Garage Roof Leaking: Repair, Overlay or Replace?

The short answer: most garage roofs are felt on a timber deck, and the deck decides the fix. Sound and dry, a repair, or a fresh layer laid over the existing felt, can be enough; soft or rotten, it comes off to the joists. Corrugated sheets are different: they may be asbestos cement, and are left alone until checked.

What is your garage roof made of?

Usually one of three things, and you can find out which without setting foot on it. Look from an upstairs window or over a fence, or use a stepladder against the wall with somebody holding it, and keep your weight off the roof itself. Old garage roofs were never meant to carry a person, and timber that has been wet can give way.

• Felt on boards or chipboard. The commonest by far. From above it is a dark sheet, gritty or smooth, folded over a timber edge. From inside the garage you see the joists with planks or large boards laid across them.
• Felt on a concrete slab. It looks much the same from outside, but inside there are no joists, only a flat concrete ceiling, and the roof edge is a solid band rather than a board.
• Corrugated sheets. A wavy profile, grey or weathered to a mottled green, held down with bolts through the crests. From inside you see the underside of the same sheets resting on timber or steel rails.

The view from inside is worth more than the view from above. A torch and a minute on a stepladder in the garage will usually show the deck, and often where it has been taking water: dark stains on the timber, chipboard swollen at its joints, or a chink of daylight.

Where does a garage roof let water in?

In a handful of predictable places, and the drip inside tends to sit close to its cause. There is rarely a ceiling or any insulation to carry water sideways first, so what you see underneath is a fair guide to where to look above.

• Where it meets the house or a wall. The felt should rise up the wall, with a flashing tucked into the brickwork above to cover its top. When that flashing comes loose or its mortar crumbles, rain running down the wall slips in behind.
• The front and side edges, and the gutter. Wind drives rain under an edge that has lifted, and a gutter that has dropped or blocked spills back onto the timber behind it.
• Splits and blisters in old felt. As felt ages it stiffens and cracks, often in lines that follow the joints of the boards below. Blisters form where moisture caught underneath lifts the sheet, and one that cracks open lets water straight through.
• Ponding on a deck that has sagged. Boards that have dipped between the joists hold a puddle after every shower, and that water works at the felt until it finds a way in. Standing water on a flat roof explains why it matters.

A slab cannot rot, so on a concrete garage roof the leak almost always turns out to be at an edge, the wall junction or the gutter rather than out in the middle.

Repair, overlay or replace: what does the deck need to be?

Each fix asks something different of the boards, which is why the deck is what an inspection is really looking at.

• A repair needs a deck that is dry and firm, a fault in one place, and felt around it that is still worth keeping.
• An overlay, a new layer laid over the old covering, needs the deck dry and firm across the whole roof, old felt that is still bonded down, and room at the edges and the wall for the extra thickness.
• A new roof is the answer once the boards are soft, rotten or swollen, or when layer upon layer has built up until one more would overload the joists. It all comes off, failed timber is renewed, and the roof is built back up. Renewing most of a covering can bring the regulations in where a repair never would; building regulations and flat roofs explains when.

For flat roofs in general, the choice is laid out in repair or replacement, and what laying a new layer involves, day by day, in our walk-through of a liquid overlay. How it plays out on garages in particular, including what tends to fail first, is on our garage roofs page.

Could corrugated sheets be asbestos?

On an older garage, yes, they may be. Asbestos cement was widely used for corrugated roofing, and the Health and Safety Executive lists roofs among the asbestos cement products people still come across. Nobody can tell from looking at a sheet, and its age does not settle the question either way.

So leave the sheets alone until they have been identified. Do not drill, cut, break, sand, jet-wash or walk on them: disturbing the material is what releases fibres, and an old sheet can be brittle enough to break under a person's weight. The HSE advises against repairing or removing asbestos materials yourself without training, and notes that suspected asbestos is often best left where it is.

We do not work on asbestos cement sheeting. If your garage is roofed in it, the first step is to have it identified by someone qualified to take a sample; taking it down, if that is what follows, belongs to an asbestos contractor. When the sheets are off and the timbers beneath have been put right, the roof that replaces them is ordinary roofing.

Is a garage roof worth doing properly?

Usually, though often for a quieter reason than people expect. A garage tends to hold whatever has no room elsewhere: tools, bikes, boxes of papers, a spare freezer, sometimes the car. Water on those is an annoyance rather than a disaster, but it is an annoyance every time it rains.

The better reason is the timber. Water that reaches the boards stays in them and spreads along them, so a small leak left for a season or two can turn a modest repair into new boarding. Doing it properly does not mean doing the most: on plenty of garages the right answer is one small repair made well. It means putting right the cause rather than brushing something over the stain.

What should you do now?

Gather what an inspection will need, protect what is underneath, and then have someone look at the deck as well as the covering.

• Take photographs outside, from wherever you can safely see the roof, and inside, under the wet patch.
• Note when it leaks: every shower, only in heavy rain, or only when the wind comes from one side. That pattern points at the cause faster than anything else.
• Clear what is stored beneath the wet patch, and lift anything that matters off the floor.
• Then arrange an inspection that looks at the deck from inside as well as the felt from above, and says what the roof is made of and where the water gets in before anyone talks about a fix.

If water is pouring in right now, the guide to the first hours of a leak covers what to do. How we look at a garage roof and price the work is set out on our garage roofs page, and how leaks in general are traced on our repairs page.