Roof Drainage Calculator
Catchment area to flow to downpipe count.
Inputs
Result
Formula
| Q = F × q5 ÷ 10000 | F in m², q5 in L/(s·ha), Q in L/s |
| Downpipes = ceil(Q ÷ capacity) | at least one |
| Effective area = plan area × slope factor | above 1 for steep roofs |
1000 m² flat roof, local q5 = 300 L/(s·ha), DN100 downpipes at 0.8 fill giving 12 L/s: design flow 30.00 L/s needs 3 downpipes (10.00 L/s each, 83% utilisation) and overflows of at least 30.00 L/s.
FAQ
Why not use one national storm intensity?
Regional variation is enormous - for the same 5-minute duration, coastal south China can exceed 400 L/(s·ha) while arid north-west runs just over 100, a three to four fold difference. A single value would undersize the south (ponding, overflow, even backflow) and oversize the north. Use the published local formula for the project city and return period.
Why add overflows if the downpipes are sized correctly?
Downpipes clog with leaves and debris, and storms exceed the design return period. Overflows are the last line of defence: once gutter level reaches the overflow, water discharges outside instead of leaking at upstands or overloading the roof. Overflows on important buildings are normally sized at no less than the design flow (confirm to GB 50015 and local rules) and must never sit above walkways or entrances.
Does this work for siphonic systems?
No. Siphonic systems run full-bore under negative pressure and a single DN100 outlet can carry three to five times the gravity flow, but only with a proper hydraulic design and proprietary outlets; pipe sizes, hanger slopes and lengths are tightly constrained, and mistakes cause vibration, noise or collapsed pipe. Use the manufacturer or dedicated software. This tool is gravity flow only.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.