Damp Proof Membranes Under Resin Flooring: When You Need One
How rising moisture destroys resin floors, when an epoxy damp proof membrane is required, and what a DPM build-up adds to cost and programme.
If a resin floor blisters, bubbles or lifts in sheets within a year, moisture is the most likely cause. Water vapour moving up through a slab exerts pressure under an impermeable coating, and eventually the bond gives way.
Which slabs are at risk
- Slabs poured before damp-proof membranes were standard detailing
- Ground-bearing slabs where the membrane was punctured or omitted
- New concrete that has not yet dried — drying typically takes about a day per millimetre of thickness in good conditions
- Basements, semi-basements and slabs below external ground level
- Buildings with a high water table or poor external drainage
Test, do not guess
Relative humidity is measured in the slab using hygrometer readings taken over a set period, or with in-situ probes. Most resin systems specify a maximum RH — commonly around 75% — above which a damp-proof membrane is required. Surface-only meters are a screening tool, not evidence: they read the top few millimetres and can look reassuring over a wet slab.
What an epoxy DPM does
A two-coat solvent-free epoxy damp proof membrane is applied directly to the prepared substrate. It tolerates far higher moisture levels than a wearing system, forms a barrier against vapour transmission, and is then blinded with quartz or over-coated so the wearing course bonds to it. Effectively the DPM becomes the waterproof layer and the resin floor above works in dry conditions.
Cost and programme
A DPM adds to both, and far less than a failed floor.
- Two additional coats of material across the whole area
- Extra curing time between coats, typically overnight each
- Usually one to two extra days on programme
- No visible difference in the finished floor
Polyurethane cementitious screeds are the exception worth knowing about. Because they are moisture-tolerant and vapour-permeable, PU HF screeds can often be installed over damper slabs without a separate membrane — one of the reasons they are specified in food production, where slabs are frequently wet and cannot be dried out.
Our approach
We moisture-test before quoting whenever there is any doubt about the slab. If a DPM is needed, it appears as a priced line in the quotation with the reason and the readings stated. If it is not needed, we do not add it. What we will not do is coat over a slab we know is too wet — that is a warranty claim waiting to happen, and it is the client who loses the floor.