Standing water on a flat roof – when does ponding become a problem?
If you can see your flat roof from an upstairs window, you’ve probably noticed puddles on it after a heavy shower. Most of the time that’s nothing to worry about. But if the same patch of water is still sitting there days later, your roof is telling you something.
Roofers call this ponding. It’s one of the most common problems we find on flat roofs over extensions, garages and dormers, and it’s much easier to deal with early than after it has caused a leak.
What is ponding on a flat roof?
“Flat” roofs aren’t actually flat. They’re built with a slight slope, called a fall, so rainwater runs off into a gutter or outlet. When part of the roof dips or the fall isn’t enough, water collects in that spot and stays there.
A common rule of thumb in the trade is that water still sitting on the roof around 48 hours after rain counts as ponding. A thin film that dries off in an afternoon is normal.
Why is standing water bad for a flat roof?
- It wears the surface faster. Constant wetting and drying, plus sunlight through the water, ages felt in particular much more quickly.
- It finds weak spots. If there’s a tiny split, a lifted seam or a nail hole under that puddle, water will get through it eventually.
- It adds weight. Water is heavy. A large pond puts extra load on the deck, which can make the dip worse over time.
- It encourages moss and algae, which hold even more moisture against the roof.
- It freezes. In winter, ice expands and can open up cracks in the surface.
What causes ponding?
Not enough fall
Some flat roofs were simply built too flat in the first place. Without enough slope, water has no reason to move and sits wherever there’s a slight hollow.
A sagging deck
Over the years, the boards under the roof covering can sag between the joists, especially if they’ve been damp for a long time. A soft or spongy feel underfoot is a warning sign, and it’s often linked to rot in the roof timbers.
Blocked outlets and gutters
Sometimes the roof is fine and the water just has nowhere to go. Leaves, moss or a blocked downpipe at the edge of the roof will hold water back. Check your guttering first, as it’s the easiest fix.
Uneven repairs
Patches and overlays can leave ridges and hollows across the roof surface, which trap water on the uphill side.
How do you fix a ponding flat roof?
It depends on the cause and how bad it is.
- Clearing the drainage. Unblocking gutters and outlets, or adding an extra outlet at the low point, can solve the problem on its own.
- Filling small hollows. On an otherwise healthy roof, small dips can sometimes be built up so water flows away.
- Re-roofing with a proper fall. If the roof is near the end of its life anyway, the best fix is to strip it and build in the correct fall using firring strips or tapered insulation, then lay a new covering.
- Replacing the deck. Where the boards have rotted, they need replacing before anything else is done.
When we re-roof, we usually recommend EPDM rubber or GRP fibreglass. Both cope with occasional standing water far better than traditional felt, though getting the fall right still matters. For a closer comparison, see is rubber roofing better than felt?. In some cases an overlay roof can be an option too.
How can you keep a flat roof draining well?
Have a look after heavy rain, especially in autumn. Keep gutters and outlets clear, brush off leaves and moss if you can do it safely from a ladder at the edge, and don’t use the roof as a walkway or a place to store things. If you do notice a pond that’s growing, or a soft spot, get it checked before the winter. Our bitumen roof maintenance guide has more tips for felt roofs.
Flat roof repairs in Kingswood, Lingfield and across Surrey
Worried about standing water on your flat roof? We’ll come and take a look, find out why it’s ponding and tell you honestly whether a repair will do or it’s time for a new roof. We cover Kingswood, Lingfield, Redhill, Reigate and the surrounding areas. Contact Collier Roofing for a free quote.