How Do Flat Roofs Drain Water?

The fall does the work

Nothing about a flat roof is truly flat. Joists are cut or firring strips are laid to give the surface a gradient, usually just a few degrees, running to a chosen point. Water follows that slope without a pump or any mechanism. If the slope is lost, through a sagging deck or a build-up laid level, the water stops moving and collects. Our page on the minimum fall for a flat roof explains how much slope is needed.

The shape of the roof decides where the slope goes. A small extension may fall one way, towards the garden. A larger roof may fall to a valley in the middle, or be divided into sections that each fall to their own outlet. The designer picks the route that keeps water away from walls, windows and doors.

Gutters at the edge

The simplest arrangement is a roof that falls to one edge, where it discharges into a gutter. The edge trim carries the water over the lip and into the gutter, which runs to a downpipe and a drain or soakaway. It is easy to build and easy to check from a ladder. The drawback is that it relies on a clean gutter. Leaves, moss and silt fill it quickly, and a blocked gutter holds water back onto the roof edge.

The trim and the gutter need to be matched. If the drip sits too far back, water misses the gutter; if the gutter is set too high, it holds water against the trim. Our guide to flat roof edge trims goes through the details.

Outlets and scuppers

Where a roof is enclosed by parapet walls, water cannot spill over the edge, so it has to leave another way. A roof outlet is a drain set into the surface, with a grating to stop debris, connected to a pipe that runs down inside or outside the building. A scupper is an opening through the parapet that lets water out into a hopper head and downpipe. Both depend on the fall leading to them, and both must be kept clear.

Many parapet roofs have a second route as well, called an overflow, set a little above the main outlet. If the outlet blocks, water rises to the overflow and spills out visibly rather than building up on the roof. An overflow that begins to run is a warning that something is blocked. Never ignore it.

What goes wrong

Blockage is the commonest fault. A leaf-filled gutter, a grating clogged with moss or a downpipe jammed with debris all stop the flow, and the water that cannot leave builds into a pool. A sagging deck produces a similar result by creating a low spot away from any outlet, which is the cause of the pools described in our guide to ponding on a flat roof.

An undersized outlet is another cause. A heavy downpour can deliver more water than a single small outlet and pipe can pass, so the roof floods briefly even though nothing is blocked. Frost can also split pipes or freeze water at the outlet, and trees overhanging the roof drop leaves and seeds that seal the grating. A flat roof beneath a tree needs more frequent attention.

Rainwater pipes and where the water ends up

Leaving the roof is only half the journey. The downpipe has to carry the water to a drain or soakaway, and a pipe that discharges onto a patio, against a wall or into a blocked gully can soak the building at the foot instead. Inside an extension, concealed pipes run through the structure, and a joint that leaks there is hard to trace. When we replace a roof, we check the whole route as well as the surface, and tell you if the pipe or gully needs work as well.

Keeping drainage working

Check gutters and outlets in autumn after the leaves fall, and again in spring. Look from a ladder or an upstairs window, not from the roof itself. Clear debris by hand and flush with water to see whether it runs freely. If a pool remains after the drains are clear, the problem is in the roof's shape rather than a blockage, and it needs a survey.

When we quote for a new flat roof, we look at where the water will go before we look at the covering. We photograph your existing outlets, gutters and downpipes in our free survey so you can see their condition, and the fixed written quote shows what each change costs. For the wider range of work, go to flat roofing in Basingstoke.