How to repair a flat roof: common problems, ponding water and when to call a roofer

Knowing how to repair a flat roof starts with knowing what has failed. A split in the felt, a lifted joint, water that sits for days, a damp patch on the ceiling: small faults on a flat roof rarely stay small for long.

Caught early, most flat roof repairs are straightforward. Left too long, the same fault turns into rotten decking, wet insulation and a much bigger bill. This guide covers the common problems, how each one is repaired, how to deal with ponding water, how to look after a felt roof, and when a patch stops making sense. For anything beyond a quick fix, our flat roof repair and replacement team can assess the deck and covering properly.

Why flat roofs fail

Flat roofs deal with standing water, temperature swings, sun, moss, debris and foot traffic. Unlike a pitched roof, they do not shed water quickly, so small weaknesses let water in over time. Trouble starts at the edges, flashings, upstands, outlets, joints and corners far more often than in the middle of the roof.

Most leaks come down to one of these:

• ageing materials
• poor installation
• standing water
• split or lifted seams
• failed flashing
• movement in the roof structure
• previous repairs done badly

That last one catches people out. A leak that has had sealant thrown at it two or three times keeps coming back because the cause was never dealt with.

The most common flat roof problems

1. Ponding water

Water that sits on the roof long after the rain has stopped wears the surface, stresses the joints and finds weak points. It has its own section below, because the fix depends entirely on the cause.

2. Cracks, splits and blisters

Coverings expand and contract with temperature, which leads to cracking, splitting and blistering, especially on older felt. A blister means air or moisture is trapped under the surface. Some stay stable for a while, but once they split they let water in. Our guide to flat roof bubbling and blistering covers them in detail.

3. Lifted joints and failed seams

Where materials overlap, the joint has to stay sealed. When a seam fails, water tracks under the covering and travels a long way, which is why the damp patch inside is not always directly below the leak.

4. Damaged flashing

Flashings around walls, skylights, roof lanterns, pipes and edges are common failure points. Once flashing cracks, lifts or pulls away, water finds its way in.

5. Surface wear and punctures

Foot traffic, falling branches, dropped tools and age all damage the surface. A small puncture does not look like much from above, but it still lets water through.

How to spot flat roof problems early

The best time to repair a flat roof is before the leak shows inside. Look out for:

• damp patches on ceilings or walls
• bubbling paint or peeling plaster near the roof line
• sagging areas on the roof
• splits, blisters or open joints
• standing water that hangs around
• loose or cracked flashing
• moss, dirt build-up or blocked outlets

Check after heavy rain or strong wind, which is when minor faults first show themselves.

How to inspect a flat roof safely

A roof check is not worth a fall. If you look yourself, only do it in dry weather, wear footwear with grip, keep off soft or damaged areas and stay away from the edges. Most of it can be done from the ground, from an upstairs window or with your phone's zoom. If access is awkward or the roof feels unstable, leave it to a roofer.

How to repair a flat roof, depending on the problem

The right repair depends on two things: the type of roof and how far the damage goes.

Small cracks or splits

Isolated splits can be repaired with a compatible patch or liquid repair system, as long as the rest of the roof is in decent condition. The area must be clean, dry and properly prepared first. A patch on a damp or dirty surface rarely lasts.

Lifted seams or joints

The section is resealed properly, or the damaged material is cut back and a new patch or joint repair is fitted. The method depends on whether the roof is felt, GRP or EPDM. A bad seam repair is the quickest way to get the same leak back a few months later.

Damaged flashing

Loose or worn flashing is resecured, resealed or replaced. If it failed because the covering around it is breaking down, fixing the edge alone will not last.

Flat roof repair by roof type

Felt: common on garages, extensions and older roofs. Repairs include patching splits, resealing joints, sorting flashings or relaying a damaged section. If the felt is brittle, badly cracked or already patched in several places, another repair is just delaying the real job.

EPDM rubber: durable, but prone to punctures, edge failure and poor detailing around penetrations. Repairs use EPDM-specific primers and patches. The wrong sealant on rubber makes a mess quickly.

GRP fibreglass: cracks around joints, trims and raised details. Repairs need proper surface preparation and compatible resin and matting. Done well, a local GRP repair lasts. Done badly, it stands out and fails.

Bitumen or asphalt: cracks with age and movement. Repairs range from local sealing and patching to an overlay, depending on condition.

Ponding water on a flat roof

Ponding is the flat roof fault homeowners ask about most, and the one most often misdiagnosed. People treat it as a surface problem when it is a level problem.

What counts as ponding?

Ponding is water still sitting on a flat roof 48 hours after rain. A shallow film that dries within a day is normal and not a concern. It becomes a problem when:

• water stays for several days
• the same puddles form after every downpour
• the puddles get deeper or wider over time
• moss, algae or silt collect where the water sits

Why the falls matter

No flat roof should be truly flat. It needs a slight fall so water runs to the gutters or outlets. When the fall is too shallow, or the deck has dropped, water collects in the low spots. The causes are:

• Not enough fall built in: the roof was designed or laid too flat, so water has nowhere to go.
• Sagging joists or decking: over the years the deck bows under load and creates dips.
• Blocked outlets and gutters: leaves, moss and silt stop water leaving the roof.
• Settlement in the deck: boards and joins sink slightly and form shallow troughs.

Standing water breaks membranes down faster, especially at seams and overlaps. It adds weight to the structure, encourages moss, freezes and expands in winter, and gives water more time to find a weak point. It can also void some manufacturer guarantees.

How to fix ponding, from minor to major

• Clear the drainage first. Plenty of ponding is simply blocked outlets or gutters. Clear them, then check again 48 hours after the next rain.
• Minor, isolated dips: on a sound roof, a small low spot can be built up locally or the drainage route improved. Levelling compounds only suit very specific situations; used wrongly, they move the water elsewhere and add weight.
• Regular ponding in the same areas: the falls need correcting, with tapered insulation laid to create a proper fall, or with extra or repositioned outlets.
• Widespread ponding or a sagging deck: the roof is stripped back, the structure corrected and the deck re-laid to the right falls before a new covering goes on.

Sealant does nothing for ponding. If the level of the roof is the problem, only correcting the level will stop it.

Looking after a felt roof

Felt is still the most common covering on garages, porches and older extensions. Sunlight dries the bitumen out, standing water eats into it, and movement cracks the edges and joints first. The damage builds slowly, so regular checks make a real difference.

Safe homeowner checks

• Inside: look at the ceilings and upper walls below the roof for brown stains, peeling paint or a musty smell. In a garage, check the underside of the deck for dark patches, soft timber or rust marks around nails.
• From the ground: look for sagging, blisters, cracks, bare patches where the mineral finish has worn away, and water still sitting a day or two after rain. Binoculars or a phone zoom help.
• Keep water moving: clear gutters and outlets of leaves and moss.
• Keep it clean: have the surface brushed off once or twice a year. Never pressure wash felt, it strips the surface.
• Keep off it: felt is not built for foot traffic. If another trade needs access, they should use boards to spread their weight.

Leave torch-on work, cutting blisters, ponding fixes, flashings and coatings to a roofer. They involve heat, height and detailing that a DIY repair rarely gets right.

Patch or replace?

A felt roof is worth patching when the damage is in one place, the surface is still flexible and the deck is firm. It is time to replace when:

• the surface is cracked or crazed across a wide area
• edges and corners are lifting in several places
• leaks appear in different spots over a short period
• the deck feels soft or sags
• old repairs are layered on top of each other
• the felt is at the end of its 15 to 25 year life

At that point a new roof costs less than chasing leaks. Our guide to the best material for a flat roof compares the options.

Temporary repair vs proper repair

A temporary repair limits water getting in and buys time. A proper repair fixes the failed section and the reason it failed. A temporary fix makes sense when a leak starts suddenly, bad weather stops a full repair, or you need to protect the inside of the house first. If a roofer tells you a repair is temporary until it is dry enough to do the job properly, that is the honest answer, not a bad sign.

When a flat roof repair is worth doing

A repair is the right call when the fault is isolated, the rest of the roof is sound, the deck and insulation underneath are dry, the roof is not near the end of its life, and there is no underlying drainage problem.

When patching is a waste of time

Another patch is money wasted when:

• the roof has multiple leaks
• there are signs of long-term trapped moisture
• the covering is breaking down across a wide area
• the roof has already been repaired several times
• ponding keeps returning because the falls are wrong
• the deck has gone soft or started to rot

As a guide, a minor repair costs £150 to £350 and a larger leak repair £400 to £1,200, while a new felt or EPDM roof on a single garage costs £1,500 to £3,000. Once repairs start stacking up, replacement is the cheaper route. For more figures, see our roofing costs guide.

Mistakes people make when repairing a flat roof

Repairing a wet roof. Repair products need a clean, dry surface to bond. On a damp, dirty or unstable surface the repair fails.

Using the wrong product. A fix that works on felt can be useless on EPDM or GRP.

Fixing the symptom, not the cause. If the flashing failed because water sat against it for months, or a seam split because the roof is moving, the leak will come back.

How to make a repaired flat roof last longer

Keep outlets and gutters clear, remove leaves and debris, check flashings and edges twice a year and after storms, and deal with minor faults early. Waiting until water comes through the ceiling is how small roofing jobs become expensive ones. Our guide on how long a flat roof lasts covers lifespans by material.

When to call a roofer

Get a roofer involved if:

• the leak keeps coming back
• several areas of the roof are damaged
• sagging or standing water does not clear
• the covering feels soft or unstable
• flashing and edges are failing in several places
• you are not sure what the roof is made of
• the repair needs heat, resins or structural work

Alliance Roofing & Building will inspect the roof and tell you straight whether it needs a roof repair, a fix to the falls or full replacement. Based in Woking, we cover Surrey, east Berkshire and south-west London, including Chobham, Guildford and Richmond. Fill out the form at the end of this page and we will arrange a visit.