Damp garage solutions: find the cause before you buy anything

Damp is a symptom, and three different problems share it

With a damp garage the fix usually gets bought before the cause is found. Damp-proof paint, a dehumidifier, a tin of sealer: each of them works on exactly one kind of damp and does nothing, or harm, to the other two. Water gets into a garage by coming up through the slab, by coming in through the walls or roof, or by condensing out of the air onto cold surfaces. They look similar from the doorway. They are not the same problem and they do not share a solution.

A lot of older UK garages were built as cheaply as a building can be: a single skin of brick or block, a slab poured straight onto the ground, often with no membrane under it and sometimes no damp-proof course in the walls. That is why a garage can be damp when the house beside it is fine. It was never built to keep water out to the standard a house is, and it is unheated, so nothing dries it.

The order of work is the same whatever you find. Identify the cause, stop the water at its source, dry the space, and only then think about paint, sealers or lining. Seal a wall that is wet from outside and the water simply stays in the brick behind the sealer. Run a dehumidifier against ground moisture and it runs forever, because the ground does not run out of water.

How to tell which one you have

Where the damp shows, and when, tells you most of it. Water coming up through the slab sits low: a darker band at the base of the walls, a floor that never quite looks dry, a white salty bloom where it has evaporated. Water coming in through the fabric leaves a patch that maps onto something outside — a cracked gutter joint, a wall junction, the roof edge — and that patch grows during rain and shrinks in a dry week.

Condensation is the odd one out because it has nothing to do with rain. It shows on the coldest surfaces first: beads on the inside of a metal door, a film on the slab on a cold clear morning, rust bloom on tools, mould on cardboard and fabric. It is worst in winter and after a wet car has been parked inside, and it does not care whether it rained yesterday.

If you still cannot tell, the foil test settles it. Tape a square of kitchen foil tightly over the wet area with all four edges sealed and leave it a day or two. Water on the room side of the foil came out of the air. Water trapped underneath came through the wall or the slab. It costs nothing and it is more reliable than a guess made from the doorway.

1. 1Tape foil over the wet areaA square of kitchen foil about the size of your hand, every edge taped down so no air gets behind it.
2. 2Leave it a day or twoLong enough for a cold night and a damp day. Do not run a heater or a dehumidifier while it is on.
3. 3Read which side is wetWet on the room side is condensation. Wet underneath is water coming through the wall or slab. Both is both, and the slab needs sorting first.

Water coming up through the slab

Ground moisture through a slab with no membrane cannot be painted away. Floor paints and sealers need a dry, sound surface to bond to, and a slab that is damp from underneath pushes moisture up behind the coating until it lifts. The lasting fix is a barrier, which for an existing floor means a surface-applied damp-proof membrane on a properly prepared slab, or in the worst cases a new slab. Both are bigger jobs than a weekend.

Before you commit to either, look outside. Soil, gravel or a path banked up against the wall higher than the floor inside bridges whatever damp-proof course there is and feeds water into the base of the wall. A downpipe that discharges at the foot of the wall does the same. Lowering the ground level along the wall and getting the rainwater away from the building are cheap, and they sometimes solve the whole thing.

In the meantime, stop the damp reaching what you own. Nothing porous on the floor: no cardboard boxes, no bags of cement, no timber laid flat. Freestanding shelving lifts everything clear of the slab without needing a fixing into a wall you may be about to treat. Interlocking floor tiles float over a slab that is slightly damp — that is one of the honest reasons to choose them over a coating — but they cover the problem rather than cure it, so read the slab first.

Water coming in through walls and roof

A single-skin wall is one brick or one block thick with nothing behind it. Rain that gets through the outer face is inside. On a house the cavity catches it; on a garage there is no cavity, so the wall relies entirely on sound pointing, an intact face and working rainwater goods to stay dry. When any of those fail you get a wet patch inside that matches the failure outside almost exactly.

Walk round the outside in the rain if you can. Look for gutters overflowing at the joints, a downpipe that has come apart, pointing that has weathered back to a groove, cracked render, and the junction where the garage roof meets a wall of the house. That junction is the most common leak point on an attached garage, and there is a separate guide on roof leaks that goes through it in order.

Fix the outside before touching the inside. Repoint, seal the gutter joint, redirect the downpipe. Once the wall has dried, a breathable masonry water repellent on the outer face helps a single-skin wall shed rain without trapping the moisture already in it. A sealing paint inside does the opposite: the wall stays wet, the paint blisters, and the damp moves to the next weakest point.

Condensation is its own problem

If the foil test showed water on the room side, nothing above applies to you. The wall is not letting water in; the air inside is carrying it and the cold surfaces are pulling it out. Membranes, repointing and repellents will change nothing. The answer is some combination of removing the moisture source, moving the air, and warming or insulating the surfaces the water forms on.

Because that is a different set of fixes with its own traps, it has its own guide. The one thing worth saying here is that the two problems can coexist. A garage with a damp slab is also feeding moisture into its own air, so it condenses on the door and the tools too. Fix the slab and the condensation eases; fix only the condensation and it keeps coming back.

Protecting what you keep in there while you sort it

Damp ruins things faster than most people expect. Cardboard wicks it, paper goes to mush, steel tools rust, and anything fabric grows mould in a season. The order of priority is simple: get things off the floor, get them away from the wet wall, and get the things that matter into closed containers. Off the floor is the biggest win and the easiest.

Freestanding steel shelving does that without a single hole in a wall you might be about to treat or reline, and it moves when the work starts. Heavy hooks put bikes, ladders and leads on the wall and out of the wet. Whatever you use, leave a gap between the back of the storage and a damp wall so air can move behind it. Storage pushed hard against a wet wall is where the mould starts.