We get this question a lot: “Do I need waterproofing or a vapour barrier?” The terms are used loosely on site and in quotations, so it is easy to see why people mix them up. Choosing the wrong one can leave your floor unprotected or have you paying for something you did not need.
Here is a plain explanation of what each one does and how to tell which your floor actually needs.
Three Different Jobs
Primer
A primer prepares a surface so the next layer bonds properly. On porous concrete, it controls suction and binds dust. On dense surfaces like tiles, it creates a key for levelling compounds. A standard primer does not stop moisture.
Vapour barrier
A vapour barrier slows or stops moisture vapour from passing through a material. Under floors, it holds back the water vapour that rises out of a damp slab so it does not get trapped under vinyl, timber or adhesive. In the flooring trade you will also hear it called a moisture vapour retarder.
Waterproofing
Waterproofing stops liquid water from passing through. It is used in bathrooms, balconies, roofs, planter boxes and anywhere water sits on or flows across a surface. A floor waterproofing membrane is designed to handle standing water and wetting from showers, rain and cleaning.
Where Damp Proofing Fits In
Damp proofing is a broader term, usually referring to a layer built into the structure to stop ground moisture rising into a building. Many newer ground floor slabs have a damp proof membrane underneath. Older buildings and some landed homes may not, which is one reason their floors stay damp.
Quick Comparison
| Primer | Vapour barrier | Waterproofing | |
|---|---|---|---|
| Main job | Improve bond | Hold back moisture vapour | Stop liquid water |
| Typical location | Under levelling compounds and adhesives | On damp slabs before flooring | Wet areas, balconies and roofs |
| Example | DP101 or DP80 | DP200 in two to three coats | Cemimax Waterproofing Membrane |
Why DP200 Sits in the Middle
Cemimax DP200 is interesting because it can act as both a primer and a vapour barrier. As one coat at 100 to 150 g/m², it primes and strengthens porous subfloors. As two to three coats totalling 250 to 350 g/m², it forms a closed film that holds back moisture vapour on slabs up to 6% CM. That is why contractors often call it a waterproof primer, even though its job is vapour control rather than full waterproofing.
Our DP200 guide goes through it in detail.
How to Decide What You Need
- Is liquid water present? If water pools, leaks or seeps through the floor, you need to solve that with waterproofing or drainage first.
- Is the slab damp but not wet? If testing shows residual moisture above what your flooring allows, use a vapour barrier.
- Is the slab dry and sound? A standard primer before levelling and adhesive is usually all you need.
Plenty of projects need more than one. A bathroom may have waterproofing under the screed and a primer before tiling. A ground floor office may need a vapour barrier, then a levelling compound, then adhesive. Our earlier post on how waterproofing membranes work explains the waterproofing side further.
Frequently Asked Questions
Is a vapour barrier the same as waterproofing?
No. A vapour barrier controls moisture vapour rising from a slab. Waterproofing stops liquid water. Some floors need both.
Can DP200 be used as waterproofing in a bathroom?
No. DP200 is a primer and moisture vapour retarder for subfloors. Wet areas need a proper waterproofing membrane.
Do I always need a vapour barrier under vinyl?
Only when the slab moisture is above the limit for your vinyl and adhesive. Test the slab to find out.
Still unsure which layer your project needs? Contact our team with a few details about the floor and we will point you in the right direction.