Gutter Size Calculator (Approved Document H)

Measure roof fascia length along the eaves and measure true rafter span from eaves to ridge using a tape or architectural drawing. Measure roof pitch angle with a spirit level inclinometer. Check gutter profile capacity on manufacturer product tables; half-round, square, deepflow, and ogee profiles offer markedly different flow capacities in litres per second.

Common mistakes when sizing rainwater systems include:

• Using simple flat ground floor area rather than effective roof catchment area, which must incorporate roof pitch multipliers for wind-driven rainfall under Approved Document H.
• Positioning a single downpipe at the far end of an unusually long gutter run, exceeding the maximum allowable draining length from stop-end to outlet.
• Installing gutters completely level rather than incorporating a slight gradient (typically 1:600 nominal fall) to direct rainwater smoothly toward downpipe outlets.

This calculator sizes gutters based on standard UK design rainfall intensity of 0.021 litres per second per square metre (75 mm per hour) under Building Regulations Approved Document H. Roof valleys, parapet gutters, curved dormers, and coastal properties exposed to severe storm gusts require individual hydraulic calculations. Ensure downpipes discharge into legal stormwater drainage or soakaways, never directly into foul sewer connections without permission.

How the estimate works

Effective roof area = eaves length × (half-span + roof rise ÷ 2). Run-off = area × 0.021 l/s per m², the 75mm-per-hour design storm. Gutter capacities with an end outlet are typical manufacturer figures for level gutters.

Worked example (the default inputs)

A 10m eaves with a 4.5m half-span at 35° has an effective area of about 61 m² and a design run-off of 1.28 l/s. That is more than a standard half-round gutter takes with one end outlet (about 0.9 l/s), so use a centre outlet or a deeper profile.

Sources

Approved Document H (Drainage).