Question answered
Water pooling on a flat roof: is ponding a problem?
The short answer
Water pooling on a flat roof is a problem when it stays long after the rain has stopped, sits over a seam or edge, or returns in the same place each time. The recommended finished fall is 1:80, so a properly drained roof should shed nearly all its water. Persistent ponding points to a dip, a blocked outlet or too little fall, and is best fixed at the cause.
A puddle on a flat roof after rain is common. What matters is whether it clears, where it sits and why it forms, because those three things decide if ponding is harmless or a leak in waiting.
When a puddle is normal and when it is not
No flat roof is perfectly smooth, and a thin film of water in a hollow after a downpour is not unusual. A shallow puddle that dries out within a day or so of fair weather, away from joints and edges, is usually just a cosmetic matter.
The signs of a real problem are different. Look for water that is still there well after the surrounding roof has dried, a puddle that is deep enough to cover a lap or an upstand, a patch that is growing, or the same hollow filling every time it rains. A tide mark of dirt or algae around an old puddle shows it has been there for a long time.
We do not put a number of hours on it, because the verdict turns on the material and on where the water lies. We judge it by position and pattern. Water over a seam or against a wall matters far more than a puddle in the middle of a plain sheet. The signs of a leaking flat roof will help you check whether ponding has already caused harm.
The fall a finished roof should have
IKO, a manufacturer, cites the 2018 edition of BS 6229 (since revised) for a recommended minimum fall of 1:80 on a flat roof. That means 12.5 mm of drop for every metre of run, enough to keep water moving towards an outlet or gutter.
The same source adds that a roof ought to be designed at 1:40, which is 25 mm per metre, to allow for building tolerances and deflection. The figure to remember is therefore a design of 1:40 and a finished minimum of 1:80. A roof designed only to 1:80 can end up flat in places once the boards have settled and the joists have taken their load.
If your roof ponds, begin by asking if any fall was ever built in. Many older flat roofs, and some on extensions, were laid with very little slope. The numbers are explained further in what fall a flat roof needs.
Why water stays: dips, blockages and ageing
Ponding has a short list of causes. The first is the fall itself, absent or too small from the start. The second is deflection: a deck or joist that has sagged under load or age, so that a hollow forms between the supports. A third is the covering, which on a felt roof can build up in ridges at the laps and hold water behind them.
Outlets cause trouble too. If an outlet sits proud of the finished surface, or is blocked with leaves and silt, water stands around it. A gutter on a flat roof that cannot take the flow does the same.
Finally, extra weight can create a low point. A paved area or planters on a roof push the deck down, which is one reason a roof used as a terrace needs careful design; see flat roof terraces. Each cause has a different remedy, which is why working out the reason comes before any repair.
What standing water does to the roof
Water is heavy. A depth of 1 mm on one square metre weighs a kilogram, so a puddle 20 mm deep across a few square metres adds a load the deck was not always sized for, and a deeper hollow becomes deeper still as the deck gives.
The covering suffers as well. Waterproof membranes are designed to resist water, but their seams, laps and edge details are the weak points, and water that sits over them is a constant test of every one. Dirt and algae settle in the hollow. Felt can soften and split. Any pinhole, however small, lets water through to the deck, where timber starts to rot unseen.
An insurer may also take an interest, because a roof that has ponded for years can look like a case of neglect. Dated photographs of the puddle, taken after rain, are a useful start if that ever arises, and they help a roofer see how the water behaves too.
Putting it right, from small fixes to a new covering
The smallest job that properly fixes ponding is clearing a blocked outlet, and it is surprising how often that is all it takes. Where a shallow hollow is the issue, a local repair that builds up the low area can help, if the covering is sound and the boards are dry.
When the slope is wrong over the entire roof, or the deck has sagged, a patch is not enough. The proper cure is to lay new falls when the roof is re-covered, using tapered insulation or timber firring strips. A new covering laid over the same dip will simply pond again.
The choice of covering matters less than the shape beneath it. At the survey we check the fall, photograph where water sits, and say whether repair or replacement is the honest answer for your roof. The full range of flat roofing work is described on its own page.
Ponding: two more questions
Is a puddle after a summer shower enough to worry about?
Usually not, if it dries within a day or so and is not sitting over a seam, edge or rooflight. Photograph it, then look again after the next rain. A puddle that comes back in the same hollow, or never fully clears, is worth a closer look.
Will a new covering stop my flat roof ponding?
Not by itself. A covering follows the shape of the deck beneath it, so a dip underneath stays a dip on top. The fix is to correct the fall as part of the re-cover, so that water has somewhere to go.
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