Roof Drainage Calculator
Catchment area to flow to downpipe count.
With the default inputs: Catchment area 1000 m² · Slope factor 1.0 · Storm intensity q5 300 L/(s·ha) etc.Design storm flow 30.00 L/s
Edit the inputs below to recalculate.Inputs
Result
| Effective catchment | 1000m² |
| Capacity per downpipe | 12.0L/s |
| Downpipes required | 3bars |
| Flow per downpipe | 10.00L/s |
| Utilisation | 83% |
| Overflow capacity reference | 30.00L/s |
Q = catchment area × intensity ÷ 10000 (area in m², intensity in L/(s·ha), result in L/s). The storm intensity must come from the local rainfall formula for a 5-minute duration and the design return period (usually 2-5 years); the default 300 here is only an example. The capacities quoted (DN75 about 8, DN100 15, DN150 30, DN200 55 L/s) are order-of-magnitude values for full-bore gravity flow; the real figures depend on stack height, pipe roughness and whether the system is pressurised - confirm to GB 50015 and product data. Significant roofs should also have overflows, normally taken as no less than the design flow but always confirmed against GB 50015 and local requirements.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
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 |
How to work it out
- Establish the catchment: the horizontal projected roof area, not the sloped area. Pitched roofs take an additional factor — 1.0 up to about 30°, 1.1-1.3 beyond.
- Look up the local rainfall intensity and take q for the design return period in L/(s·ha). Five years for housing, ten for important buildings, and this value drives the pipe size.
- Flow Q = q × F × ψ ÷ 10000, with F in m² and ψ the runoff coefficient (0.9-1.0 for roofs); the 10000 converts m² to hectares.
- Look up the capacity of one downpipe by diameter: gravity flow roughly 8-10 L/s at DN75, 12-20 at DN100, 30-45 at DN150, depending on gutter depth and stack height.
- Downpipes = Q ÷ (capacity × reduction factor), rounded up. The factor allows for blockages and is normally 0.8.
Where it goes wrong
- Using the sloped roof area as catchment — catchment is measured on plan and the slope adds 5-15%, so mixing them throws the flow either way.
- Guessing q at 300 — intensity is regional, over 400 in the far south and barely 100 in dry northwest regions; a guess oversizes or undersizes every pipe.
- One total divided by the number of outlets — a large roof must be split into gutters and sub-catchments, each outlet draining its own area; averaging floods the far corners.
- No overflow provision — the code requires overflow for storms beyond the design return period, either overflow weirs or overflow pipes; omitting it is a design gap.
Material and cost
uPVC downpipe DN100 25-45 CNY/m, cast iron 60-120, HDPE 40-80. Roof outlets 80-200 each, installation 15-30 CNY/m. Siphonic systems cost 1.5-2 times gravity systems.
On site
- Keep outlets clear of movement joints and parapet corners, and keep the flow path short — an outlet should not drain a radius beyond 25 m.
- Put a rodding eye at the foot of each stack and a strainer at every outlet; blocked leaves are the leading cause of roof leakage.
- In cold regions protect downpipes from freezing: keep stacks off shaded external walls or use internal drainage.
Basis
- GB 50015-2019 Standard for design of building water supply and drainage, Chapter 5
- GB 50345-2012 Technical code for roof engineering, Chapter 4
- GB 50400-2016 Technical code for rainwater control and use in buildings and communities
FAQ
Why not use one national storm intensity?
Why add overflows if the downpipes are sized correctly?
Does this work for siphonic systems?
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Roof Drainage Calculator. 2026-10-25. https://jianzhujisuan.com/en/roof-drainage