Why is my concrete floor wet, and is it rising damp or condensation?

Solid floors built before damp proof membranes became standard have nothing stopping ground moisture rising through them, and plenty of later floors have a membrane that was punctured, stopped short at the edge or never lapped to the wall's damp proof course. Condensation looks identical from above and needs the opposite treatment, so the test comes first.

Safety

What you'll need

• a piece of clear polythene about 500mm square
• strong tape that sticks to concrete
• a hygrometer (optional)
• a moisture meter (optional)

Step by step

1. Clean a patch of bare slab.Sweep and, if there is old adhesive residue, scrape a patch clean. The tape has to make a genuinely airtight seal or the test tells you nothing.
2. Tape the polythene down on all four edges.No gaps. Do it in more than one place — the middle of the room and near an external wall behave differently and the comparison is informative.
3. Leave it for several days, ideally longer.Two or three days will often show a result, but a week gives a much more reliable one, especially in cold weather when evaporation is slow.
4. Read the result.Condensation on the underside, or a darkened patch of concrete beneath the sheet, means moisture is coming up out of the slab. Condensation on the top face, in the room, means the slab is cold and the room air is humid. A dry patch under the sheet while the surrounding floor is damp is the classic condensation signature.
5. If it is coming from below, do not cover it.The routes are a surface damp proof membrane — either a liquid epoxy or polyurethane damp proof coating applied to the prepared slab, or a loose-laid or bonded sheet membrane — and the membrane has to be continued to the walls and connected to the wall's damp proof course, or moisture simply comes around the edge.
6. If it is condensation, change the temperature or the humidity.Heat the room so the slab is above the dew point, reduce moisture production, and improve ventilation. Insulating over the slab before laying the covering is the durable answer, because it puts the cold surface below the insulation where room air cannot reach it.
7. Check the obvious sources of new water before either.A leaking pipe in or under the slab, a failed external gully, ground falling towards the house, or a slab poured hard against a wall with no perimeter. A slab that is wet in a patch rather than generally is often a leak, not a membrane failure.
8. If the slab is new, remember it is still drying.Fresh concrete and screed release a great deal of water and take a long time to lose it. The correct step there is a proper moisture measurement to the manufacturer's stated method rather than a judgement call, and flooring manufacturers publish the reading they require.

Common mistakes

• Laying a plastic membrane over a damp slab and putting the floor straight back — It stops the damp reaching the covering but pushes it sideways into the walls, where it appears as rising damp on the plaster a few months later.
• Assuming a cold, damp-feeling slab must be rising damp — In an unheated room in spring, a cold slab will condense room moisture out of the air and be genuinely wet with no ground water involved at all. The cure — heat and ventilation — is the opposite of a membrane.
• Testing on a warm dry week in summer and declaring the floor sound — Slabs are driest when the ground is driest. Test in a wetter season, or accept that a clear result in August is weaker evidence than a clear result in February.

Questions people ask