Why Is My Garage Roof Leaking?

Work out where the water is coming from

Start inside, with a torch. Note where the water appears and what the weather was doing. A drip that begins an hour into a heavy shower points to an actual opening in the roof or a flashing. A damp patch after a cold night that dries by midday looks like condensation. Stains with a tide mark mean repeated wetting. Wet insulation or timber near a wall suggests the flashing where the roof meets the house. Take photographs from inside, and look at the roof from an upstairs window or a safe vantage rather than climbing up.

On a flat roof the water can travel along the deck before dripping, so the visible drip is not always under the hole. Do not assume it. Mark the spot and track the wet area outwards.

The usual causes by covering

Each type of garage roof has its own habit of failing:

• Felt: splits along laps, blisters, cracks at the corners and at the wall junction. Old felt is brittle and lets go at the edge.
• EPDM rubber: punctures from branches or tools, lifted edges where the adhesive failed, and failed tapes at corners.
• GRP fibreglass: hairline cracks at board joints or at the edge, where the deck moves slightly.
• Cement or metal sheets: cracked sheets, rusted or missing fixings, and gaps at overlaps. On asbestos cement, do not climb on or disturb the sheets; read whether an asbestos garage roof is dangerous.
• Bitumen corrugated sheets: sagging, sun damage and cracks around fixings, explained in are corrugated bitumen sheets any good.

Age is a factor. A roof that has simply run out of life leaks in many places at once, and the leaks multiply every winter. A younger roof that leaks in one spot has usually suffered a specific failure, such as a puncture, a lifted lap or a blocked outlet. The difference between the two decides whether a repair has a future, and it is the first thing we judge when we stand in the garage and look up.

Flashings, gutters and condensation

Many garage leaks are not in the roof surface at all. A garage attached to the house has a flashing, a strip of lead or other metal that seals the roof edge against the wall. If it has slipped, cracked or lost its pointing, rain tracks behind it. A blocked gutter overflows and wets the fascia and the end of the deck, which then rots. Poor falls leave water sitting in a pond that finds any weakness.

Condensation is the great imitator. Warm damp air from a car, a tumble dryer or damp walls meets a cold underside, and water forms and drips. It occurs on metal and fibre cement roofs more than on a lined timber deck. If drips follow cold mornings and the outside of the roof is intact, read condensation under a metal roof before you pay for a repair that cannot work.

Garages with a pitched roof add another set of suspects: slipped or cracked tiles, a failed ridge, and a worn felt underlay below. The same logic applies. Find the line the water takes, and look at the highest point it could have entered. For a brick garage with a flat roof, the parapet or the coping stones along the top of a wall are also worth checking, since water enters through open joints and runs down into the building.

Repair or replace

A single split, a lifted edge or a failed corner can be repaired, and a patch is often enough for a roof that is otherwise sound. A roof with several leaks, soft deck boards or a surface that crumbles has little life left, and patching costs more over time than it saves. Rotten boards or joists change the decision, because new covering over a weak deck fails early, and rotten garage roof joists explains how to judge it. Our guide to whether to repair or replace a garage roof sets out the sums.

The free survey includes photographs of the leak, the deck and the roof surface, and a plain account of what failed and why. If a repair will do, we say so, and either route comes with fixed written figures. More on the service is on our garage roof replacement page, and we prioritise leaking roofs, making them safe first.