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.

Approved Document H's gutter and outlet sizing table

In Approved Document H Table 2, read the maximum effective roof area against the half-round gutter and outlet sizes. The listed capacity assumes a level eaves gutter draining through a sharp-edged outlet at one end:

• 75 mm gutter with a 50 mm outlet: up to 18 m² (0.38 L/s)
• 100 mm gutter with a 63 mm outlet: up to 37 m² (0.78 L/s)
• 115 mm gutter with a 63 mm outlet: up to 53 m² (1.11 L/s)
• 125 mm gutter with a 75 mm outlet: up to 65 m² (1.37 L/s)
• 150 mm gutter with an 89 mm outlet: up to 103 m² (2.16 L/s)

The table holds only for the conditions it states: a level half-round gutter with a sharp-edged outlet at one end, and no more than 50 times the water depth from stop end to outlet; over a longer distance the capacity is reduced. A gutter laid to a fall, or fitted with a round-edged outlet, can sometimes take a larger area. A sharp bend close to an outlet can introduce energy loss and reduce the rated gutter capacity; use the applicable design allowance. Leaf guards limit debris entry but still need clearing, and their effect on flow must suit the manufacturer’s capacity conditions. Do not assume fitting a guard increases the rating.

Sharing the flow between outlets

A long gutter does not have to drain through a single outlet. Divide the contributing area between the outlets and calculate each share’s design flow. Each is half only when the catchment is shared equally; verify both gutter arms, outlets and downpipes against their ratings. At a single central outlet, size the gutter using the larger contributing area on either side, but the common outlet and downpipe must carry the sum of both flows. Shorter gutter arms do not automatically double the same outlet’s rating.