How Do Flat Roofs Drain Rainwater?

If you’re considering installing a flat roof on your home you may well be wondering the obvious question – how do flat roofs drain? How do you avoid having water pool on the top and cause damage over time?

Well we’ve put together the basic facts here so you can rest assured that a properly installed flat roof will not have any problems draining water.

The Slope

The first thing to say about how flat roofs drain water away is that they’re not actually totally flat.

Flat roofs are designed to appear level, but have enough of a slope that rainwater naturally flows into the guttering. The surface should be even and a professionally installed flat roof should never have pools of water collect on top of it. If water does sit on the roof for long periods of time it can cause a build up of silt, which can damage the membrane if it subsequently freezes.

A flat roof should have a slope of 1cm for every 60cm of width at the minimum. This is achieved by installing firrings, which are battens of wood at different heights to create a slight slope on the roof.

Guttering

After the slope, on most flat roof designs the water heads to guttering in order to control its descent into the drains and avoid accidental deluges landing on unsuspecting people standing below the roof edge.

Guttering needs to be sturdy and tends to be a standard size so it can withstand adverse weather condition. It will usually be placed on one or two roof edges where the flat roof is designed to drain. There also needs to be an overhang from the roof to the guttering to avoid splashing or spilling.

Gutters are cheap to install and effective, so are one of the most common draining methods for flat roofs in residential homes. It does need regular cleaning to avoid clogging with leaves however!

Integral Drains

For larger flat roofs, like those used for flats, townhouses, commercial buildings or schools, it’s common to have a drain placed inside the roof edge.

The roof is designed to have a gentle slope towards that drain, from where the water is piped away from the walls and foundations. These are often chosen because they can look better than guttering in some circumstances, and all the drainage is hidden.