Water in a garage tends to be noticed late. Nobody sits in there watching the ceiling, so by the time a drip is spotted, the stored boxes are damp, the timber is dark and the leak has been running through a few winters. That makes the first job detective work. Water rarely comes out directly below the hole in the covering. It runs along a rafter, drops at a joint, tracks down a wall and appears several metres from the cause. This guide walks through the usual culprits on each type of garage roof, from felt and rubber on a flat roof to profiled sheets and tiles on a pitched one. It explains what you can check safely from the ground, what a roofer looks at once up there, and how the choice between repairing and replacing usually falls. Where a quote is involved, the figures quoted are typical UK ranges, not an Empire price list.
Find where the water really gets in
A drip on the floor tells you where gravity ended, not where the leak began. Water follows the underside of whatever it lands on: a rafter, a purlin, a plasterboard ceiling, the top of a wall plate. It may run a good distance before it drops.
Begin inside, in daylight, with the door shut. Look for daylight through the covering, which marks a hole or a failed lap. Look for staining, which shows the path water has taken, and follow it uphill to the highest damp patch. Rot in the timber, dark tide marks and rust streaks on fixings all point back toward the entry.
Then look from outside, from the ground or from a ladder held by someone else. Do not walk on a garage roof. Many have a deck or sheets that will not take a person's weight, and corrugated cement or old rooflight panels can give way without warning. Check from below what you can see: slipped tiles, a sagging line along the ridge, a gutter hanging off the fascia, a mat of moss sitting wet on the covering, and a flashing at the house wall that has lifted.
Timing helps too. A leak that shows only when rain is blown in from one side suggests a joint or an edge on that side. One that runs after every shower, however light, points to a hole in the covering or a blocked outlet. A drip that comes on cold mornings with no rain at all may be condensation and not a leak, and our page on garage roof condensation shows how to tell the two apart.
Flat garage roofs: where felt, EPDM and GRP fail
Most flat garage roofs have one of three coverings. Torch-on felt is bitumen sheet melted onto the deck with a gas flame. EPDM is a synthetic rubber membrane, usually glued down in one piece. GRP is a fibreglass skin, glass strands set in resin, laid over a deck. Each fails in its own way.
Felt cracks and blisters as it ages. The surface splits, the grit wears off, and seams at the laps open up. A blister that has burst lets water in at once. Typical lifespans reported by cost guides run from 10 to 20 years, and a poorly laid felt roof can fail sooner. EPDM rarely splits across its field, but it can be punctured, and the glued edges and corners lift where the adhesive has given up. GRP can crack at the joint between the deck boards, where the structure moves, and at the edge trim.
Ponding
Standing water is both a symptom and a cause. A flat roof is supposed to have a slight fall toward the outlet. British Standard 6229 sets 1:80 as the recommended minimum fall, and designers aim for 1:40 so that settling and construction tolerance still leave a working slope. If your roof holds a puddle for days after rain, the deck has either sagged or was never built to fall. Water that sits finds every weak point sooner or later.
Edges, upstands and outlets
The field of the roof is rarely where the water gets in. An upstand is the membrane turned up a wall at the roof edge, and a flashing seals its top. If that seal has cracked, or the sealant behind it has dried and shrunk, water runs behind the upstand and into the structure. At the front edge, a drip trim should throw water into the gutter. If it is missing or bent, water runs back under the membrane. Outlets block with leaves and moss, and then the water finds the lowest weak point instead.
Pitched garage roofs: tiles, sheets and ridges
A pitched garage roof sheds water quickly, so leaks are nearly always at an interruption: a broken piece, a joint, a fixing, an edge or a junction with another building.
Tiled and slated roofs
On a tiled roof, the covering is shielded by an underlay, the membrane below the battens. Slipped, cracked or missing tiles let wind-driven rain get past the covering, and old underlay can then fail beneath. Battens are the narrow timber rails that the tiles hook onto, and rotted ones let tiles slide. The ridge is a regular source. Mortar-bedded ridge tiles crack with movement and weather, and once the bedding opens, rain is driven underneath. A ridge held only by mortar is an old-fashioned detail, and repairing it properly means adding mechanical fixing.
Corrugated and box-profile sheets
Metal or fibre sheets leak at their laps and their fixings. The end lap, where one sheet overlaps the next up the slope, relies on enough overlap and a sealing strip. The side lap depends on the sheets being laid with the right edge to the weather. Screws go in at the high point of the profile, each with a sealing washer. When the washer perishes or the screw works loose in an oversized hole, each fixing becomes a tiny leak. Sheets can also rust at cut edges and under the paint. Our page on box-profile roof sheets shows how those roofs are put together.
Asbestos-cement sheets
Older garages may have corrugated asbestos-cement sheets. They become brittle with age and crack under a single footstep, and a cracked sheet leaks. Do not climb, scrub or pressure-wash one, and do not drill or screw into it. If you suspect a crack, our guide to a broken asbestos roof sheet explains what to do next.
The house wall, the gutter and other places leaks start
Garage roofs that are not leaking in the middle often leak at the edges, where one part of the building meets another.
On an attached garage, the sloping garage roof butts against the house wall, and a flashing covers that joint. Think of a flashing as a folded lead strip that stops water getting behind the roof edge. Its upper edge sits in a raked-out mortar joint along the wall, held by wedges. Over time the mortar crumbles, the lead lifts or splits at a fold, and the sealant someone applied as a quick fix cracks. Rain then runs down the wall face into the roof. This is one of the first places we examine on any attached garage, because the damage can sit unseen behind the cover strip for years.
On a detached garage with a pitched roof, the equivalent spots are the verge (the roof's outer edge beside a gable wall) and the eaves. An open verge lets wind-driven rain in under the end tiles or sheets. An eaves detail without a proper tilting fillet or fascia board lets water run behind the gutter and into the wall plate.
Gutters are a frequent cause that is easy to overlook. A gutter that overflows because of a blockage or a sag throws water against the fascia and wall, which looks like a roof leak from inside. Cast sections joined with failed seals drip at every joint. Check that it runs freely in a downpour and is held on sound brackets; our page on garage gutters goes through the options.
Finally, a garage that shares a roof or a wall with a neighbour can leak because of something happening on their side. A split parapet or an open joint between the two roofs is a shared problem, and the practical steps appear in our guide to a shared garage roof.
Quick fixes that work and quick fixes that do not
Water coming in calls for a temporary measure, and some are better than others.
On a flat roof, a patch of matching membrane, stuck or welded to a clean, dry surface, can last. A coat of liquid sealant over a cracked felt roof buys a little time at best. Bitumen paint is sold as a cure and rarely is one; it hides the problem, then cracks in the next frost. On sheet roofs, replacing a failed sealing washer or a screw is a proper repair. Smearing sealant over a lap is a stopgap, because the lap should shed water by overlap, not by glue.
On a tiled roof, replacing a slipped tile and re-fixing it is simple. Re-bedding a ridge with fresh mortar is a short-term repair that may need repeating. A mechanical fixing is the better answer, because movement is what cracked the mortar in the first place.
If the leak is active and you cannot get it looked at straight away, protect the contents rather than fighting the roof yourself. Move things away from the drip and put a bucket or a sheet under it. Never stand on the roof in wet weather to throw a tarpaulin over it. A leaking roof is slippery, and an unseen fragile section can fail under you.
A roof open to the weather is made safe first, and a live leak goes to the front of our queue. That might be a secured sheet or a temporary patch, followed by a proper repair once the cause is clear.
Repair or replace: how the decision usually falls
A roofer's job on a leaking garage is to say whether mending will hold. Several signs favour repair. The leak is in one place and the rest of the covering is sound. The fault is a failed detail, such as one flashing, one outlet or one lap. The deck underneath is dry and solid.
Signs that favour a new covering are different. The leak has spread, so water now enters in several places. The covering is brittle, crazed or thin. The deck has softened, because a damp board crumbles when pressed with a screwdriver. Or the roof has already been patched repeatedly, and each patch sits on top of the last.
Costs shape the choice. MyBuilder's roof repair cost guide (2026) reports typical UK ranges: £100 to £300 for a minor job, £500 to £1,000 for a moderate one, and £2,500 and upward for major work. For replacement, MyBuilder's garage roof guide (2026) gives typical UK ranges of £700 to £1,000 for a single flat garage roof, £900 to £1,400 for a double, and £2,500 to £4,500 for a pitched one. The price you sign is fixed after the survey, and the quote states which of the two jobs yours turns out to be.
Two further things tip the balance. First, age: felt typically lasts 10 to 20 years, EPDM 20 to 30 years or more, and GRP 25 to 40 years, as ranges from cost guides that depend on exposure and workmanship. A roof nearing the top of its range gains most from new covering. Second, the water already inside: a leak that has run for years may have rotted the deck, and then the repair becomes a replacement of the structure under the covering. The full checklist is in our garage roof repair-versus-replace page.
What the survey looks at, and the paperwork that may follow
When we survey a leaking garage, we trace the water from the inside first, then climb up to inspect the covering, the edges, the junctions, the outlets and the gutter. We probe suspect timber and lift laps to see the fixings, and you get pictures of your own roof showing what we found. After that comes a written fixed price that names the cause and the remedy.
Where a patch will hold, a patch is what you are quoted. If the answer is a new covering, the paperwork question comes up. A complete replacement can bring Building Regulations into play, and our guide to Building Regulations for garage roofs explains the tests. The outbuilding of a house with a listing, or one inside a designated conservation area, may need consent first, as our guide to listed outbuilding roofs explains.
Insurance is the other thing to think about, and the wording matters. Standard policies often exclude damage from wear and tear, so a roof that has simply aged out may not be covered where a storm-damaged one would be. Pictures and a written roofing report support a claim, but no one can promise the outcome. Our page on garage roof insurance covers how those claims are usually argued.
Before the visit, clear a path to the inside of the garage so the deck and joists can be seen from below, and at the end we clear up and carry the rubbish off. Our garage roof replacement service page describes the types of garage we work on across Bath and the towns within 30 road miles.