Roof Drainage Flow — Method
The flow a roof delivers to its gutters and downpipes is the catchment area times the design rainfall intensity, adjusted by a runoff coefficient: Q (L/s) = area (m2) x intensity (mm/hr) x runoff / 3600. For a pitched roof use the horizontal plan (projected) area, not the sloped area. The downpipe is then selected so its capacity (running part-full, per BS EN 12056) exceeds the design flow; large roofs are split across several outlets and downpipes so each stays within capacity.
Frequently Asked Questions
Find the design flow Q (L/s) = roof area x rainfall intensity x runoff / 3600, then pick a downpipe whose capacity exceeds Q. A 120 m2 roof at 75 mm/hr gives 2.5 L/s, handled by a 75 mm downpipe.
Use the local design value, often a 1-in-50-year short-duration intensity (commonly 50-150 mm/hr depending on region). Higher categories of building use a more severe return period.
Use the horizontal plan (projected) area of the roof for the catchment, since rain falls vertically. Add allowances for wind-driven rain on walls draining onto the roof if relevant.
Divide the total flow by the capacity of one downpipe and round up, but also respect the maximum gutter run to each outlet. Spreading outlets keeps gutters shallow and avoids overflow.
The fraction of rain that runs off rather than being absorbed or evaporated. For an impervious roof it is about 1.0; reduce it slightly only for special surfaces.
Rainwater Downpipe and Roof Drainage
Roof drainage has to carry away the heaviest rain the building is designed for without the gutters overflowing. The design flow comes straight from the catchment area and the local design rainfall intensity.
The design flow
Q = area x intensity x runoff / 3600 gives litres per second. Use the projected plan area and a region-appropriate intensity (often a 1-in-50-year value for housing, more severe for critical buildings).
Choosing the downpipe
Pick a downpipe and matching gutter outlet whose part-full capacity exceeds the flow; split large roofs over several outlets. For the connected underground drain, size with the pipe sizing calculator.
Related: Water Demand, Pipe Sizing, Pump Head.