Is water pooling on my flat roof a problem?

By Charlie the Roofer for Roofline · Published 1 November 2026 · Checked against the sources listed below

How much standing water is normal on a flat roof?

A shallow film that drains or dries within about 48 hours of the rain stopping is normal. LABC Warranty notes that heavy downpours often leave temporary standing water on flat roofs. Richard Clennell of Bauder draws the line at 48 hours. A small amount held behind a lap that clears within two days is standing water and does no harm, while water deeper than the laps that is still there after 48 hours is ponding.

Water should not linger because the roof should have a fall. BS 6229, the British Standard code of practice for flat roofs, revised in 2025, recommends a finished fall of at least 1:80, which is 12.5mm of drop for every metre. LABC Warranty says normal practice is to design at 1:40 so the finished roof still reaches 1:80 after site tolerances and small deck movements.

You can check without going up. Look from an upstairs window or the garden after rain, take a photo each day, and note how long the water lasts.

Why does ponding water matter?

Ponding matters because it adds weight to the deck and wears on the covering until something gives. LABC Warranty lists the risks:

• Sagging that feeds itself. Water adds load, the deck bends, and the deeper dip holds more water, until the deck is damaged or the roof is put right.
• Faster leaks. If the covering is punctured under a pond, the depth of water builds a head of pressure that drives it into the building more quickly.
• Blocked outlets. Standing water collects leaves and algae, which then wash into the outlet and make the ponding worse.
• Frost damage. Water that freezes stiffens the membrane. On the thaw, the parts still under ice move more slowly than the rest, which can force a lap open. That is a real risk through a Pennine winter of repeated frosts.

What causes water to pool on a flat roof?

Most ponding happens because water cannot reach the outlet, either because the outlet is blocked or set too high, or because the roof surface dips.

• Blocked outlets or gutters. Debris and silt around a drain guard slow the flow so water backs up. Where a roof drains over its edge, a blocked gutter can hold water back in the same way.
• Raised outlets. When the covering around an outlet is finished after the main roof, the extra layers form a step that holds water back and traps silt. The 2018 edition of BS 6229 recommended recessing these laps or fitting the outlet in a sump.
• Falls that are too shallow. A roof built flatter than 1:80, or one with back falls, cannot shed water properly. Where two falls meet the valley is shallower still: LABC Warranty's example has two 1:80 falls meeting in a valley of only 1:113.
• A sagging deck. The covering follows the deck beneath it, so a pool in the middle usually means the deck or joists have bowed. LABC Warranty notes that ponding can follow where no allowance was made for the deck to deflect. Insulation crushed under the covering leaves a low spot in the same way.
• Obstructions. Water often sits behind a rooflight unless a raised wedge, called a cricket, turns it aside.

How is ponding fixed, and what will a roofer check?

The fix depends on the cause, so it starts with an inspection rather than a product. From above, a roofer checks the outlets and the laps around them, where the water sits and how deep it gets, and the state of the covering under it, then measures the actual falls. When its surveyor finds ponding on a new roof, LABC Warranty requires an independent level survey. From below, the roofer looks at the ceiling for staining or sag and, where there is access, at the joists and deck for damp or rot. Our guide to finding a roof leak covers what to look for indoors.

Typical fixes, from smallest to largest:

1. Clearing the outlet or gutter, where a blockage is the only fault.
2. Recessing a raised lap, forming a sump or adding an outlet at the low point.
3. Lifting the covering and rebuilding the fall with tapered firrings or tapered insulation. Laying flat insulation boards over a poor fall does not cure it.
4. Stripping back to the joists and re-decking where the boards have sagged or rotted.

Avoid filling the dip with levelling compound on a timber deck. If the deck has sagged, the compound adds weight to a structure that is already struggling. LABC Warranty explains that added load on a deck increases its deflection, so the dip deepens. If water is still sitting on your roof two days after rain, send a photo on WhatsApp to 07413 797731 or book a free inspection. Roofline will send you photos of what it finds with a free itemised quote. Our flat roofing page explains the systems we fit.

Sources

1. LABC Warranty, Technical Update: Ponding on flat roofs (TS-3242-2.00)
2. RICS Modus, Bauder sponsored content (December 2024): Richard Clennell on standing water and ponding
3. NFRC, BS 6229:2025 update: key points for members
4. BSI Knowledge, BS 6229:2018 Flat roofs with continuously supported flexible waterproof coverings
5. Building Centre / SIG Design & Technology, BS 6229:2018 the changes explained
6. IKO, Ponding water on a flat roof: how to stop it

General information about roofing in England, checked against the sources above when published. It is not a substitute for advice about your own roof.