Strictly speaking no flat roof is level. It is a gently sloping surface, and its job is to deliver rainwater to an outlet. When it fails to do that, water collects in shallow pools, called ponds, and stays there. A pond is a symptom, and finding its cause decides whether the fix is a quick clear-out, new tapered insulation, a revised outlet or a replacement deck.
This guide is about the causes and cures. It does not set out the minimum fall figure, which has its own page on the fall a flat roof needs. Here we deal with the three usual culprits (a deck that sags, falls that were never right, and outlets that are blocked or badly placed), how tapered insulation corrects a roof, how gutters take the water away, and what standing water does to the covering. The wider service sits on our flat roofing page.
Three reasons water stays on a flat roof
Water that lingers more than a day or so after rain nearly always comes down to one of three faults, and sometimes a combination.
Deflection in the deck
Deflection means the structure has bent under load. The joists or boards sag between their supports, and the lowest point of the sag becomes a pond. Approved Document A recognises the problem: it refers to a roof structure that cannot carry an increased load because of overstressed members or unacceptable deflection leading to ponding, and says strengthening follows. The causes include joists that are too small for the span, rot at the wall ends, and loads that have been added since the roof was built.
Falls that were never right
Some roofs were built flat, or with a fall too shallow to survive the tolerances of ordinary building work. A deck laid dead level, or with a fall that reverses in a corner, holds water from the day it is finished. This is often found on older extensions and on garage roofs where drainage was an afterthought.
Blocked or misplaced outlets
An outlet that sits above the lowest part of the roof leaves a permanent pond around it. An outlet that is full of leaves, moss and silt backs water up across the roof. Here the pond is temporary and can usually be cleared, but if the outlet is the wrong size or the wrong place, it will return.
Telling them apart
A pond that appears in the same place every time, and stays for days, points to the deck or the falls. A pond that appears after heavy rain and goes down slowly points to an outlet. Photographs taken the day after rain, from above, are a great help when we survey, and we take our own as part of every inspection.
What a pond does to a membrane
Water is not an automatic enemy of a waterproof covering. Rubber, fibreglass and felt are all made to keep it out. What a pond adds is time, weight and temperature swings, and those work on a roof in several ways.
Constant wetting
A covering that is wet for days at a time ages faster at the surface than one that dries after each shower. Algae and silt settle in the pond, hold moisture against the surface and make the pond harder to clear. On felt, standing water finds any lifted lap or hairline split and works it open.
Freezing and thawing
Water that freezes expands, and ice in a pond can lift a seam or stress a weak spot. When the ice melts, the cycle starts again. A roof with no ponding is spared much of that.
Weight
Water is heavy, and a pond adds load to the part of the deck that is already sagging, making the dip deeper. That is how a small deflection can become a larger one over several years.
Warranties and ponding
Each manufacturer writes its own guarantee terms, so read the ones for your product with drainage in mind. The guarantee we give on our own workmanship runs for 10 years and is in writing.
Overall, ponding shortens the life of a covering by increasing the wear it receives, which is why our guide to flat roof lifespan treats drainage as a first question.
Tapered insulation and how it corrects a roof
When the deck is sound but the falls are wrong, the usual answer is to build the fall into the insulation. Tapered insulation is supplied as boards cut to a slope. Laid in a planned pattern, the boards turn a level deck into one that drains to the outlets, without having to rebuild the joists.
How the scheme works
The insulation supplier prepares a layout from the roof's dimensions and the positions of the outlets. The layout shows each board, from thin to thick, so the roof falls toward the outlet. Rockwool's guidance on tapered falls, citing BS 6229:2018, explains a design fall of 1:40, so that tolerances still leave finished falls of 1:80. IKO's advice for single-ply roofs makes the same point. The extra allowance in the design is deliberate.
Cross-falls and crickets
On a roof with a large flat area, the water is steered with cross-falls and with small ridges called crickets, which push water round obstacles such as a chimney, an upstand or a rooflight kerb. Without them, water backs up against the obstruction.
Where tapered boards fit in a replacement
Because the insulation is part of the roof build-up, the best time to correct the fall is when the covering is being renewed anyway. It also lines up with the thermal rules: renewing the whole covering can bring the roof under Approved Document L, as described on our page about insulation upgrades. We specify the tapered scheme at the survey and include it in the quote.
Outlets, gutters and getting the water away
A roof that falls well still has to put its water somewhere. There are two common ways: an outlet through the roof into a downpipe, and an open edge that discharges into a gutter.
Roof outlets
An outlet is a drain set into the roof surface, with a grating to keep leaves out. It must be fixed at the lowest point, with the covering dressed neatly into it, and big enough for the area it serves. Undersized outlets cope on a drizzly day and flood on a wet one. A good outlet is also reachable, because a blocked one needs clearing, and nobody will keep it clear if they cannot get to it.
Overflows
On a roof surrounded by a parapet, an overflow through the upstand gives a second way out if the main outlet blocks. Without it, the water level can rise until it spills over the damp-proof detail at the wall. We pay attention to this on extensions with a high wall around three sides.
Gutters at the edge
Where the roof drains over an edge trim into a gutter, the gutter has to be able to receive it. Gutter makers generally ask for a slight fall toward the outlet, somewhere from 1:600 to 1:350 depending on the system. FloPlast gives 500 mm as the widest bracket spacing on straight lengths of its MiniFlo range, and Brett Martin suggests supports at no more than 900 mm. Where a gutter droops between brackets, water sits in it and spills behind the trim. Our guidance on flat roof edge trims shows how the lip and the gutter meet.
Clearing, inspecting and testing
Many ponding problems on Stroud roofs begin with debris. Houses on wooded hillsides and streets with mature trees drop leaves, seed cases and twigs onto roofs, and an unattended roof collects them. This is a plain fact of leaf fall, and it says nothing about the weather.
What a regular clear-out involves
Leaves are lifted from the surface and the outlet grating, the outlet is checked for silt, and gutters are flushed. Moss and algae are brushed away, using a soft brush that will not scratch the surface. A pressure washer can damage some coverings, and MyBuilder's cleaning guide warns it may harm delicate surfaces. That guide, updated August 2026, puts flat roof cleaning at £300 to £600 in its national ranges.
Checking the cause
After a clear-out, a roof that still holds a pond has a structural or fall problem. We check the deck for sag by sighting along it and by using a straight edge, and we measure how far the pond reaches. If the pool is shallow and small, repair or a local tapered patch may be enough. If the whole roof holds water, the covering is probably at the end of a sensible repair life.
A simple test you can do
Check the roof the morning after a downpour, or ask us to. Take photographs from an upstairs window or from a ladder, without stepping onto the roof. Send them with the date and a note of the rainfall, and we can tell you whether the fault looks like a blockage or a structural problem. Our guide to flat roof leak signs shows what to look for inside the house.
Repair, re-cover or rebuild: choosing the fix
The fix depends on the cause, and the options range from a morning's work to a full strip.
Clearing and re-fixing
If the problem is a clogged outlet or a sagging gutter, clearing and re-aligning is usually all that is needed. This is the least costly outcome and the one we all hope for.
Local build-up
A shallow pond over a sound roof can sometimes be treated by building up the low area with a tapered section before the covering is renewed. This depends on the covering being repairable, and it is a judgement we make on site.
Re-cover with new falls
When the covering is near its end, the best time to correct drainage is the re-cover. The old surface is stripped, the deck is repaired, and a tapered insulation scheme or new firring strips set the falls. A re-cover that touches more than half the roof surface can trigger a thermal upgrade, so the two jobs are best planned together. Our guide to extension flat roofs shows how drainage fits into the build-up of a new roof.
Structural repair
If the deck has sagged because the joists are too small or rotten, the cure is structural: new joists, sistering, or a new deck. Where the existing structure cannot carry what is proposed, a structural check comes first, and the Planning Portal suggests asking a structural engineer or surveyor first.
The decision and the quote
Our page on whether to repair or replace a flat roof sets out how we weigh the options. The written quote after the survey covers scaffold, strip, deck work, tapered insulation, outlets, covering, trims, labour and waste.