This comes up constantly, and the honest answer is that for most interlocking tile installations you do not need a membrane, and adding one can occasionally make things worse.
Here is how to work out which situation you are in.
Why interlocking tiles are different
A membrane exists to stop moisture rising out of a slab and damaging the floor covering above it. That matters enormously for timber, laminate, carpet and anything glued down.
Interlocking plastic tiles are a floating floor. They are not bonded to the slab, they are not damaged by moisture, and air can move around and under them. Moisture that rises out of the concrete evaporates and leaves.
That is the key point. The tiles do not care about damp. The question is whether you care about what the damp does underneath them.
When a membrane is genuinely worth it
You have measurable rising damp
Not a slightly cold floor. Actual moisture coming up through the slab, usually in an older garage with no original damp proof membrane beneath it.
You are storing moisture sensitive things at floor level
Cardboard boxes, tools, timber, upholstery. If damp is rising and you are standing things directly on the floor, it will find them.
The garage is becoming habitable space
A conversion into a gym, office or room needs proper damp control regardless of the floor covering. Different rules apply and it is worth getting advice.
You are laying something else on top
If tiles are the base for a further layer, or if you are laying rubber matting or a surface that traps moisture, the calculation changes.
When a membrane causes problems
This is the part that gets missed.
A membrane does not remove moisture from a slab. It redirects it. Seal the top of a damp slab and that moisture goes somewhere else, typically sideways into the walls at the base, which is a considerably more expensive problem than a damp floor.
Three further issues in a garage specifically.
- It becomes a slip layer. Sheet material under a floating floor lets the tiles shift, which shows up as gaps opening at the joints.
- It traps water from above. A car brings in rain and road salt. Water that gets between tiles now cannot evaporate downward and sits on the membrane instead.
- It hides the problem. Covering up a damp issue without understanding the cause tends to mean discovering it later, worse.
How to test your slab
Two simple tests, both free.
The plastic sheet test
Tape a piece of clear polythene, roughly 500mm square, tightly to the floor on all four edges. Leave it for two to three days, ideally longer.
Condensation forming on the underside, or a darkened patch of concrete beneath it, indicates moisture rising through the slab. A dry sheet and dry concrete indicates you are fine.
Do this in two or three positions, including near the walls, because damp is rarely uniform.
The look and feel test
Persistent dark patches, white powdery deposits on the surface, a musty smell, or a floor that is damp in areas that never see water from outside. Any of these suggest moisture coming from below rather than above.
What to do if you find damp
Understand the cause before covering it.
- Check external ground levels. If soil or a path sits above the internal floor level, water has an easy route in.
- Check gutters, downpipes and drainage around the building.
- Look at ventilation. Many damp garage floors are actually condensation problems, which is a completely different fix.
- For genuine rising damp in a slab with no original membrane, a liquid applied damp proof membrane or a surface treatment is the usual answer, and it should go down before any floor covering.
If you are unsure whether you are looking at rising damp or condensation, that is worth a professional opinion. The two look similar and need opposite treatments.
What most people should actually do
For a typical UK garage with a reasonably sound slab and no visible damp:
- Clean the floor thoroughly
- Fill significant cracks and grind off high spots
- Lay the tiles directly onto the concrete
- Leave an expansion gap at the perimeter
- Ventilate the garage sensibly
No membrane. The floating floor and the air movement around it do the job.
Frequently asked questions
Will damp damage plastic floor tiles?
No. PVC and polypropylene tiles are unaffected by moisture. The concern is what happens underneath them and to whatever you store on them.
Can I use a builder's polythene sheet as a membrane?
It is not a substitute for a proper damp proof membrane, and under a floating floor it creates a slip layer. If you genuinely need a DPM, use a product designed for the job.
My garage floor sweats in winter. Is that rising damp?
Often it is condensation rather than rising damp. A cold slab plus warm moist air equals water on the surface. Ventilation and insulation address that, and a membrane will not.
What about underfloor insulation?
Insulated subfloor systems exist and are worth considering for garage conversions, though they raise the floor level, which affects door clearance.
Browse the garage flooring collection, or describe your floor to us and we will tell you honestly what preparation it needs.