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Cast iron rainwater

What size gutter does your roof need?

Enter your postcode and a few roof measurements. We size the gutter and downpipe for a heavy UK downpour, and show how much rain your area gets in an average year.

Average rain a year, mm

Your roof

m
m
The length of gutter that this roof slope drains into.
40°
Steeper roofs catch more wind-driven rain. Most UK houses are between 30° and 45°.
Design rainfall
The heaviest two minute storm expected about once a year where you live. Choose the UK maximum for extra margin.
Outlets
m
0 m puts the outlet at the end. Half the eaves length puts it in the middle, which halves the water each side carries.
Other details
A corner takes 15% off the capacity. A guard or grating halves it.

Every style for this roof

StyleSizeOutletRated l/sResult
How we worked it out

How we size it. We follow BS EN 12056-3 and Approved Document H. Gutters are sized for a short, heavy storm, not for average rainfall. The design figure is the heaviest two minute storm expected about once a year at your postcode, read from Approved Document H Diagram 1. Pitch is allowed for by adding half the roof's vertical height to its plan depth. Capacities are Heritage Foundry flow rates for each gutter and outlet, for a level run up to 50 times the gutter depth. We reduce them for longer runs, corners and guards. Where the outlet is not at the end, we size the gutter for the longer side.

This is guidance for simple eaves gutters that overflow outside the building. For valley or parapet gutters, gutters over a doorway or where an overflow could get inside, runs over 20 m, or Scotland and Northern Ireland, contact us and we will check it for you.