Damp Proof Membranes for Resin Floors: When You Need One and What Happens Without It

What is a damp proof membrane?

A damp proof membrane, or DPM, is a barrier that stops water vapour from moving up through a concrete slab into whatever is bonded on top of it.

A sheet damp proof membrane is the polythene laid under the slab at construction stage, before the concrete is poured. It sits between the ground and the concrete and is part of the building’s original design.

A surface damp proof membrane is applied on top of the finished, prepared concrete. It is usually a low viscosity, solvent-free epoxy damp proof membrane, rolled or squeegeed on in one or two coats, and it cures into a continuous film bonded to the slab. 

In a resin floor build-up, the surface DPM sits between the prepared concrete and the primer. It becomes part of the system rather than an extra layer sitting loose beneath it, and once the floor is finished, you cannot tell it is there.

When do you need a damp proof membrane?

A membrane is specified on the basis of a test result, not a hunch. That said, the situations that produce high readings are predictable.

Moisture readings above 75% RH

This is the usual trigger. Hygrometer testing to BS 8203 gives you the figure, and most resin systems will not go down above it.

A new or fast-tracked slab

Concrete takes months to dry, and construction programmes rarely allow for it. If the floor has to go down before the slab has given up its construction moisture, a membrane bridges the gap.

No original damp proof membrane

Common in older industrial and warehouse buildings, where the slab was poured straight onto hardcore. Ground moisture has a permanent, uninterrupted route upwards and it is never going to stop.

A ground-bearing slab

Anything in direct contact with the ground is at higher risk than a suspended slab, particularly where the water table is high or external ground levels sit above internal floor level.

A previous floor that failed through moisture

If the last system blistered, lifted or came away with damp concrete beneath it, the cause has not gone anywhere. Re-laying without a membrane produces the same result.

What happens without a damp proof membrane?

The failure sequence is consistent and worth understanding, because it explains why the problem is not fixable from the top.

Water vapour rises through the slab and reaches the underside of the resin floor. The resin is impermeable, so the vapour cannot pass through it. Instead it collects at the interface, and as more arrives, pressure builds.

That pressure works against the bond between resin and concrete. The bond is strong, but it is being loaded continuously, twenty-four hours a day, across the whole floor. Eventually it gives way somewhere, and once it does, the failure spreads, because every square metre of bonded floor around it is under the same load.

What you see is blistering first: small domes, often with moisture visible inside if you cut one open. Then delamination, where the blisters join up and larger areas separate. Then lifting, where traffic catches a loose edge and takes the sheet with it. Alkaline moisture can also attack the resin itself and the bond line, so it is not purely a mechanical problem.

A membrane is applied in a day or two, as part of a floor that is going down anyway. Putting right a floor that failed through moisture means grinding off the failed system, preparing back to sound concrete, testing, applying the membrane you should have had, and laying the floor again.

That is the full cost of the original floor, plus the removal, plus a second round of downtime in a building that has to stop working while you do it.

Our process

We test as part of the free survey before we quote, so the specification reflects your slab rather than an assumption.

As approved suppliers for the major UK resin manufacturers, we specify from a range of damp membrane systems, applied by our own team over the prepared substrate. Our team completes a full inspection on handover and provides maintenance information for the system installed.