Can screed be laid on a damp substrate?

The answer depends entirely on the source of the moisture. A slab that is still damp from its own construction and a slab affected by rising damp are two unrelated situations, and confusing them is costly.

‍

Two types of moisture you shouldn't confuse

Theresidual construction moisture is found in a recent concrete slab that hasn't finished drying. It decreases over time and does not rule out laying a screed, provided you respect the drying times and break the contact with a membrane.

The rising damp is a completely different matter: water from the ground rises continuously through the slab due to the lack of a damp-proof membrane underneath. It will never disappear on its own. Pouring a screed over it just traps the problem: six months later, the tiles come loose, the parquet warps, and the skirting boards turn black.

‍

A simple test before you decide

Tape a square of plastic film about 50 cm wide onto the substrate, sealing the edges, and leave it in place for 24 to 48 hours. If condensation appears under the film or if the concrete darkens in that spot, the substrate is releasing moisture. This test doesn't replace a professional measurement, but it is enough to trigger the right reaction: find the cause before covering it up.

‍

What to do before pouring

• On a slab-on-grade without waterproofing: install a membrane before the insulation, and treat the cause if the water is coming from the walls.
• Above a crawl space that is damp: check its ventilation before insulating.
• In all cases: install a separation film, the role and limitations of which are detailed in our guide do you need a vapor barrier under a screed.

The choice of insulation then depends directly on the substrate: our guide which floor insulation to choose based on the substrate details the various scenarios.