A guide to roof drainage

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Drainage is a subtle art when it comes to building & repairing flat roofs and it can be achieved in a variety of ways. The aim of course is to allow rainwater to run off the surface of the rook and swiftly into the gutters without letting water gather to form little ponds on the roof surface. However there are sometimes problems and even small ponds can cause trouble. A small pool begins a slow process of pooling leading to the gathering of silt which in turn leads to organic growth that causes stress on the roof membrane and slowly degrades it. Ice and animal life can add wear to a roof surface and does the worst damage where a roof is home to a pond or two. We’ve seen neglected flat roofs complete with tufts of grass and moss surrounding a pond of a few inches deep, the damage in a severe case like this is complete to the roof and the timbers below.

No such thing as a flat roof

The secret to drainage on a flat roof is not to build a flat roof. The correct term is ‘low incline roof’ but we don’t hold it against anyone for calling them flat as a typical minimum ‘fall’ for a roof is 1:80. So for every 8 meters of roof you need at least 10cm of drop. In actual fact the minimum drop varies depending on the materials & configuration used and an experienced roofer will choose to build a far greater drop into a low incline roof where conditions as for it. The drop can be achieved in a variety of ways depending on roof construction but typically there’ll be level joists to create the flat ceiling underneath and angled timbers called firrings attached to the top of the joists onto which the roof deck can be built.

Now if you have a flat roof you may find it interesting to muse about it’s construction but knowing how it was built will not help you much in diagnosing a drainage problem on your existing flat roof. You’re likely to notice if the roof is retaining water but working out why is much trickier. We cannot contain in this article full details of all you need to know to make the right judgment in every flat roofing situation but here are a few basics.

Why isn’t my flat roof draining?

If you’ve seen water pooling on your roof it’s possible to look for the right signs and get an idea of why the problem is occurring.

Guttering & drains

If the guttering and drainage system is obstructed in some way it will likely cause some issues on top of the roof itself. If you don’t have a good view of your gutters and can’t remember the last time you looked and then your see water gathering on the roof, it’s probably time to have a check. Clearing gutters is not rocket science but you do need to be safe, please take care and see our notes on safety below.

The lay of the roof

If you can get a view of your roof from the right angle, you’ll be able to see how it lies. With your eyes at roof level you will be able to see the fall of the roof. When the roof is dry it’s easiest to see how the overall shape of the roof lies. You may be able to identify depressions or bowing in the roof surface and work out how it’s causing the pooling you’ve seen. If the roof surface is uneven it may be that some timbers have slipped or that the deck is getting too hot or too wet in particular areas, it’s hard to tell for sure and to get it fixed, a qualified roofer had best take a look at it.

The roofing membrane

It’s possible that the way the roofing membrane has been installed or how it’s bowed over time can lead to a lip at the edge of the flat rooftop. Even a small lip can cause sizeable pool that should be tackled. If your roofing membrane is faulty it’s best approached by a qualified roofer. Some issues have quick fixes and sometimes quick fixes will make the issue worse and the cost greater.

Obstruction & detritus

It might sound obvious but people sometimes underestimate how much difference a blanket of leaves makes to the drainage a ability of a flat roof. We’ve been called out to inspect roofs that were thought to have drainage problems to find that brushing down the surface with a wide soft nylon broom and clearing the gutters revealed a fully functional roof capable of draining well in normal conditions. If your roof isn’t draining well but has a scattering leaves, twigs or other organic bits it could certainly do with a brush down. If you decide to clear debris from your flat roof, please take care & have a read of our safety section below.


We can’t stress enough how careful you should be in going up on ladders or out on top of flat roofs. If in doubt, don’t stand on a flat roof and always take care using ladders and have a mate down below capable of holding that ladder if it slips. Wear proper clothing & avoid strong winds.