How do I test a floor for damp before laying flooring?

Every floor failure caused by damp was preventable with a test that costs the price of a bin bag. Laying a floor over a slab that is still giving up moisture ruins the floor and often the skirting with it.

What you'll need

• A metre square of polythene sheet
• Strong tape
• A moisture meter or a hygrometer box, if going further
• A torch

Step by step

1. Understand what you are testing for.Two different things: residual construction moisture in a new slab, and ongoing damp rising through an old one. The test looks the same; the remedy does not.
2. Do the polythene test.Tape a 1m square of clear polythene down on all four edges, in the middle of the room and near an external wall. Leave it 48–72 hours.
3. Read the result.Droplets on the underside of the sheet, or a visibly darker patch of concrete beneath it, means moisture is moving up through the slab.
4. Test more than one spot.Slabs are not uniform. Test the centre, near an external wall, and near any suspect area such as a former door or a plumbing route.
5. Use a hygrometer for a proper figure.A surface hygrometer box sealed to the slab and left for at least 72 hours gives a relative humidity reading. Below 75 percent is the widely used threshold for most floor finishes.
6. Treat a pin moisture meter with caution on concrete.They are calibrated for timber and give an indication only on masonry. Useful for comparison between areas, not for a pass or fail decision.
7. Allow a new slab to dry properly.The traditional rule of thumb is around a day per millimetre of thickness for the first 50mm and considerably longer beyond that. A 100mm slab can take six months in good conditions.
8. Decide on a membrane.Any solid floor with no visible damp proof membrane, any ground-bearing slab in an older house, and any suspended timber floor over a damp void needs one under a moisture-sensitive finish.
9. Choose the membrane type.A liquid-applied epoxy damp proof membrane for a slab that is still drying, or a polythene sheet DPM lapped up the wall behind the skirting for a dry slab that needs isolating.
10. Do not rely on the underlay's foil.The foil or plastic backing on a combined underlay is a vapour check, not a damp proof membrane. It is not designed for water rising from a slab.
11. Find the cause before covering it.Rising damp, a leaking pipe, a bridged damp course or an external ground level are all fixable. Sealing a live damp problem under a floor moves it into the walls.

Tips

• Do the test before you buy the flooring, not after it is delivered. It takes three days and it is the cheapest insurance available.
• Almost all engineered wood and laminate manufacturers void their warranty if the subfloor moisture exceeds their stated figure. That figure is in the fitting instructions.
• Underfloor heating must be commissioned and cycled before the moisture test, and the flooring laid to the heating manufacturer's schedule.

Common mistakes

• Skipping the test because the floor looks dry — A slab can be visually and touch dry at the surface while holding a great deal of moisture below. Once a floor and its underlay cover it, that moisture comes up into the boards, which cup, swell and delaminate.
• Using a liquid DPM over a slab that is actively wet from a leak — A membrane holds back construction moisture, not a live water source. Sealed in, the water finds the walls instead and produces damp at skirting level around the room.

Questions people ask