Damp concrete garage floor: why it happens and what you can honestly put on it

Why a British garage slab is damp from underneath

A house floor has a damp-proof membrane under it, a sheet of polythene between the ground and the concrete, so moisture in the soil cannot rise into the room. A great many garages, particularly older detached ones and anything built as an afterthought, do not. The slab was poured straight onto hardcore and the ground has been feeding water into it ever since. Concrete is porous. Water moves up through it, reaches the surface and evaporates into the garage, continuously, all year, with the rate going up in a wet winter.

This is invisible most of the time. The surface looks dry because the water leaves as vapour. What gives it away is a white powdery bloom on the concrete, which is salt left behind as the water evaporates; patches that go darker in winter; cardboard boxes that feel limp on the bottom; and the way a rubber mat left on the floor for a month has a wet shadow underneath it when you lift it.

It is worth separating this from condensation, which is a different problem with a different cause. Condensation is water from the air landing on a cold surface, and it happens on top of the slab. Rising moisture comes from underneath. The test below tells them apart, and it matters because the fixes are not the same. A slab can suffer from both at once, and in an unheated garage in January it usually does.

The plastic sheet test: is it rising or condensing?

The standard way to find out what a slab is doing is a square of polythene taped to the floor. It costs nothing and it is the one test worth doing before you spend money on any floor covering. A piece of heavy polythene about half a metre square, a roll of tape that will stick to concrete, and a day or two of patience.

Read the result carefully. Wet on the underside of the sheet, or a dark patch on the concrete where the sheet was, means moisture is coming up through the slab and the sheet stopped it evaporating. Wet on the top of the sheet only means condensation from the room air, and the slab itself may be fine. Both at once is common in an unheated garage in winter. Nothing under the sheet after a couple of cold days is a slab you can treat as dry.

1. 1Cut a square of polytheneHeavy polythene, about half a metre square. A thin bin liner tears and lets vapour through; use something substantial.
2. 2Pick a representative spotNot by the door and not where a rug has been. A clean patch of bare slab in the middle of the floor, brushed free of dust so the tape will hold.
3. 3Tape every edge downSeal all four edges completely so no air gets under. The test only works if the moisture has nowhere to go but the underside of the sheet.
4. 4Leave itAt least a day; longer in cold weather, because everything happens more slowly. Do not lift it early to check.
5. 5Lift it and look at both sidesWater or beads on the underside, or a dark patch on the concrete, means the moisture is rising through the slab. Water on the top side only is condensation from the room.

Why paint and epoxy lift on some floors and not others

This is the question that confuses people, because the same tin of floor paint gives one man a floor that lasts a decade and his neighbour a floor that peels in a winter. The difference is usually not the paint and not the preparation. It is what is underneath the slab. A bonded coating has to hold onto the surface of the concrete, and on a slab with no membrane there is water arriving at that surface constantly, pushing against the coating from behind.

For a while the coating holds. Then a patch blisters where the moisture found the weakest bond, the blister lifts a flake, water gets under the edge of the flake, and the whole thing goes in strips. A membrane-protected slab never puts the coating under that pressure, which is why the same product behaves so differently on two floors that looked identical on the day they were painted.

That is the honest case for a loose-laid tile on a damp slab, and it is a strong one. The tile does not depend on adhesion to anything. A wet surface underneath it is not trying to push it off, because nothing is holding it on in the first place. It just sits there, indifferent. That is also why the floors that most need covering are the ones that shed a coating fastest: the damp slab is the one you wanted to hide, and it is the one that will not let you.

What loose-laid tiles do and do not fix

What they fix: the covering itself no longer fails. You get a surface that stays put on a slab that would have shed a coating, and that lifts if you ever need to look underneath. That is genuinely useful, and it is the reason tiles are the sensible covering on a great many British garage floors. Nothing that follows is a criticism of the tile; it is a description of what a covering is.

What they do not fix: anything about the damp. The slab is still bringing moisture up. Under a solid tile that moisture has nowhere to go, so it sits there. Lift a solid tile after a winter on a damp slab and expect a wet underside, a dark slab, and often a musty smell. A perforated or open-grid tile lets air move under and through the floor, so the moisture keeps evaporating into the room. That is better for the slab and for the tiles, and worse if what you wanted was a floor with no holes in it.

And the room is still damp. A tile changes what you stand on, not the air, and a cardboard box on a tiled damp floor is a cardboard box in a damp room. Anything you care about still wants to be off the floor. Shelving lifts a box a shelf's height above the wettest surface in the room, and a lidded plastic box keeps the damp air off what is inside it; between them they solve the problem the tile was never going to.

Storing things on a damp floor

The floor covering is the smaller decision. The bigger one is what touches the floor. On a slab that is feeding moisture upward, anything sitting directly on it is sitting in the wettest part of the room: cardboard goes soft, a steel toolbox rusts from the base up, timber offcuts warp, and a bag of cement sets in the bag. Tiles do not change that. The bottom of a box on a tile is still the bottom of a box in a damp garage.

Get storage off the floor. Freestanding shelving lifts the lowest shelf clear of the slab, and the first shelf up is dramatically better than the floor even in the same room. Long awkward things go on wall hooks. Anything that genuinely cannot get wet goes in a sealed plastic box with a lid, off the floor, and that is a rule for the whole garage rather than a fix for the floor.

There are surface-applied damp-proof coatings sold for exactly this problem. They are specialist products with their own preparation requirements, and whether one suits your slab is a question for the maker, not for this guide. What we can say is that a loose-laid tile does not need one to stay down. If you do go that way, do it before the tiles rather than after, because the coating needs the bare slab and the tiles will hide whether it worked.