Garage roof leak: what to check before you call anyone

First, make sure it is a leak

Drips from a garage roof on a cold clear morning, with no rain for two days, are not a leak. They are condensation on the underside of a metal or fibre-cement roof, and people spend a winter hunting for a hole that is not there. A leak arrives with rain and stops after it. Condensation arrives with cold and stops when the sun warms the sheet. The pattern underneath is different too: a leak leaves a stain with a tide mark spreading from one point; condensation is an even film of droplets across the whole underside.

If it is condensation, nothing in this guide helps, and the condensation guide does. If it is a leak, the drip point is not where the water gets in. Water runs along the underside of a sheet, down a rafter or along a board until something makes it let go, so the hole is uphill of the drip, sometimes by a long way. That is why the inside check comes before the outside one: it narrows down where to look.

Check from inside on a wet day

Go in while it is raining, or straight after, with a torch. Look up and find the wet, then follow it uphill along the slope of the roof until it stops being wet: that is close to the entry point. On a felt roof over boards the boards show it as a dark patch. On a sheet roof the water tracks along the corrugation, so check the sheet laps and every fixing along that line. On a pitched roof with a ceiling, the stain on the ceiling is the drip point, and the leak can be a metre or more uphill of it.

Note what you find rather than fixing it from inside. Sealant on the underside of a roof does nothing, because the water is already through, and a bucket is a plan for a night, not a repair. What you are after is a short list of places to look at outside, in order of likelihood, before anyone goes up a ladder. A photo of each wet patch, taken while it is wet, is worth having when a roofer asks what you saw.

1. 1Find the highest wet pointWater runs downhill before it drips. Trace the wet track up the slope until it stops; the entry is near there.
2. 2Check the wall junctionWhere the roof meets a wall of the house, look for water running down the wall from the top edge rather than dripping from the roof.
3. 3Check every fixing on the wet lineOn a sheet roof, a rusted or missing fixing washer on the wet track is the usual culprit.
4. 4Look below the gutter lineWet at the very edge of the roof, above the wall, points to the gutter or the eaves rather than the roof itself.

Outside: the five usual suspects

Garage roofs fail in the same handful of places, and most of them are easier to check from a ladder against the wall than from the roof itself. Do not walk on a fibre-cement roof, ever. They are fragile, they give no warning, and falling through one from garage height is the kind of accident that ends badly. A felt roof on soft, bouncy boards is the same warning in a different material: the deck is rotten, and it will not hold you.

The junction with the house wall is the most common leak on an attached garage. Rain runs down the house wall, hits the top of the garage roof, and looks for the way past whatever seals that joint, whether it is lead flashing, a mortar fillet or a bead of sealant that gave up years ago. Check it first, and check the gutter next, because an overflowing gutter dumps water down the wall behind the fascia and it appears inside as a roof leak.

Felt flat roofs: patch or replace

The standard UK garage roof is felt on boards with a fall so slight that water ponds on it, and it has a working life at the end of which it fails everywhere at once. A single split or a lifted lap on a roof that is otherwise sound is a patch: clean, dry, a bitumen-based repair product or a self-adhesive patch over the split, on a dry day, and expect it to hold for a while rather than forever. It is a way to stop water while you plan.

Several blisters, bare areas where the surface has weathered off, a soft deck underfoot, or a leak that moves each time you patch it, are a roof at the end of its life. The options for replacing it are a new felt system torched on, a single sheet of rubber membrane glued to a new deck, or a fibreglass roof laid over boards. Each has its advocates; all of them start with a sound, dry deck, which is the part most often skipped. Get more than one price, and ask what happens to the rotten boards.

Sheet roofs and the asbestos question

Corrugated cement sheets on a garage built before asbestos was banned in the UK in 1999 may well be asbestos cement. They are safe left alone and dangerous when broken, drilled, cut or pressure-washed, because that is when the fibres are released. So the rules are: do not walk on it, do not drill it, do not cut it, do not clean it with a pressure washer. If it needs replacing, that is a job for someone who handles it properly, and disposal is controlled; your council will tell you where it can go, and it is not the skip.

Modern metal and fibre-cement sheets leak at the fixings and the laps. A fixing with a rusted or missing sealing washer lets water straight through the hole it made; replacing it with a new fixing and washer from a ladder is a reasonable job on a roof you can reach safely. An opened lap wants a sheet-lap sealant, and a cracked sheet wants replacing rather than filling. Remember the point from the start: a metal roof that drips on a cold dry morning is not leaking.

Stop it happening again

Clear the gutters every autumn after the leaves and again in spring, cut back anything overhanging the roof, and after a big storm look at the junction and the fixings before the next one. Look at the ceiling inside once a month in winter for a new stain. Most garage roof leaks announce themselves months before they get serious, and a stain the size of a hand is much cheaper than a stain the size of a door.

Inside, do not store things where a leak would land. Cardboard, timber and anything that rusts belong off the floor and away from the wall the roof drains towards, and nothing precious goes on the top shelf under a felt roof that is past its best. Freestanding shelving gets things off a slab that a leak will wet; a floating tiled floor copes with getting wet in a way a painted one does not. That is a holding position. The fix is on the roof.