Follow the water all the way away from the building.
Steep-slope and low-slope drainage basics
What makes a roof drainage system work?
A working system provides a continuous route from the roof surface to a suitable discharge point. Gutters collect water, outlets and downspouts carry it away, and ground drainage keeps it from returning to the foundation. Low-slope roofs may use drains or scuppers instead. Check the entire route; a larger gutter alone cannot correct a blocked outlet or poor discharge.
Draw a route before choosing a gutter.Section link
Begin with a simple roof plan. Mark each plane, valley, gutter run, outlet, and downspout. Then continue the line onto the ground. A system that carries water neatly to the bottom of a wall but releases it into a depression beside the foundation has stopped its work too soon.
PNNL’s gutter guidance explains the connection between concentrated roof runoff and wet foundations. This is why a basement moisture investigation can begin at the roof edge. Roof covering, guttering, grading, and below-ground drainage are separate pieces of work, but their effects meet at the same building.
A valley or an upper roof discharging onto a lower roof can concentrate flow in a small area. Avoid dividing the total roof area equally among downspouts unless the actual drainage layout does that. The question is how much area feeds each outlet, not how evenly the pipes are spaced around the house.
Name the part and its job.Section link
| Part | Job | Question to resolve |
|---|---|---|
| Gutter | Collects water at an eave. | Does it direct flow to its outlet without spilling at joints or low spots? |
| Outlet and downspout | Transfers collected water downward. | Is the opening clear, and is the whole route sized for the contributing area? |
| Roof drain | Collects low-slope roof water into piping. | Where does the pipe go, and how is it inspected and maintained? |
| Scupper | Passes water through a parapet or roof-edge opening. | Is it a primary outlet or an overflow, and where does it discharge? |
| Ground discharge | Moves water beyond the building perimeter. | Will it flow away without ponding, erosion, or an unsafe walking surface? |
Read this table one item at a time
Gutter
- Job
- Collects water at an eave.
- Question to resolve
- Does it direct flow to its outlet without spilling at joints or low spots?
Outlet and downspout
- Job
- Transfers collected water downward.
- Question to resolve
- Is the opening clear, and is the whole route sized for the contributing area?
Roof drain
- Job
- Collects low-slope roof water into piping.
- Question to resolve
- Where does the pipe go, and how is it inspected and maintained?
Scupper
- Job
- Passes water through a parapet or roof-edge opening.
- Question to resolve
- Is it a primary outlet or an overflow, and where does it discharge?
Ground discharge
- Job
- Moves water beyond the building perimeter.
- Question to resolve
- Will it flow away without ponding, erosion, or an unsafe walking surface?
Primary drainage handles normal rainfall. A designed overflow route provides another way out if the primary system cannot do its job. On a roof enclosed by parapets, that distinction matters because water can accumulate above the membrane. Follow the project’s drainage design; do not casually raise an overflow opening or connect away an intentional warning discharge.
Worked example: the same volume can arrive much faster.Section link
Take a roof with a 100-square-meter horizontal collection area. Assume 25 millimeters of vertical rainfall and, for this simplified example, that all of it becomes runoff. Convert the depth to meters: 0.025. The volume is 100 × 0.025 = 2.5 cubic meters, or 2,500 liters.
If that rain arrives evenly over one hour, the average flow is about 41.7 liters per minute. If the same amount arrives in 15 minutes, it averages about 166.7 liters per minute. The collection area and total volume are unchanged, but the average delivery rate is four times greater. Real storms vary within those periods, so these averages are not design peak flows.
This is why annual rainfall is an inadequate basis for choosing drain sizes. A drainage design needs the applicable local rainfall intensity and the system’s geometry and capacity. The example uses horizontal collection area for vertical rain; it is not the sloped roofing-material quantity from a takeoff.
Record where the route breaks.Section link
From a safe indoor or ground-level position, note where water appears during rain: over a gutter edge, behind the gutter, from a joint, at a downspout connection, or beside the foundation. Record the weather and take a photo that shows the location. “Rear corner overflows during heavy rain” gives an assessor more to investigate than “gutters bad.”
Do not infer that every overflow requires a larger gutter. Debris, a restricted outlet, a disconnected pipe, poor alignment, or concentration from an upper roof can produce different repair scopes. PNNL notes that larger gutter capacity depends on having enough downspout drainage.
Keep roof-level cleaning and drain inspection with people equipped for that access. After winter, distinguish a blockage from ice-dam water backup. Finally, include maintenance access and the discharge destination in any replacement proposal; both remain important after the new metalwork looks finished.
Sources and further readingSection link
Understanding Roofing / Updated
Scope and limitations
- The rainfall example calculates volume, not required gutter, drain, or pipe size.
Gutters and Downspouts
Pacific Northwest National Laboratory
Roof runoff must be directed away from foundations; gutter performance depends on adequate downspout drainage and maintained discharge paths.
Managing Snow Loads on Roofs and Decks
Pacific Northwest National Laboratory
Snowmelt, ice dams, and blocked drainage can retain water and add roof loading.