A damp patch on the garage ceiling, a drip onto the car, a bloom on the wall: the first sign of a leak is almost never the leak itself. Water enters at one point, runs along a rafter, a purlin or the underside of a deck, and drops somewhere else. Chasing the stain with sealant is the commonest mistake, and the one that costs most.
Here we cover every kind of garage roof leak, whatever the covering, and the test we apply to choose between patching and starting again. If yours is a felt roof and the question is patching, our answer on repairing a felt garage roof goes into the method. Roofs on converted garages are a special case, covered in garage conversion roofs. The broader range of garage work is on the garage roof replacement page. When water is coming in now, a roof that is letting water in jumps the queue with us, and we stop the ingress before planning the lasting fix.
Trace the water before you touch the roof
Start inside, with a torch, on the next dry day after rain. Check the roof timbers or sheet from below, not only the ceiling lining if there is one. Follow any tide marks and dark streaks upwards, since water leaves a trail. Check where it stops: at a joint, a fixing, a wall junction or a gap.
Is it a leak at all?
Condensation is the great impostor. Cold steel or cold felt over a damp garage drips in much the same way as a hole does, and it appears in cold weather rather than after rain. If the drips arrive on dry, cool days, or the underside of a metal roof is wet all over, you are probably looking at moisture forming inside. Our guide to box-profile sheet roofs explains the backing that deals with it on steel.
Where the stain is not the source
On a pitched roof, the entry point is usually higher than the stain, and offset towards the ridge. On a flat roof, the entry may sit well away from the drip, since water creeps along the deck and under the felt before it finds a joint or a nail hole. Sealing the spot above the stain on a flat roof therefore often misses.
Take photographs
Photographs of the inside and outside in daylight, with a date on them, are worth far more to a roofer than a description. They also help you see whether a stain is growing or is old and dry. Send them with a short note on when the water appears: after any rain, only in driven rain from one direction, or only after a long wet spell.
Leaks from cement and sheet coverings
On corrugated cement, fibre-cement or metal sheet, the usual entry points are the fixings and the laps. Screws and bolts work loose or corrode, and the washers under them perish. Side laps open when a sheet bows, and end laps fail where the overlap was too short to begin with. Cracked sheets, often from someone treading on a brittle panel, leak straight through.
If the sheet may be asbestos
Do not patch, drill, scrub or walk on cement sheet until you know what it is. Under HSE guidance, intact asbestos cement that is removed carefully counts as non-licensed work, and substantially broken sheeting moves into the notifiable category. A cracked sheet is a reason to call us for a survey, not for a DIY repair. Our answer on whether your garage roof is asbestos sets out the next step.
What a repair can do
On sound metal sheet, replacing perished fixings and resealing laps is a proper repair. On brittle cement sheet, patching is a short-term measure: the sheets are old, the next crack is a matter of time, and every repair adds weight to a fragile surface. Going over the top is sometimes the answer, with limits that we explain in our guide to overlaying asbestos roofs.
A note on gutters
Sheet roofs often discharge into small gutters that overflow in heavy rain. Water running down the wall behind them looks like a roof leak. Check the gutter and the downpipe before you blame the sheet.
Leaks from tiled and pitched garage roofs
With tiles, the covering is rarely the culprit; the details around it are. Water gets in at the ridge, the verge, the abutment where the garage meets the house, or through a slipped tile. We usually look in that order.
Ridge and verge
Old mortar cracks and falls out, and the tiles it held start to move. Since BS 5534:2014, the practice has been to fix ridge and hip tiles mechanically too, which is why repair is often a matter of re-bedding with fixings, not another dab of mortar. MyBuilder's 2026 guide puts ridge tile re-bedding or replacement between £300 and £600, and repointing at £200 to £400.
Flashings
Flashing, normally lead, is the strip that seals a roof against the wall beside it. Where an attached garage meets the house wall, the flashing is the leak point more often than not. It lifts, cracks or loses its pointing. Checkatrade's repair guide quotes Code 5 lead flashing repairs at £50 to £70 per metre, ex VAT; full replacement is bigger work.
Underlay and battens
An old underlay that has perished lets wind-driven rain reach the timbers even when the tiles are intact. Rotten battens drop their tiles. If the leak comes back after a tile repair, the underlay and battens are the next place to look.
Our wider guide to pitched garage roofs covers how these roofs are built.
Leaks from flat garage roofs: felt, EPDM and GRP
Flat garage roofs leak for a handful of repeated reasons, and most start with water that has nowhere to go.
Ponding
BS 6229 calls for a design fall of 1:40, which allows for tolerances and deflection and leaves a working fall of 1:80. Many older garages were built flatter than that. A roof that holds a puddle for days after rain is ageing faster than it should, and splits, blisters and seam failures concentrate where the water sits.
Felt
Felt splits at folds, blisters where moisture is trapped and opens at laps. Edges are the weak spot: where the felt turns up a wall or over the kerb, it can crack or pull away. A patch works only if the surrounding felt is sound. Where the surface is crazed all over, patches fail one after another.
EPDM
EPDM runs as one sheet across the field, so a leak is nearly always at an edge, a corner, a penetration or an outlet. Poorly glued laps and corner folds are the usual faults. A repair is often a local re-bond or a new patch with the right adhesive.
GRP
GRP cracks when the deck flexes or an edge trim moves. Hairline cracks near upstands and drips are common. They can be ground out and repaired, but a spreading crack pattern points to a deck problem, not just a skin problem.
What the numbers look like
MyBuilder's 2026 garage roof guide gives £700 to £1,000 for a single flat garage roof and £900 to £1,400 for a double. Checkatrade's repair guide, last updated in June 2026, lists bitumen flat roof repairs at £100 to £140 per m², EPDM at £120 to £170 and GRP at £130 to £180, ex VAT and excluding access.
The gutters, walls and details that fake a roof leak
Not every garage ceiling stain comes from the roof surface. Several other defects deliver water to the same place, and they are cheaper to fix.
Gutters and downpipes
A blocked or sagging gutter overflows behind itself, soaking the fascia, the eaves and the wall head. Manufacturer guidance sets gutters to fall gently towards the outlet, roughly 1:600 to 1:350 depending on the system, and that fall is easily lost when brackets slip or the run droops. Cleaning and re-aligning a gutter is a modest job; leaving it can rot the rafter ends that carry the roof.
Rising and penetrating damp in the walls
A wet patch low on a garage wall may be ground moisture, not the roof. A patch near the top of a wall where the roof meets it is more likely a flashing or gutter. Telling the two apart saves the wrong repair.
Doors and vents
Water can blow under a garage door or through a vent and appear on a ceiling that is perfectly dry above. A wet patch on a wall beside the door, rather than under the roof, fits that picture.
Rooflights
A garage with a polycarbonate or glass rooflight may leak at the frame, the sealant or the flashing, long before the rest of the roof wears out. Our answer on adding a rooflight to a garage roof covers how one should be fitted.
Plumbing
A garage with a washing machine or a boiler can leak from pipework. If the drips stop whenever it is dry outside, the roof is the likely source; if they come regardless of the weather, look at the plumbing.
Repair or replace: a working rule
The decision comes down to how much of the covering is failing, what the timbers underneath are like, and how old the roof is against the typical life of the material. MyBuilder's roof repair guide gives £150 to £1,500 on average for a roof repair, with minor repairs at £100 to £300, moderate at £500 to £1,000, and major repairs at £2,500 or more. For a pitched garage roof replacement, its garage guide shows £2,500 to £4,500. Treat all of them as national benchmarks, not our prices.
When a repair is the right call
- One clear fault: a slipped tile, a lifted flashing, a failed lap, a split at one edge.
- A deck and timbers that are firm and dry.
- A covering within the middle of its typical life.
When replacement is the better spend
- Splits, blisters or cracks across much of the surface.
- A deck or battens that are soft, stained or black.
- Repeat leaks after several patches.
- Brittle cement sheets, or sheets whose condition makes any contact risky.
If you have paid for the same roof to be patched three times, the fourth patch is a replacement paid for in instalments.
What to expect from us
Our survey follows the water to its source and photographs your own roof. The written quote then states plainly whether we recommend a repair or a replacement and why. VAT is shown on the quote. Where the job is large enough to be notifiable under the Building Regulations, we explain the thresholds and handle the notification. Our guarantee on the workmanship runs for ten years on paper, and the waste goes with us. Send photographs and a note on when it leaks through the form or WhatsApp, or ring 01453 933613.