Rainwater design flow from roof area (worked example)

To size guttering you need the design flow rate: how many litres per second the system must carry in a heavy storm. It comes from how hard it rains and how much roof drains to that gutter. The figures here are chosen for the example.

The key fact. 1 millimetre of rainfall over 1 square metre of area is 1 litre of water. So rainfall in mm/h multiplied by area in square metres gives litres per hour.

The situation. We use a design rainfall intensity of 75 mm/h and an effective roof area of 48 square metres draining to one gutter.

Step 1 — litres per hour. 75 mm/h x 48 m2 = 3600 litres per hour.

Step 2 — convert to litres per second. There are 3600 seconds in an hour, so divide by 3600: 3600 / 3600 = 1.0 litre per second.

Using the result. The gutter, its outlet and the downpipe must each be able to carry at least 1.0 L/s, so you select sizes from the capacity tables that meet or exceed that figure, usually with a margin.

Takeaway. Flow (L/s) = intensity (mm/h) x area (m2) / 3600. Choose the design intensity and the way effective area is measured from current guidance; Approved Document H sets out the method and capacities for a real design.