A green roof needs a waterproof roof underneath it.

A green roof may be intended to manage stormwater, improve a view, support planting or create usable outdoor space. Those goals lead to different projects. The EPA overview distinguishes extensive systems with shallower planting from intensive systems supporting a wider range of landscape uses. Vegetation can shade surfaces and cool through evapotranspiration; it can also retain and slow rainfall runoff.

Write down the main outcome you want and how you will judge it. For a visible roof, seasonal appearance may matter most. For a stormwater project, ask for the performance calculation required by the local program. For a roof terrace, include occupancy and access in the initial brief. Calling every option a “roof garden” can obscure very different requirements.

The Whole Building Design Guide describes vegetative roofs as coordinated systems for waterproofing, insulation, drainage, plant support and protection of the underlying roof. The table names functions, not a universal construction sequence. Some products combine functions; the actual order depends on the selected roof assembly.

Ask the supplier to mark these functions on a project section. A list of branded products is harder to check than a drawing that shows their relationship to drains, parapets and penetrations.

Request the complete system weight at the stated design moisture condition, plus other applicable loads. Structure, snow, maintenance access and vegetative overburden are among the design factors identified by WBDG. Public access introduces further loading and access requirements. A roof intended only for maintenance is a different brief from an occupied terrace.

Consider this invented planning example: one proposal describes empty trays, while another describes planted trays with saturated media. Their numbers cannot be ranked until both describe the same condition. Create a line for each component and ask what is included. Do not assume the lighter-looking planting design gives the lower design load.

Have the building assessment identify the usable area and any restrictions before finalizing the planting layout. Record how materials will be delivered and staged during construction. The permanent arrangement and the temporary stockpile arrangement deserve separate attention.

EPA identifies ongoing plant care and maintenance as part of green-roof ownership. Ask for a schedule that covers establishment, irrigation checks where applicable, unwanted vegetation, plant replacement and inspection after unusual weather. Request a clear handover from the installation period to ordinary operation.

Use a maintenance responsibility sheet with five columns: task, frequency or trigger, responsible party, access method and record location. Add the supplier’s required tasks to it. Assign someone to retain inspection photographs and service reports so a change in building manager does not erase the roof’s history.

Also ask how drains remain reachable, how a leak would be investigated, where removed media could be stored temporarily, and who reinstates the planting afterward. These are practical service questions that should be priced while the project is being compared.

Request separate prices for building preparation, waterproofing, vegetation, access provisions and maintenance. Ask which warranty covers the membrane, which covers planting, and whether removal and reinstatement of overlying materials is included in a covered repair. Avoid comparing a tray-only price with a complete roof project.

If the primary aim is reducing solar heat gain, also read the cool roofing guide. If the immediate issue is an aging low-slope covering, start with the membrane options. The next useful deliverable is a feasible roof assembly and operating plan, followed by a planting design that fits them.