Green Roof Curve Number Planning
A green roof changes how rain leaves a building. Soil media, plants, slope, drains, and previous wetness all affect runoff. A curve number method gives a practical design estimate when detailed hydrologic modeling is not required. It links rainfall depth to runoff depth through storage potential. Lower curve numbers usually mean more retention. Higher curve numbers act more like hard roofing.
Why Drainage Matters
Drainage design protects the roof assembly. It also protects people below the building. A green roof can hold water for plant health, but it must also release excess water safely. Standing water can overload structural elements. It can also damage membranes, root barriers, and insulation. Good drainage planning checks runoff volume, peak flow, retained depth, and emergency overflow capacity.
Using Curve Numbers
The calculator uses a weighted curve number. It blends the vegetated portion with hard roof details, paths, edging, skylight zones, or mechanical pads. The result can be adjusted for dry, normal, or wet antecedent conditions. This is useful because a roof after several wet days can create more runoff than the same roof after a dry period.
Design Inputs
Start with roof area and design storm depth. Enter the curve number for the planted zone. Then enter the curve number for the non planted zone. Add the vegetated share. A storage credit can represent media pore space or detention layers already proven by product data. Rainfall intensity helps estimate peak drainage flow. Drain count and drain capacity help compare required and installed capacity.
Construction Use
The output helps early construction planning. It is not a replacement for local code, structural review, or product testing. Designers should verify loading, waterproofing, overflow routes, scuppers, inspection access, and maintenance needs. Use conservative inputs when data is uncertain. Check both frequent storms and major design storms.
Reading Results
Runoff depth shows water leaving the roof. Retained depth shows the storm depth controlled by storage and curve number behavior. Runoff volume supports tank sizing, downpipe checks, and detention review. Peak flow supports drain checks. The conventional roof comparison shows the expected benefit of the green roof system in simple design terms. Use the export tools to document assumptions for drawings, reports, and site records.