In short
Water sitting on a flat roof days after rain means the roof has no fall, or the deck has sagged and made a dip. On EPDM rubber or GRP the water itself does little harm; on old felt it finds every lap and blister and shortens the roof's life. If the pool is over a joint, a lap or a seam, it needs sorting before winter.
This is a job for our Flat roofing service.
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What ponding is telling you
A flat roof is never truly flat. It should be laid to a fall so rain runs to the outlet or gutter and is gone within a day or so. When we see a puddle that is still there two days after rain, one of three things has happened: the roof was built with too little fall, the decking has sagged between the joists, or the outlet has silted up and the water simply cannot get away. We see all three on the back additions and garages around Norwich and Great Yarmouth, and the sagging deck is the most common on older felt roofs where the boards have been wet from underneath for years.
Whether it matters depends on the covering
Standing water is hardest on felt. It sits on the laps, works into any blister, and freezes in winter. The covering itself will not fail because of one puddle, but the puddle is usually where the first leak turns up.
- Torch-on felt: the laps and blisters under a pool are where leaks start. A pool over a lap needs attention.
- EPDM rubber: the membrane does not mind sitting in water. Elevate, who make the RubberCover system we fit, describe it as a single sheet with no field seams on most house roofs, so there is usually no joint under the puddle to worry about. The risk is at upstands and outlets, not in the middle.
- GRP fibreglass: the same. A cured GRP roof is one continuous surface, and a pool in the middle of it is a nuisance rather than a leak waiting to happen.
What matters on every covering is the edge of the pool. If water is standing against a flashing, a rooflight kerb, a wall abutment or a blocked outlet, it is testing the one place a flat roof is weakest.
What to check yourself
- Look at the outlet or the gutter the roof drains into. Leaves, moss off the main roof and tennis balls block more flat roofs than anything else.
- From an upstairs window, note where the puddle is. In the middle is a deck or fall problem. At the edge is a drainage problem.
- Look at the ceiling underneath after heavy rain. A stain that grows is a leak; a stain that never changes may be old.
What we do about it
If the covering is sound and the outlet is blocked, we clear it and leave the roof alone. If the deck has sagged under felt, the honest fix is to strip it, replace the soft boards, put the fall back with firring strips and re-cover. Where the roof is being re-covered anyway, that is the moment to build the fall in. We fit EPDM and GRP for exactly this reason: neither depends on the water leaving quickly. Cure It's GRP system is designed for application across the seasons, with additives for cold and warm weather, so a re-cover does not have to wait for summer.
One thing we will not do is pour a liquid coating over a ponding felt roof and call it fixed. It does nothing about the fall, and the water comes back to the same place.
