainage plane — the weather-resistive barrier, house wrap, or building paper behind the siding. The Department of Energy’s Building America Solution Center describes the correct relationship plainly: “the upturned leg of the step flashing is behind the vertical drainage plane (house wrap) on the wall or sealed to it with an adhesive membrane and sheathing tape.”

Behind. If the house wrap laps over the outside of the step flashing instead, every piece of that flashing is now a small shelf collecting water and delivering it into the wall. The metal can be perfect and the roof will still rot the wall, because the lap runs the wrong way. It is a common reason a roof-to-wall detail fails while looking perfectly correct from the ground, and it is the reason roof-to-wall work is a scope conversation about two trades rather than one.

The reference

Every flashing location, what it does, how it fails, and what the failure looks like from insideSection link

Thirteen locations. The fourth column is the one worth reading first, because it is the only column you can observe without leaving the ground.

Flashing locations on a steep-slope roof. Code dimensions cited are 2024 IRC model text, not the adopted law of any jurisdiction.
Location	What it does	How it fails	What the failure looks like from inside or from the ground
1 · Drip edge — eave	Carries water off the edge of the deck and into the gutter rather than letting it wick back under the sheathing and onto the fascia. Model code: extend not less than 1/4 in below the sheathing and not less than 2 in back onto the deck, with laps of not less than 2 in.	Omitted entirely on older roofs. Installed in the wrong order relative to an ice barrier, so the membrane drains behind the metal. Fastened too sparsely to hold in wind.	Rotting fascia board and paint failure along the eave line. Staining at the top of the exterior wall directly below the eave. Usually no ceiling stain at all, which is why it goes unreported for years.
2 · Drip edge — rake	The same job along the sloped gable edge, plus holding the shingle edge down against uplift. Roof edges see the highest wind pressures on the roof.	Omitted at rakes, which was common before drip edge was required at both edges. Fastened at wide spacing, so wind lifts the covering along the gable.	Shingles visibly lifted or curled along the gable edge after wind. Water entering behind the rake board. Staining at the upper corner of a gable-end wall.
3 · Open valley	A metal or roll-roofing lining carries the combined flow of two roof planes down an exposed channel. Model code: open metal valley lining not less than 24 in wide.	Lining too narrow for the flow arriving. Fasteners driven through the lining inside the flow path. Debris damming and pushing water sideways under the shingle cut.	A ceiling stain that runs as a line rather than a spot, below the line of the valley, appearing in prolonged or heavy rain and not in brief showers.
4 · Closed-cut or woven valley	The shingles themselves carry the water across the valley; the lining underneath is the backup. Model code: closed valley lining of smooth roll roofing or a self-adhering polymer-modified bitumen underlayment complying with ASTM D1970, not less than 36 in wide.	The cut made too close to the centerline, or the upper shingle not clipped and sealed. A weave attempted with a laminated shingle too thick to lie down through it. No lining under the shingles at all.	The same line-shaped stain as an open valley, sometimes migrating along the ceiling as the shingle edge lifts season by season.
5 · Step flashing — sidewall	Where a roof runs up a wall along the slope, one bent piece per shingle course, each lapping the one below, so the shingle-lap logic continues up the wall. Model code: base flashing against a vertical sidewall is continuous or step flashing, not less than 4 in in height and 4 in in width.	Continuous L-metal used on a shingle roof where step flashing belongs, with sealant standing in for the laps. Installed outside the wall's weather-resistive barrier instead of behind it. Face-nailed through the vertical leg.	Wet insulation and dark staining inside the wall cavity, often with no ceiling stain at all. Frequently discovered only when the siding comes off for other reasons.
6 · Counterflashing (cap flashing)	Covers the top edge of the base flashing so water running down a wall or masonry face is shed over the base flashing rather than behind it. Let into a reglet cut in masonry, or lapped behind the cladding.	Surface-mounted to the face of the wall and sealed along its top edge, so a sealant bead is the only thing keeping water off the base flashing.	A leak that stops when someone re-caulks it and returns once the sealant has aged again. From the ground, a visible line of sealant along the top edge of metal at a chimney or wall.
7 · Kickout (diverter) flashing	At the lower end of a roof-to-wall run, turns the concentrated flow out away from the wall and into the gutter. The 2024 IRC never uses the word: R903.2.1 requires “a flashing… to divert the water away from where the eave of a sloped roof intersects a vertical sidewall.”	Omitted altogether — the most consequential omission on this list. Present but too short or too shallow to clear the cladding. Present, but the gutter does not extend under it.	Often nothing visible for years. Then staining, bubbling paint, or soft, punky trim and siding on the wall directly below the intersection, and rot in the sheathing and framing behind it.
8 · Apron / headwall flashing	One continuous piece where a roof plane dead-ends into a wall across the slope. The downslope face of a chimney is the same detail in a different place.	Vertical leg too short. Ends not turned up or hemmed, so water runs off the ends and into the wall. The whole detail relying on sealant at its terminations.	Staining that appears at the ceiling-and-wall junction directly below the intersection, often across most of its width rather than at one point.
9 · Chimney cricket (saddle)	A small ridged structure on the upslope side of a chimney that splits water and snow around it instead of letting them pond against the masonry. Model code: required on the ridge side of any chimney or penetration more than 30 in wide measured perpendicular to the slope.	Omitted on a wide chimney. Built but not flashed into both the roof and the masonry. Built to the minimum on a roof with a long upslope run above it.	Staining beside or below the chimney chase, worst during snowmelt or after prolonged rain. From the ground, a wide chimney with flat shingle upslope of it and no ridged structure.
10 · Chimney apron, step and counterflashing set	A chimney needs all four: apron across the downslope face, step flashing up both sides, a cricket above if it is wide, and counterflashing let into the masonry covering the tops of all of them.	Any one of the four missing. Counterflashing surface-mounted rather than let into a reglet. The masonry itself failing — open mortar joints and a deteriorated crown pass water regardless of the flashing.	Recurring staining around the chimney chase in the attic ceiling or the room below. Efflorescence or damp patches on an interior chimney breast.
11 · Skylight flashing	A kit matched to the specific unit and the specific covering: head flashing at the top, sill at the bottom, step pieces up the sides, all integrated with the underlayment.	Kit omitted and the perimeter bedded in roof cement. Wrong kit for the covering. Head flashing not lapped under the upslope underlayment.	Water in the skylight well and shaft that tracks rainfall. Condensation looks similar but appears on cold clear nights with no rain — the timing is the diagnostic.
12 · Vent-pipe boot and collar	A membrane collar plus a metal flange, shingle-lapped into the courses so the flange's upper edge is under the shingles above and its lower edge lies over the shingles below.	The elastomeric collar splits under ultraviolet exposure — it is the one component with nothing over it. Or the flange is face-nailed inside the flow path and sealed with mastic.	A small, sharply defined ceiling stain that reappears with every rain and grows slowly. Often below a bathroom, kitchen, or laundry, because that is where vent stacks are.
13 · Slope transitions and roof-to-roof junctions	Where a steeper plane discharges onto a shallower one, or a low-slope porch or addition roof meets a steep-slope main roof, flashing bridges two different water behaviors and two different coverings.	The shallower plane finished with a material below its minimum slope — asphalt shingles are limited to 2:12 and steeper in the model code, with doubled underlayment between 2:12 and 4:12. Transition metal too short for the flow it receives from above.	Staining well away from the transition itself, because water that gets in here runs along framing before it drops. This is the location most often misdiagnosed from inside.

Read this table one item at a time

1 · Drip edge — eave

What it does: Carries water off the edge of the deck and into the gutter rather than letting it wick back under the sheathing and onto the fascia. Model code: extend not less than 1/4 in below the sheathing and not less than 2 in back onto the deck, with laps of not less than 2 in.
How it fails: Omitted entirely on older roofs. Installed in the wrong order relative to an ice barrier, so the membrane drains behind the metal. Fastened too sparsely to hold in wind.
What the failure looks like from inside or from the ground: Rotting fascia board and paint failure along the eave line. Staining at the top of the exterior wall directly below the eave. Usually no ceiling stain at all, which is why it goes unreported for years.

2 · Drip edge — rake

What it does: The same job along the sloped gable edge, plus holding the shingle edge down against uplift. Roof edges see the highest wind pressures on the roof.
How it fails: Omitted at rakes, which was common before drip edge was required at both edges. Fastened at wide spacing, so wind lifts the covering along the gable.
What the failure looks like from inside or from the ground: Shingles visibly lifted or curled along the gable edge after wind. Water entering behind the rake board. Staining at the upper corner of a gable-end wall.

3 · Open valley

What it does: A metal or roll-roofing lining carries the combined flow of two roof planes down an exposed channel. Model code: open metal valley lining not less than 24 in wide.
How it fails: Lining too narrow for the flow arriving. Fasteners driven through the lining inside the flow path. Debris damming and pushing water sideways under the shingle cut.
What the failure looks like from inside or from the ground: A ceiling stain that runs as a line rather than a spot, below the line of the valley, appearing in prolonged or heavy rain and not in brief showers.

4 · Closed-cut or woven valley

What it does: The shingles themselves carry the water across the valley; the lining underneath is the backup. Model code: closed valley lining of smooth roll roofing or a self-adhering polymer-modified bitumen underlayment complying with ASTM D1970, not less than 36 in wide.
How it fails: The cut made too close to the centerline, or the upper shingle not clipped and sealed. A weave attempted with a laminated shingle too thick to lie down through it. No lining under the shingles at all.
What the failure looks like from inside or from the ground: The same line-shaped stain as an open valley, sometimes migrating along the ceiling as the shingle edge lifts season by season.

5 · Step flashing — sidewall

What it does: Where a roof runs up a wall along the slope, one bent piece per shingle course, each lapping the one below, so the shingle-lap logic continues up the wall. Model code: base flashing against a vertical sidewall is continuous or step flashing, not less than 4 in in height and 4 in in width.
How it fails: Continuous L-metal used on a shingle roof where step flashing belongs, with sealant standing in for the laps. Installed outside the wall's weather-resistive barrier instead of behind it. Face-nailed through the vertical leg.
What the failure looks like from inside or from the ground: Wet insulation and dark staining inside the wall cavity, often with no ceiling stain at all. Frequently discovered only when the siding comes off for other reasons.

6 · Counterflashing (cap flashing)

What it does: Covers the top edge of the base flashing so water running down a wall or masonry face is shed over the base flashing rather than behind it. Let into a reglet cut in masonry, or lapped behind the cladding.
How it fails: Surface-mounted to the face of the wall and sealed along its top edge, so a sealant bead is the only thing keeping water off the base flashing.
What the failure looks like from inside or from the ground: A leak that stops when someone re-caulks it and returns once the sealant has aged again. From the ground, a visible line of sealant along the top edge of metal at a chimney or wall.

7 · Kickout (diverter) flashing

What it does: At the lower end of a roof-to-wall run, turns the concentrated flow out away from the wall and into the gutter. The 2024 IRC never uses the word: R903.2.1 requires “a flashing… to divert the water away from where the eave of a sloped roof intersects a vertical sidewall.”
How it fails: Omitted altogether — the most consequential omission on this list. Present but too short or too shallow to clear the cladding. Present, but the gutter does not extend under it.
What the failure looks like from inside or from the ground: Often nothing visible for years. Then staining, bubbling paint, or soft, punky trim and siding on the wall directly below the intersection, and rot in the sheathing and framing behind it.

8 · Apron / headwall flashing

What it does: One continuous piece where a roof plane dead-ends into a wall across the slope. The downslope face of a chimney is the same detail in a different place.
How it fails: Vertical leg too short. Ends not turned up or hemmed, so water runs off the ends and into the wall. The whole detail relying on sealant at its terminations.
What the failure looks like from inside or from the ground: Staining that appears at the ceiling-and-wall junction directly below the intersection, often across most of its width rather than at one point.

9 · Chimney cricket (saddle)

What it does: A small ridged structure on the upslope side of a chimney that splits water and snow around it instead of letting them pond against the masonry. Model code: required on the ridge side of any chimney or penetration more than 30 in wide measured perpendicular to the slope.
How it fails: Omitted on a wide chimney. Built but not flashed into both the roof and the masonry. Built to the minimum on a roof with a long upslope run above it.
What the failure looks like from inside or from the ground: Staining beside or below the chimney chase, worst during snowmelt or after prolonged rain. From the ground, a wide chimney with flat shingle upslope of it and no ridged structure.

10 · Chimney apron, step and counterflashing set

What it does: A chimney needs all four: apron across the downslope face, step flashing up both sides, a cricket above if it is wide, and counterflashing let into the masonry covering the tops of all of them.
How it fails: Any one of the four missing. Counterflashing surface-mounted rather than let into a reglet. The masonry itself failing — open mortar joints and a deteriorated crown pass water regardless of the flashing.
What the failure looks like from inside or from the ground: Recurring staining around the chimney chase in the attic ceiling or the room below. Efflorescence or damp patches on an interior chimney breast.

11 · Skylight flashing

What it does: A kit matched to the specific unit and the specific covering: head flashing at the top, sill at the bottom, step pieces up the sides, all integrated with the underlayment.
How it fails: Kit omitted and the perimeter bedded in roof cement. Wrong kit for the covering. Head flashing not lapped under the upslope underlayment.
What the failure looks like from inside or from the ground: Water in the skylight well and shaft that tracks rainfall. Condensation looks similar but appears on cold clear nights with no rain — the timing is the diagnostic.

12 · Vent-pipe boot and collar

What it does: A membrane collar plus a metal flange, shingle-lapped into the courses so the flange's upper edge is under the shingles above and its lower edge lies over the shingles below.
How it fails: The elastomeric collar splits under ultraviolet exposure — it is the one component with nothing over it. Or the flange is face-nailed inside the flow path and sealed with mastic.
What the failure looks like from inside or from the ground: A small, sharply defined ceiling stain that reappears with every rain and grows slowly. Often below a bathroom, kitchen, or laundry, because that is where vent stacks are.

13 · Slope transitions and roof-to-roof junctions

What it does: Where a steeper plane discharges onto a shallower one, or a low-slope porch or addition roof meets a steep-slope main roof, flashing bridges two different water behaviors and two different coverings.
How it fails: The shallower plane finished with a material below its minimum slope — asphalt shingles are limited to 2:12 and steeper in the model code, with doubled underlayment between 2:12 and 4:12. Transition metal too short for the flow it receives from above.
What the failure looks like from inside or from the ground: Staining well away from the transition itself, because water that gets in here runs along framing before it drops. This is the location most often misdiagnosed from inside.

Dimensions are 2024 IRC model text. They are not the requirement in any jurisdiction until that jurisdiction adopts that edition, and adoptions amend. The fourth column describes patterns, not diagnoses: several of these produce similar-looking stains, and the location of a stain is weak evidence about the location of the entry point.

One decision worth understanding

Open, closed-cut, or woven: the valley method is a real choiceSection link

Valleys carry more water per square foot than anywhere else on a roof, and there are three ways to build one. None of them is universally right, and the shingle's own instructions frequently decide it for you.

Valley methods for asphalt shingle roofs, with the lining the 2024 IRC model text requires for each.
Method	How it is built	Lining the model code requires	Where it is strong	Where it is weak
Open metal valley	A metal lining runs the length of the valley and stays exposed. Shingles are cut back on both sides to leave a visible channel of metal.	Metal valley lining not less than 24 in wide.	Highest flow capacity and the fastest self-clearing of debris. The channel is inspectable from the ground: you can see whether it is clean.	The metal is visible, which some owners dislike. Fasteners placed inside the flow path — or a lining too narrow for the roof above it — turn the strongest method into the weakest.
Open valley, roll-roofing lining	The same exposed-channel geometry, but the lining is two plies of mineral-surfaced roll roofing rather than metal.	An 18 in wide bottom ply and a 36 in wide top ply.	Cheaper than metal and workable on a roof where matching metal is impractical.	A bitumen product in the most abraded, most UV-exposed, wettest part of the roof. Its service life is not the metal's.
Closed-cut valley	Shingles from one plane run through the valley; shingles from the other are cut back in a clean line a few inches off the centerline. The lining underneath is never seen.	Smooth roll roofing or a self-adhering polymer-modified bitumen underlayment complying with ASTM D1970, not less than 36 in wide.	No exposed metal, a continuous shingle appearance, and the most commonly installed method on laminated shingles.	The water crosses a cut shingle edge. If the cut is too close to the centerline, or the upper shingle is not clipped and sealed, that edge collects water instead of shedding it. Debris cannot self-clear the way it does in an open channel.
Woven valley	Shingles from both planes are alternated across the valley so the courses interlace, with no cut at all.	The same closed-valley lining: not less than 36 in wide.	No cut edge in the flow path, and no exposed metal.	It requires a shingle flexible enough to lie flat through the weave. Many laminated shingles are not, and their manufacturers' instructions may not permit a weave at all. Because the model code defers asphalt shingle flashing to those instructions, the product decides this, not preference.

Read this table one item at a time

Open metal valley

How it is built: A metal lining runs the length of the valley and stays exposed. Shingles are cut back on both sides to leave a visible channel of metal.
Lining the model code requires: Metal valley lining not less than 24 in wide.
Where it is strong: Highest flow capacity and the fastest self-clearing of debris. The channel is inspectable from the ground: you can see whether it is clean.
Where it is weak: The metal is visible, which some owners dislike. Fasteners placed inside the flow path — or a lining too narrow for the roof above it — turn the strongest method into the weakest.

Open valley, roll-roofing lining

How it is built: The same exposed-channel geometry, but the lining is two plies of mineral-surfaced roll roofing rather than metal.
Lining the model code requires: An 18 in wide bottom ply and a 36 in wide top ply.
Where it is strong: Cheaper than metal and workable on a roof where matching metal is impractical.
Where it is weak: A bitumen product in the most abraded, most UV-exposed, wettest part of the roof. Its service life is not the metal's.

Closed-cut valley

How it is built: Shingles from one plane run through the valley; shingles from the other are cut back in a clean line a few inches off the centerline. The lining underneath is never seen.
Lining the model code requires: Smooth roll roofing or a self-adhering polymer-modified bitumen underlayment complying with ASTM D1970, not less than 36 in wide.
Where it is strong: No exposed metal, a continuous shingle appearance, and the most commonly installed method on laminated shingles.
Where it is weak: The water crosses a cut shingle edge. If the cut is too close to the centerline, or the upper shingle is not clipped and sealed, that edge collects water instead of shedding it. Debris cannot self-clear the way it does in an open channel.

Woven valley

How it is built: Shingles from both planes are alternated across the valley so the courses interlace, with no cut at all.
Lining the model code requires: The same closed-valley lining: not less than 36 in wide.
Where it is strong: No cut edge in the flow path, and no exposed metal.
Where it is weak: It requires a shingle flexible enough to lie flat through the weave. Many laminated shingles are not, and their manufacturers' instructions may not permit a weave at all. Because the model code defers asphalt shingle flashing to those instructions, the product decides this, not preference.

Which methods are permitted for a given shingle comes from that product's approved installation instructions, which 2024 IRC R905.2.8 explicitly defers to. Ask to see the page rather than accepting a general answer, and note that a valley method chosen for appearance still has to be one the product allows.

The glossary carries short definitions of each: open valley, closed valley, and woven valley.

One practical consequence: a valley is the only flashing location whose condition an owner can genuinely monitor without help. From an upstairs window or a zoomed photograph, an open valley either has a clear channel or it does not. A closed valley gives you far less to look at, which is a fair reason to prefer open metal on a roof under trees even when the appearance argument runs the other way.

The expensive omission

The kickout: one missing piece, one rotted wallSection link

Of everything on this page, the omitted kickout does the most damage for the least visible warning. Here is the geometry that explains why, and the arithmetic that sets the scale.

Picture the ordinary condition: a garage roof, a porch roof, or the lower slope of a two-story house runs down alongside a wall and ends at the eave. Water on that plane runs straight down the slope. Step flashing does not steer it sideways toward the wall, and nothing else does either. What concentrates at the bottom corner is the channel along the intersection itself: the band of roof running against the wall, plus everything the wall face above sheds onto that line, collected over the whole length of the run and delivered to one point a few inches wide. That corner is where the kickout goes. The 2024 IRC asks for it in R903.2.1 without ever using the word: “a flashing shall be installed to divert the water away from where the eave of a sloped roof intersects a vertical sidewall.”

A worked example: the scale of the water

Start with how much water the plane is handling at all. In still air rain falls vertically, so the catchment of a roof plane is its horizontal projection — its footprint on the ground, what this site calls plan area — not its sloped surface area. Take a plane 24 ft long against the wall with 12 ft of horizontal run: 288 sq ft of catchment.

One inch of rain over one square foot is one-twelfth of a cubic foot, which is about 0.623 US gallons. So:

gallons per hour = plan area (sq ft) × rainfall rate (in/hr) × 0.623

At a rate of 1.0 in/hr:

288 × 1.0 × 0.623 = 179 gallons per hour
which is about 3.0 gallons per minute — a five-gallon bucket filled every hundred seconds
and over a forty-five-minute rain, about 135 gallons

Double the rate to 2.0 in/hr and it is roughly 6 gallons per minute.

That is the whole plane, and the whole plane is not what the kickout handles — most of that water leaves along the eave, spread over its full length. What reaches the corner is the intersection channel: the band of roof against the wall, plus the runoff off the wall face above, which on a two-story elevation is several hundred square feet of vertical surface draining onto one line whenever the rain has any horizontal component at all. There is no honest way to put a single number on that share. It moves with wind direction and speed, wall height and cladding, roof slope and run length, and the rainfall event itself.

So the useful thing the arithmetic establishes is scale, not a flow rate for the corner: a modest roof plane moves gallons per minute in ordinary rain, and the geometry of a roof-to-wall run collects a share of it over the full length of the intersection and delivers that share to a single point. And 1.0 in/hr is an arithmetic placeholder, not a design value. Actual rainfall intensities for a specific address, by duration and return period, come from NOAA’s Precipitation Frequency Data Server. Sizing anything from those numbers is engineering work.

What the missing piece actually does

A kickout does not stop that flow — nothing stops it. It changes its direction by a few degrees, out and away from the cladding and into the gutter. Without it, everything the channel has collected runs down the face of the wall at a single point, and the wall assembly behind that point has to handle a concentration nobody detailed it for. Some of it gets behind the cladding at the first joint, nail hole, or lap it finds.

The Building America Solution Center describes the resulting chain directly. Water leaks into the wall, and the damage it lists reaches the “wall sheathing, framing, insulation, and mold inside the wall cavities.” It attributes “thousands of dollars worth of damage” to the missing or inadequate detail.

The reason it goes on for years is the same guide’s second point: older painted wood siding announced water damage by peeling. Modern claddings — vinyl, fiber cement, stucco, manufactured stone — do not. Between the concentration of flow, the concealment of the cladding, and the fact that the damage is in the wall rather than the ceiling, an omitted kickout can rot a wall through without ever producing the ceiling stain that would have sent someone looking.

What you can check without leaving the ground

Find every place a roof edge ends against a wall. Look at the bottom corner. Is there a small piece of metal turning out toward the gutter, or does the roof edge simply run into the siding?
Does the gutter actually extend under that corner, or does it stop short of it?
Look at the wall directly below, from the intersection down to the ground. Discoloration, bubbling paint, warped or soft siding, and soft trim are the signs BASC lists.
Press — gently, from the ground — on trim below that corner. Softness is worth a professional opinion; it is not a DIY repair.
Check the same corner in any photographs you already have of the house: a listing photo, an insurance photo, a pre-purchase inspection report.

If a kickout is missing on a house that has stood for a decade, the question is no longer just about roofing. What is behind that wall becomes the question, and answering it means someone opening the cladding.

The scope conversation

“We’ll reuse the existing flashing”Section link

This sentence — or, more often, the absence of any sentence about flashing at all — is where two apparently comparable proposals stop being comparable.

The model reroofing provisions permit flashing to be reused at a re-roof in most circumstances — your jurisdiction’s adopted edition and its amendments decide whether that holds where you live — and it is common, and sometimes the right decision. It is also one of the easiest line items to quietly leave out of a bid, and the decision becomes permanently invisible the moment the new covering goes over it. Both of those things are true at once, which is why it belongs in a written scope rather than in a conversation on a driveway.

What the model code actually restricts

Less than most people assume. The reroofing provisions in the model codes say that existing vent flashing, metal edgings, drain outlets, collars, and metal counterflashings shall not be reinstalled where rusted, damaged, or deteriorated. The NRCA’s Mark Graham summarizes the same rule in Professional Roofing: those specific materials “are not permitted to be reinstalled when they are rusted, damaged or otherwise deteriorated.”

Read it carefully and notice what it does not say. It does not require new flashing. It sets a condition test — not rusted, not damaged, not deteriorated — and a piece of metal can pass that test comfortably while still being the wrong choice.

The four questions the condition test does not answer

Is it the right size for the new covering? The model code dimensions sidewall base flashing at 4 in by 4 in and then defers the rest of asphalt-shingle flashing to the manufacturer’s approved installation instructions. A different shingle can want a different flashing size or profile, and an existing piece that is sound may simply be wrong for what is going over it.
What is under it? Old flashing is exactly where deck rot concentrates, because that is where water has been getting in. Leaving the flashing in place means nobody looks under it. That is a real argument for removal that has nothing to do with the metal’s condition.
Are the laps still right? A reused step flashing is reused in place, which means the cladding stays on and the relationship between the flashing and the wall’s weather-resistive barrier stays exactly as it was — including if it was wrong. If there was no kickout before, there will be no kickout after.
What is the sealant doing? If the existing detail depends on a bead of sealant, reusing it reuses the dependency. The sealant is older than the roof that is being replaced.

When reuse is genuinely the better answer

Sound copper or lead counterflashing let into historic masonry is the clearest case: cutting it out risks the masonry, the replacement is specialist work, and the existing metal will outlive the shingles going on above it. Recently installed flashing from a documented repair is another. A cladding that cannot be removed and reinstated is a third. In each of those, reuse is a decision with a reason, which is different from reuse as a default.

How to put it in a scope

Ask for the flashing line items listed by location with new or reused stated against each, and the metal and thickness named. A proposal that does this is not necessarily the more expensive one — it is the one you can compare. The hiring and quotes path and the checklist in the roof buying kit are built around forcing exactly this kind of specificity before price enters the conversation.

Diagnosis, safely

Reading a flashing failure from inside the buildingSection link

The stain is evidence about the building's framing, not about the roof's geometry. Here is how to use it anyway.

Water entering at a flashing lands on the top of the deck or the underlayment and then does what the glossary calls water travel: it runs down the underside of the sheathing until it meets a rafter, travels along that rafter, crosses a top plate or a ceiling joist, and drops at the first place something stops it. By the time it appears on a ceiling it may be several feet from the entry point, on the other side of a wall, and — on a hip roof or a complex plan — in a different room.

That is why the useful signal is almost never where. It is when.

Timing tells you more than location

Appears in any rain, small stain, repeatable: think penetration — a pipe boot or a small flashing. These leak on a predictable schedule because the defect is a hole, not a lap.
Only in prolonged or heavy rain: think valley, or a detail that only fails once flow exceeds what it can carry, or a debris dam.
Only in wind-driven rain, and only from one direction: think wall-side: step flashing, counterflashing, headwall, or the cladding above them. Vertical surfaces only get loaded when the rain arrives sideways.
Only during snowmelt or a thaw: think eave and ice backup, or the upslope side of a wide chimney with no cricket.
On cold clear nights with no rain at all: that is not a leak. That is condensation, and it is a ventilation, air-sealing, and moisture question rather than a flashing one.
Nothing on the ceiling ever, but damage on an exterior wall: think kickout, step flashing, or drip edge. This is the pattern that gets found last.

Documents that answer questions a ladder would not

The previous inspection report, if the house was bought recently. Flashing observations are usually in it.
Listing photographs and insurance photographs, which are often taken from angles you no longer have access to.
The permit record for the last roof, which tells you whether it was inspected at all.
The previous roofing proposal or invoice — what it did and did not include is frequently the answer.
Photographs taken by any contractor who has been on the roof since. Ask for them; most will send them.

Once you have a timing pattern and a rough location, the next decision is scope rather than diagnosis: whether this is a contained repair or evidence of a roof at the end of its service. That is the question the repair-or-replace path is for, and the answer is different for a split pipe boot than for a wall that has been taking a roof-to-wall run’s concentrated flow at one point for eight years.

Money

What flashing costs — and why this page does not publish a numberSection link

Publishing a national dollar figure for flashing would be inventing one. Here is what actually drives the cost instead, which is more useful anyway.

Flashing is a small fraction of a roof’s material cost and a large fraction of the labor that decides how long the roof lasts. That combination makes it uniquely badly suited to a per-square or per-linear-foot national average: the metal is cheap and roughly comparable everywhere, and almost all the variation is in reaching it.

The variables that decide the number for a specific building:

Linear feet at each location. A simple gable has an eave, two rakes, and perhaps a pipe or two. A cross-gabled house with two dormers, a chimney, and a porch roof has valleys, sidewalls, headwalls, kickouts, a cricket, and transitions — the same roof area, several times the flashing.
Metal and thickness. Galvanized steel, aluminum, copper, and lead-coated copper differ widely in material cost, and differ again in how long they last in a given climate and runoff condition. This page does not publish a figure for either, because no defensible national one exists.
Whether cladding comes off. The single largest multiplier on the wall side. Siding that unclips is one price; stucco or manufactured stone that has to be cut and patched by another trade is a different project.
Masonry work. A reglet cut for counterflashing, repointing an open joint, or rebuilding a chimney crown is masonry labor priced as masonry labor, not as roofing.
Access and height. A second- or third-story wall intersection may need staging rather than a ladder.
Whether it is inside a re-roof or standalone. The same step flashing replaced during a tear-off, when the covering is already off and the crew is already there, versus as a discrete repair with its own mobilization, is not the same job at all. This is the strongest cost argument for doing flashing work at re-roof time.
What is found underneath. Deck rot at a flashing is likely enough that it should have a written allowance and a unit rate before the job starts, not a phone call on day two.

For how this site builds and states any cost figure — units, scope, geography, as-of date, and confidence — see the cost methodology. For where flashing sits inside a whole roof price, start at the roof cost hub. Neither will give you a flashing line item, because an honest one does not exist at national scale.

Considerations

What changes this on a real buildingSection link

Code and jurisdiction

There is no nationwide building code for site-built houses in the United States. Everything quoted on this page from the 2024 International Residential Code is model text: language a state or municipality may adopt, amend, delay, or decline. Your jurisdiction may be on a different edition, may have amended the flashing sections, and may enforce them differently on a re-roof permit than on new construction.

The kickout requirement is a good illustration of why the edition matters. The Building America Solution Center traces it through IRC section R703.8 in the 2009 and 2012 editions, and R703.4 and R903.2.1 in 2015 and 2018. A house built in 2004 may well have been built without one, entirely legally.

Model-code text is not the law where you live. Record the jurisdiction, the adopted edition, the amendments, and the effective date, and confirm with the authority having jurisdiction before relying on any dimension on this page.

Moisture and ventilation

Flashing sits at the junction between the roof’s drainage plane and the wall’s. Whether the assembly beneath is a vented attic or a correctly designed unvented one changes what happens to the water and vapor that do get in, and how long the damage stays hidden. Both assembly types are legitimate; neither excuses a flashing detail, and neither makes one optional.

Climate

In cold climates, the failure that matters most at the eave is not rain running down but meltwater backing up. An ice barrier — a self-adhered membrane at the eave — is the assembly that is supposed to handle that, and it interacts directly with the drip edge and the valley lining.

The 2024 IRC model text requires an ice barrier only “in areas where there has been a history of ice forming along the eaves causing a backup of water as designated in Table R301.2,” and where required, extends it from the lowest edges of all roof surfaces to a point not less than 24 inches inside the exterior wall line — 36 inches measured along the slope on roofs of 8:12 and steeper.

Whether an ice barrier is required at your address, and how far up the roof it must run, is set by your jurisdiction's adopted code edition and its local amendments, its climate designation, and the specific assembly. There is no universal rule, and no number on this page is one.

Wind

Roof edges take the highest pressures on a roof. The Building America Solution Center states it directly: “the greatest stress roof assemblies experience in high wind zones is at roof edges. That is where roofs experience the highest positive and negative air pressures,” and it recommends drip edge at both eaves and rakes in high-wind zones, mechanically fastened to the deck at a maximum of 4 inches on center. Edge metal is therefore doing two jobs at once — shedding water and holding the covering down.

Wind performance is site- and building-specific. Basic wind speed, exposure, building height and geometry, pressure zone, enclosure, risk category, attachment, and the tested assembly all matter. A marketing mph number on a product is not a code determination for your building.

Maintenance

Two things on this page are maintenance items rather than permanent construction. The first is sealant: every bead of it in a flashing detail has a service life and will need renewing. The second is debris. Valleys, kickouts, and the upslope side of chimneys are exactly where leaves and grit collect, and a dam in a valley pushes water sideways under the shingle cut, which is a path the detail was never designed to resist.

Metal also moves. IBHS notes that when metal flashings move under thermal expansion and contraction “they can loosen or damage fasteners, disengage from cleats and clips, and allow water infiltration at joints,” and that metals expand substantially more than the masonry and wood they are attached to. A long run of edge metal with no allowance for movement is a joint that will open.

Metal choice and dissimilar metals

Flashing metal has to be compatible with the fasteners through it, the other metals it touches, and the metals whose runoff lands on it. IBHS, in the RICOWI roof guide, states that “when two dissimilar metals (e.g., copper and aluminum) come in contact in an electrolyte solution (e.g. salt water), galvanic corrosion will occur,” and that the risk rises “the further apart the metals are on the galvanic series.” That is not a detail to settle on site. It belongs in the proposal, along with the metal, the thickness, and the fastener.

Fire

A roof’s Class A, B, or C fire classification is a property of a tested assembly — deck, underlayment, and covering together under a defined test — and not of any single component.

No flashing detail on this page changes a roof's fire classification, and no flashing product carries one on its own. If a fire classification matters to you, the question is which tested assembly was specified and installed, not what the flashing is made of.

Hail and impact

Impact-resistance classifications describe how a covering performed in a laboratory impact test. They are not statements about the flashings, which are usually the thinnest and most easily deformed metal on the roof.

“Class 4 impact resistant” does not mean hail proof, and it says nothing about flashing at all. After a hailstorm, dented edge metal and a bruised covering are two separate findings and should be documented separately.

Access and site conditions

Almost everything on this page can be checked from the ground with binoculars, from an upstairs window, from photographs an installer takes for you, or from the documents in a proposal. None of it requires you to be on the roof, and the fall exposure is real: OSHA’s residential fall-protection guidance states that “falls are the leading cause of death for workers engaged in residential construction,” with conventional fall protection required for workers six feet or more above lower levels. Those are trained workers with equipment.

Do not climb onto a roof or into an attic to check any of this. Ground-based, window-based, document-based, and installer-supplied evidence answers every question on this page.

Warranty and repair

Warranties rarely land where flashing failures happenSection link

The three documents people mean by “the warranty” cover different things, and flashing tends to fall between them.

Whose product is the flashing?: A shingle manufacturer’s product warranty covers that manufacturer’s product. Step flashing, counterflashing, drip edge, valley metal, and pipe boots are frequently bought from somebody else. Before assuming a flashing leak is a warranty claim, find out from the actual document whether the flashing is a covered accessory under that specific warranty, and what happens if it is not.
Workmanship warranty: Most flashing failures are installation failures, not product failures, which puts them in the installer’s workmanship warranty rather than the manufacturer’s. Read that document for its length, what triggers a callback, whether a diagnostic visit is chargeable, whether it survives the sale of the house, and what happens if the company stops trading.
System or enhanced warranties: Extended warranties offered through a manufacturer’s contractor program often require that manufacturer’s own accessories and a credentialed installer, and often carry registration deadlines and inspection or maintenance obligations. Ask which components in this specific proposal qualify and which do not — the answer is often the reason a proposal specifies one brand’s starter and ridge but says nothing about the flashing.
What tends to void or exclude: Reused flashing, installation over an existing roof covering, details that depend on sealant instead of a mechanical lap, and unapproved repairs by others are common exclusions. None of that is universal — it is written differently in every document — which is exactly why the document, not a summary, is the thing to read.

Repairability

Flashing repairs vary enormously in cost and disruption, and the variation is almost entirely about what has to be removed to reach the flashing rather than about the flashing itself.

Pipe boot: the cheapest correct repair on a roof. Lift a few shingles, replace the boot, re-lap, done.
Valley: shingles have to come off up both slopes for the length of the valley, and the replacements will not weather to match.
Step flashing: requires opening the cladding above it, because the pieces run behind the siding and behind the weather-resistive barrier. A step-flashing repair that does not open the wall has not repaired the step flashing.
Counterflashing in masonry: needs a saw cut for a reglet, or a rebuilt mortar joint. This is masonry work and should be priced as masonry work.
Kickout retrofit: the piece costs very little; the work is opening the corner of the cladding, correcting the barrier laps, and replacing whatever the missing kickout has already damaged.

A warranty is a contract between a reader and whoever wrote it. What it covers, what voids it, whether it transfers, and how it is enforced are set by that document and by the law where the reader lives. Read the actual warranty for the product and the installer in front of you — not a summary of one, including this one.

Ask before you sign

Questions to ask an installerSection link

None of these require you to know roofing. All of them are answerable in one sentence by someone who is planning to do the work properly, and evasively by someone who is not.

Which flashings are you replacing, and which are you reusing?
The only bad answer is a vague one. “We reuse what’s in good shape” is not a scope; “new step, new drip edge at eaves and rakes, new valley metal, new pipe boots, existing chimney counterflashing retained because it is sound copper let into the masonry” is.
Is there a kickout flashing where the roof runs into the wall, and if not, will you be adding one?
This is the highest-consequence question on the list. If the answer is that one is not needed, ask where the water at the bottom of that intersection goes instead.
Will the siding come off where the roof meets the wall, and will the step flashing go behind the house wrap?
A step-flashing job done without opening the cladding cannot get the laps right. If the answer is that the siding stays on, that is a legitimate decision with a cost attached — but it should be a stated decision, not a silence.
What metal, what thickness, and what fasteners — and are they compatible with each other and with the covering?
This is where galvanic corrosion is either prevented or created — and it is where the difference between a flashing that outlives the covering above it and one that fails under it is decided. This page does not publish a service-life number for any flashing detail, because none is honestly available; the metal, the fastener, and the runoff landing on both are what set it.
Which valley method are you using, and does the shingle manufacturer's instruction sheet permit it for this product?
The model code defers to the manufacturer’s approved instructions for asphalt shingle flashing. A competent installer knows which valley methods their product allows and can show you the page.
Is the chimney wider than 30 inches across the slope, and is there a cricket on the upslope side?
A number you can check yourself with a tape measure from a window or a photograph. If the chimney is wide and there is no cricket, ask what is being done about it.
How is the counterflashing attached — into a reglet, behind the cladding, or surface-mounted and sealed?
Surface-mounted and sealed is not always wrong, but it converts a permanent detail into a maintenance item, and you should know that before it becomes your maintenance item.
Will you photograph the flashing details before the covering goes on, and give me the photographs?
Every detail on this page becomes invisible the moment the shingles go over it. Photographs are the only inspection a homeowner can perform afterwards, and a willingness to take them is itself informative.
If the deck under a flashing turns out to be rotten, how is that priced?
The wood under old flashing is the most likely place to find rot, and mid-job discoveries are the most common source of a bill that does not match a quote. The allowance and the unit rate should be in writing before work starts.
Was this house built before 1990, and if so, how are you handling the old roof cement and felts?
Old mastics, felts, and roofing cements may contain asbestos. The right answer involves testing before disturbance, not reassurance.

Require these in writing

Each flashing location listed separately — eave drip edge, rake drip edge, valleys, sidewall step, headwall/apron, chimney apron, chimney step, chimney counterflashing, cricket, skylight, pipe boots, transitions — with new or reused stated for each.
Metal type and thickness for each location, and the fastener metal.
Valley method by name, and the specific shingle product whose instructions permit it.
Whether the cladding is being removed at roof-to-wall intersections, and who reinstates it.
Whether a kickout is being installed at every roof-wall run that ends above a wall.
The ice-barrier extent, if one is required in your jurisdiction, and how it relates to the drip edge.
A deck-repair allowance with a unit rate and the method for documenting what was replaced.
Who pulls the permit and what the inspection covers.
A commitment to pre-cover photographs of every flashing location, delivered to you.

What goes wrong

Misconceptions and failure modesSection link

Common misconceptions

Common belief
Flashing is the shiny metal around the chimney.
What is actually true
The chimney is one of at least a dozen locations. Drip edge at the eave and the rake, valleys, sidewalls, headwalls, kickouts, skylights, pipe boots, and slope transitions are all flashing, and most of them are invisible from the ground once the roof is finished.
Common belief
Sealant is flashing.
What is actually true
Sealant is a gasket with a service life. Flashing is a geometry: a piece lapped so that gravity, not adhesion, keeps water out. A detail that only works because of a bead of sealant will work until the sealant does not — and sealant is renewed on a maintenance cycle, not on the building’s. Where sealant appears in a correct detail, it is a secondary defense behind a lap that would work without it.
Common belief
The leak is directly above the stain.
What is actually true
Water entering at a flashing runs along the underside of the deck, down a rafter, along a top plate, and drops wherever something stops it. A stain in the middle of a ceiling can originate at a wall intersection several feet uphill and to one side. This is the single biggest reason self-diagnosis from inside goes wrong, and the reason a competent diagnosis starts at the interruptions rather than above the stain.
Common belief
A new roof automatically means new flashing.
What is actually true
It very often does not. The model reroofing provisions permit existing flashing to be reused in most circumstances — whether your jurisdiction’s adopted text does is a question for the authority having jurisdiction — and reuse is common, and is frequently invisible in a proposal because the proposal simply does not mention flashing at all. The section on reuse below sets out what the model code actually restricts, which is narrower than most people assume.
Common belief
It only leaks in wind-driven rain, so it is not really a roof problem.
What is actually true
A leak that appears only in driven rain is a very strong indication of a flashing or cladding problem rather than a covering problem. Vertical surfaces and the joints between them are exactly what driven rain loads, and a detail that relies on water traveling straight down will fail when the water arrives sideways. It is a narrower diagnosis, not a smaller one.
Common belief
The roof is under warranty, so a flashing leak is covered.
What is actually true
Possibly, but not for the reason implied. Flashing is often not the shingle manufacturer’s product, and a flashing failure is usually workmanship rather than product. Which document applies depends on what each one says, and the answer changes with the specific warranty in front of you.

How it actually fails

Kickout flashing omitted: The bottom of a roof-to-wall run has no diverter, so the flow collected along the whole length of that intersection runs down the face of the wall instead of into the gutter, and gets behind the cladding at the first joint, nail hole, or lap it finds. The Building America Solution Center attributes “thousands of dollars worth of damage” to this omission, with water reaching “wall sheathing, framing, insulation, and mold inside the wall cavities.”What you can see: Often nothing at all for years. BASC notes that modern claddings mask the damage that older painted wood siding would have shown as peeling paint. Look for staining, bubbling paint, or soft trim on the wall directly below where a roof edge meets it, and for a gutter that does not extend under the intersection.
Continuous flashing used where step flashing belongs: A single continuous L-shaped piece is run up the slope behind the siding rather than one stepped piece per shingle course. Continuous flashing is a legitimate detail — the model code permits base flashing at a sidewall to be “continuous or step flashing” — but the Building America Solution Center is specific about which goes where: “step flashing is used with shingle roofs; continuous flashing is used with metal and rubber membrane roofs.” On a shingle roof the courses themselves create the laps, and a continuous piece has every course crossing it at a joint with nothing lapping that joint.What you can see: Usually invisible without opening the cladding. Suspect it where a roof-to-wall leak recurs after repeated sealing, and ask directly whether step flashing or continuous flashing was used.
House wrap lapped over the step flashing instead of behind it: The wall’s drainage plane runs on the outside of the flashing, so every piece becomes a shelf that collects water and delivers it into the wall. BASC specifies the opposite: the upturned leg goes behind the drainage plane, or is sealed to it.What you can see: Damage inside the wall with a roof that looks perfect. Frequently discovered only during re-siding.
Counterflashing surface-mounted and caulked: Instead of being let into a reglet cut in the masonry or lapped behind the cladding, the cap flashing is screwed to the face of the wall and sealed along its top edge. The joint is a sealant bead, and the sealant is the only thing keeping water off the top of the base flashing.What you can see: A leak that stops when someone re-caulks it and comes back once the sealant has aged again — the “we fixed that last year” pattern. From the ground, a visible line of sealant along the top of metal at a chimney or wall.
Rusted or deformed flashing reinstalled at re-roof: Existing metal is bent back, the new covering goes over it, and the old pinholes, rust, and fastener holes go back with it. Model reroofing text is explicit that existing vent flashing, metal edgings, drain outlets, collars, and metal counterflashings shall not be reinstalled where rusted, damaged, or deteriorated.What you can see: A leak in the first two or three years of an otherwise new roof, located at a penetration or an edge rather than in the field. Ask for the pre-cover photographs.
Valley debris dam: Leaves and grit collect in the valley, water backs up behind the dam and spreads sideways, and it travels under the shingle cut or over the edge of the lining — a direction the detail was never built to resist.What you can see: A ceiling stain running as a line rather than a spot, appearing in long or heavy rain and not in short showers. Visible debris in the valley from an upstairs window or a photograph.
Pipe-boot collar split by ultraviolet light: The elastomeric collar that seals against the pipe is the one part of the assembly with nothing over it, and sunlight is what it gets. When it cracks, water runs down the outside of the pipe, through the deck penetration, and into the ceiling below.What you can see: A small, sharply defined, repeatable ceiling stain that appears with every rain — often below a bathroom, kitchen, or laundry, because that is where vent stacks are.
Cricket omitted behind a wide chimney: On a chimney more than 30 inches wide across the slope, the upslope face becomes a dam. Water and snow pile against it, sit, and freeze. Model code requires a cricket or saddle on the ridge side of any such chimney or penetration.What you can see: Staining beside or below the chimney chase, worst during snowmelt or after prolonged rain. From the ground, a wide chimney with a flat shingled area upslope of it and no ridged structure.
Drip edge omitted, or the eave sequence reversed: Without drip edge, water wicks back under the sheathing edge and onto the fascia. Where an ice barrier is present, the order in which the two are installed decides whether the membrane drains onto the metal or behind it.What you can see: Rotting fascia, paint failure along the eave, and staining at the top of the exterior wall below — usually with no ceiling stain at all, which is why it goes unreported for years.
Skylight set in mastic instead of its flashing kit: The head, sill, and step components matched to the unit and the covering are skipped, and the perimeter is bedded in roof cement. The detail becomes a sealed joint rather than a lapped one.What you can see: Water in the skylight well that tracks rainfall. Condensation, by contrast, appears on cold clear nights with no rain — the timing is the diagnostic, not the appearance.

Sources and further readingSection link

Understanding Roofing / Published August 13, 2026 / Updated August 14, 2026

Scope and limitations

It cannot tell you where your roof is leaking.
Water travels along framing before it appears, and a stain's location is evidence about the building's structure, not about the roof's geometry.
It cannot tell you what your jurisdiction requires.
Every code dimension here is model text from the 2024 IRC, which is a document a government may adopt, amend, or decline.
Your adopted edition, its amendments, and your authority having jurisdiction govern.
It does not publish a cost figure for flashing.
Flashing is priced by linear foot at each location, by the metal, and above all by what has to be removed to reach it — and there is no defensible national dataset that separates those from the rest of a roofing job.
It does not publish a service-life number for any flashing detail.
The metal, the fasteners, the sealant, and the substrate age at different rates, in different climates, with different runoff landing on them.
It cannot quantify how much of the leak problem flashing represents.
No national dataset separating leak origins by location could be found.
The figure that circulates online — that some very large percentage of roof leaks originate at flashing, usually attributed to the NRCA — has no traceable NRCA publication behind it, and this page does not repeat it.
The argument here rests on the mechanism, on where the model code concentrates its requirements, and on what the named sources below actually say.

2024 International Residential Code, Chapter 9: Roof Assemblies — R903.2, R903.2.1, R903.2.2, R905.1.2, R905.2.2, R905.2.8 through R905.2.8.5
UpCodes — commercial code aggregator, reproducing model IRC text (General Services Administration adoption). Not an official jurisdiction source; ICC publishes the authoritative version at codes.iccsafe.org. / 2024 edition
Flashing locations and the kickout requirement; 0.019 in metal thickness; 4 in × 4 in sidewall base flashing; drip-edge dimensions; open and closed valley lining widths; the 30 in cricket threshold; ice-barrier extent; the deference of asphalt-shingle flashing to the manufacturer's instructions.
Model-code text. It is not the law in any jurisdiction until that jurisdiction adopts it, and adoptions routinely amend it. The International Code Council publishes the authoritative version at codes.iccsafe.org. Confirm the adopted edition, amendments, and effective date with your authority having jurisdiction.
“Reinstallation of materials” — the reroofing restriction on reusing metal flashings
UpCodes, reproducing the reinstallation-of-materials provision as adopted across jurisdictions (IRC R908.5, IBC 1511.5, and their equivalents)
That existing vent flashing, metal edgings, drain outlets, collars and metal counterflashings shall not be reinstalled where rusted, damaged or deteriorated.
This is a cross-jurisdiction listing, not a single adopted code page: the jurisdiction and section number it shows will not always be the one that governs where you live, and it may change as UpCodes changes what it surfaces. The sentence itself is stable across the residential and commercial model codes; the section number is not. Confirmed on this date against the same provision as reported in the Professional Roofing article below, which cites IBC 2018 Section 1511.5. Your jurisdiction's adopted text governs.
Step and Kick-Out Flashing at Roof-Wall Intersections
U.S. Department of Energy, Building America Solution Center (PNNL)
What step and kick-out flashing do; the 4 in up the wall and 4 in onto the deck figures; the damage a missing kickout causes to sheathing, framing, insulation and wall cavities; the code history of the kickout requirement through IRC R703.8 and R903.2.1; and that modern claddings hide the damage older painted wood siding would have shown.
Best-practice guidance for builders, not adopted law. Its references to specific IRC editions are to model text, and to a 2020 Florida amendment that applies only in Florida.
Flashing of Roof-Wall Intersections in Existing Homes
U.S. Department of Energy, Building America Solution Center (PNNL)
The definition of kick-out flashing; the difference between step flashing and continuous flashing by covering type; and that the upturned leg of the step flashing goes behind the wall's drainage plane or is sealed to it.
Written for retrofit work on existing homes. It is guidance, not a code determination, and it does not address every cladding type.
Flashing of Penetrations in Existing Roofs
U.S. Department of Energy, Building America Solution Center (PNNL)
That penetrations are a weak point in the roof; the shingle-lap principle for penetration flashing; and the membrane-collar-plus-metal-flange sequence at a vent pipe.
Guidance for retrofit work. Product-specific instructions still govern any particular boot or skylight kit.
Look for Missing Roof and Wall Flashing
U.S. Department of Energy, Building America Solution Center (PNNL)
The from-the-ground symptoms of missing flashing: discoloration and peeling paint, warping siding, water stains running down walls where roofs meet walls, and bubbling paint or soft trim.
Written as a homeowner inspection guide and it lists a ladder among its equipment. This page does not adopt that part of it; nothing here requires leaving the ground.
Roof Edge Protection
U.S. Department of Energy, Building America Solution Center (PNNL)
That roof edges experience the highest positive and negative pressures in high wind; drip edge fastened at a maximum of 4 in on center; and drip edge recommended at both eaves and rakes in high-wind zones.
A high-wind best-practice guide. It is not a wind-design determination for any building; that is site- and building-specific engineering.
Metal Flashing (RICOWI Roof Guide)
Insurance Institute for Business & Home Safety / 2019
The definitions of counterflashing, drip edge, drip kick out and valley flashing quoted from the NRCA glossary; galvanic corrosion between dissimilar metals and the galvanic series; and the effect of thermal expansion on fasteners, cleats and clips.
Written primarily around low-slope and commercial edge metal and copings. Its material tables are guidance, not a specification for any particular building.
Flashing and vents — The NRCA Roof Builders Handbook
National Roofing Contractors Association
That flashing prevents water entry at all junctions of the roofing to a wall, skylight, chimney, parapet, dormer, vent, or other roof penetration, and that without flashing most roofs would leak like a sieve.
Trade guidance written for installers. It is not adopted code and it does not quantify how often flashing is the origin of a leak.
Reroofing by the book
Mark S. Graham, Professional Roofing (National Roofing Contractors Association) / 1 February 2019
That the model codes require removal down to the deck when an existing roof is removed, and that vent flashings, roof edge metal flashings, metal counterflashings, drain outlets and collars may not be reinstalled when rusted, damaged or otherwise deteriorated.
Written against the code editions current in 2019. Use it for the principle and check your jurisdiction's adopted edition for the text.
Fall Protection in Residential Construction
U.S. Occupational Safety and Health Administration
That falls are the leading cause of death for workers engaged in residential construction, and that conventional fall protection is required for residential construction work six feet or more above lower levels.
An occupational-safety standard for employers and workers. It is not homeowner guidance, and the fact that trained workers use fall protection is a reason for an untrained reader to stay off the roof entirely, not a procedure to copy.
How do I know if I have asbestos in my home?
U.S. Environmental Protection Agency
That shingles and similar home products may contain asbestos; that testing is warranted where material is damaged or where a renovation would disturb it; and that sampling should be done by a properly trained and accredited asbestos professional.
General homeowner guidance. It does not identify which specific roofing products contain asbestos, and state and local rules on testing, notification, and disposal vary.
Precipitation Frequency Data Server (NOAA Atlas 14)
NOAA National Weather Service, Hydrometeorological Design Studies Center
That site-specific precipitation frequency estimates by location and duration exist and are published, which is why the rainfall rate used in the worked example on this page is an arithmetic placeholder rather than a design value.
A hydrology dataset, not a roofing design tool. Using it to size drainage or design a detail is engineering work for a qualified professional.

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