Damp Concrete Floor Or Damp Carpet On The Ground Floor

What's happening?

A solid concrete floor sits on the ground, and ground moisture will come up through it unless a damp proof membrane lies in or under it. Many older floors have none or have a membrane that has failed, and some newer ones were laid over a damp or compacted base. The result shows in the carpet or underlay: damp patches, a musty smell, mould on the underside and lifting laminate or tiles. Water may also appear after heavy rain when the water table rises. A slab can look damp from condensation on a cold surface or from a leaking pipe under the screed, so a patch with no link to the weather is a plumber's first check. A new screed or slab that has not dried can also release moisture for months and ruin a floor covering laid too soon.

What usually causes it?

• No damp proof membrane in an older slab, or a damaged or unlapped one
• Groundwater rising after heavy rain, or a high water table
• Condensation on a cold slab, especially under rugs, furniture and underlay
• A leaking pipe, heating pipe or waste in or below the screed
• A new slab or screed still drying when the floor covering was laid
• Rising damp or penetrating damp at the edge of the floor, from a failed damp proof course

Is it dangerous?

Low as a leak but not as mould: a damp carpet and underlay grow mould you breathe in, and laminate and timber floors cup and rot. Throw out a carpet and underlay that has been wet and smells.

Can I fix it myself?

Lift a corner of the carpet or the covering and look at the underlay and the slab. Tape a square of polythene to the slab for two days: moisture under it is coming up through the floor, moisture on top is condensation. Note whether the damp changes with the weather. Ventilate and heat the room. Do not lay new flooring over a damp slab.

When should you call a damp proofer?

Call a plumber first if the patch does not follow the weather, in case of a leak. Call a damp proofer if the slab is damp throughout or after rain, or the floor covering is rotting. Ask for a moisture test and a membrane option before laying a new floor.

What will the damp proofer actually do?

• Lift the floor covering and measure the slab's moisture over a wide area
• Check for a leaking pipe or drain with pressure and flow tests
• Check the outside ground levels, drainage and wall damp proofing near the floor edge
• Lay a liquid or sheet damp proof membrane over the slab, linked to the wall damp proof course
• Fit a screed or floor finish suited to a floor with a membrane
• Advise on ventilation and heating to keep the slab dry

Who fixes this

• Internal damp proof membrane
• Damp survey
• Damp proof course installation

Questions people ask

Sources & References

1. Designing Buildings: damp proof membrane
2. Property Care Association: rising damp
3. Designing Buildings: rising damp