In short
Water pools on a flat roof because the roof no longer has enough fall to drain: the deck has sagged, the outlet is blocked, or it was never laid with a proper slope. A flat roof should still finish at a fall of at least 1:80 under BS 6229, and standing water that does not clear after rain is adding weight and finding every weak seam. Re-covering the roof without correcting the fall leaves the same puddle on a new membrane.
This is a job for our Flat Roofing service.
Why the water is not draining
"Flat" roofs are not meant to be flat. The Liquid Roofing and Waterproofing Association's guidance note on falls quotes BS 6229: the minimum finished fall at any point should be 1:80, and roofs are normally designed steeper, at 1:60 or 1:40, so that once the timber settles they still end up at 1:80. On the garage and extension roofs we see in Spalding and Holbeach the usual story is a deck laid to a token fall in the first place, joists that have sagged under decades of wet felt, and an outlet sat in the one spot the water cannot reach.
The LRWA note is blunt about older roofs: they were designed with a fall, they deflect over time, and re-waterproofing alone will not address a pre-existing ponding issue. That is the sentence to remember when someone offers to overlay your roof in a morning.
Why a puddle matters
- Weight. The LRWA note works it out: water is 1 kg per square metre per millimetre of depth, so 25 mm of water over 20 square metres is an extra 500 kg sitting on the joists. The deck sags further, the puddle deepens.
- Leaks find the puddle. Any pinhole, split lap or lifted edge under standing water has a reservoir on top of it rather than rain that runs off.
- Dirt and algae. The water leaves silt and green growth, which blocks the outlet and makes the ponding worse.
- Frost. A sheet of ice on a felt roof lifts the laps as it expands.
What we do about it
The first job is to find out whether the deck is sound. We lift a corner of the covering, check the boards and joist ends for rot, and measure the fall with a level. Then the options are:
| What we find | What we do |
|---|---|
| Blocked outlet, deck sound, fall present | Clear or enlarge the outlet, fit a leaf guard, repair the covering |
| Deck sound, fall lost | Firrings or tapered insulation to rebuild the fall, then a new EPDM or GRP covering. The LRWA names tapered insulation as the preferred fix where the roof is being insulated at the same time |
| Deck rotten | Strip to the joists, replace what is soft, new deck laid to a real fall, new covering |
Checkatrade's 2026 roof repair guide prices EPDM repairs at £120 to £170 per square metre and GRP at £130 to £180, and notes older flat roofs are more prone to leaks because of poor drainage. We would rather quote you once for the fall and the covering together than patch a roof that will pond again next month. A new flat roof from us carries our ten-year guarantee, and that guarantee is only worth something if the water leaves.