Rainwater
Gutter flow and number of outlets or downpipes
- where
- design flow from the roof (l/s)
- effective roof area (m²)
- design rainfall intensity (l/s per m²)
- maximum effective area per outlet (m²)
- flow capacity of the gutter with one outlet (l/s)
- outlets and downpipes needed
Explain this equation
Checks whether a gutter and outlet can take the rain off a roof, and how many outlets or downpipes are needed. Use it with the effective area from the catchment calculator.
Rainfall intensity i is the litres of water landing on each square metre every second during a design storm. Multiplying by the effective area gives the flow Q the gutter must carry. Q_c is what one outlet with that gutter can handle. Dividing gives the area one outlet can serve, and the outlets needed, rounded up.
Flow is area times intensity; outlets are then flow divided by capacity per outlet.
Sense check: 60 m² at 0.021 l/s/m² is 1.26 l/s; a 0.78 l/s outlet copes with about 37 m², so you need 2 outlets.
Watch out: capacity drops if the outlet is not at the end, if the gutter has bends, or if it is not laid level. Use the manufacturer's figures for the actual layout.
Works out the rainwater flow from a roof and how many outlets and downpipes a gutter needs for its rated capacity.
Enter your values to see the result.
Good to know
Approved Document H Table 2 lists half-round gutters laid level with a sharp-edged outlet at one end: 75 mm 0.38 l/s (18 m²), 100 mm 0.78 l/s (37 m²), 115 mm 1.11 l/s (53 m²), 125 mm 1.37 l/s (65 m²) and 150 mm 2.16 l/s (103 m²). Those pairs equal about 0.021 l/s per m², which is the default here. Manufacturer data for deeper profiles, falls and running outlets is often higher; use it where you have it. A downpipe must be at least the size of the outlet it serves.
Common mistake
Sharing outlets evenly without checking each gutter length. An outlet not at the end must be sized for the larger area draining to it, and stop ends should be no more than 50 times the water depth from an outlet.
Limits
Simplified Approved Document H method for eaves gutters. Internal and valley gutters, siphonic systems and buildings needing a higher design storm need BS EN 12056-3.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the roof covering manufacturer’s fixing specification, BS 5534, BS 8000-6, BS 6229, the Lead Sheet Training Academy guidance and Building Regulations. Wind uplift and structural design need a competent designer.