There is a sentence on a great many microcement websites that is simply not true: "microcement is waterproof." It is repeated so often that clients arrive believing it, contractors quote on the basis of it and a certain number of wet rooms fail because of it. So this is the correction, in detail, along with what a properly built wet room actually looks like layer by layer.
The claim and what is wrong with it
Microcement is a polymer-modified cementitious coating applied at two to three millimetres and finished with a polyurethane seal. Sealed, it is highly water-resistant - water sits on it and runs off it rather than soaking in. That is a real and useful property.
What it is not is a waterproofing membrane. It is applied by hand in coats, it is thin, it has no elasticity to speak of and it cannot be tested. A membrane is a continuous engineered layer with a stated performance, taped at every junction, capable of accommodating movement and verifiable before anything covers it. Those are different objects doing different jobs.
The distinction matters most exactly where it is least visible: internal corners, the floor-to-wall junction, around the shower waste and at every pipe penetration. Those are the places water finds and they are the places a surface coating cannot protect on its own.
What a correct build-up looks like
From the substrate outwards:
1. Substrate preparation
Everything starts here. Boards must be rigid and screwed at close centres - deflection under load is the single most common cause of failure downstream. Existing tile is tap-tested across the whole field and anything hollow comes off. Screed is checked for moisture with a meter, not with a date on a programme. Falls to the waste are formed and confirmed with a level before anything is covered up.
2. Primer
Chosen for the substrate, not from habit. Low-porosity surfaces such as porcelain need a grip primer; absorbent surfaces need a stabilising one. Getting this wrong produces a membrane that is stuck to dust rather than to the building.
3. The waterproofing membrane - this is what keeps it sealed
A liquid-applied or sheet membrane over the whole wet zone. Two coats where liquid-applied and the second at right angles to the first so pinholes cannot line up. Reinforcing tape bedded into every internal corner, every floor-to-wall junction and around every pipe, waste and fixing penetration. Pre-formed corner pieces where the geometry is complex.
This layer is the one that stops water. Everything above it is finish.
4. Test it before covering it
A flood test where the geometry allows, or a documented visual inspection of every taped detail where it does not. This is the step most often skipped and skipping it is indefensible - once the microcement goes on, a defect in the membrane is invisible and can only be found by the damage it eventually causes downstairs.
5. Base microcement, with mesh
Two base coats with a fibre mesh embedded, which distributes stress across the surface and bridges the joints of any tile beneath. Mesh does not make the surface waterproof; it makes it stable, which is a different and equally necessary thing.
6. Finish coats
Two fine coats, compressed, carrying the colour and the character. Continuous across the floor, up the walls, into the niche, around the bench, with no joint anywhere.
7. The seal
A two-component polyurethane, applied in multiple coats, with an anti-slip additive in the shower zone and wet-room floor. This is the second line of defence - it makes the finish dense, non-porous and cleanable - and it is a consumable. It will need renewing at some point in the surface's life and the membrane underneath is what means that is a maintenance task rather than an emergency.
Why the two-layer principle matters
Every well-built wet room in any material works on the same principle: a waterproof layer you cannot see and a durable surface you can. Tiles are not waterproof either - grout is porous and tile joints move - which is exactly why tiled wet rooms are sealed too. Nobody argues about that. The confusion only exists with microcement because it looks continuous and continuity gets mistaken for impermeability.
How these bathrooms actually fail
In our experience of being called to look at other people's work, in order of frequency:
- No membrane at all. Microcement applied straight onto board because the surface was believed to be waterproof.
- Membrane without taped details. The flat areas are fine and every corner is a route.
- Movement. A deflecting floor or an unsupported board cracks the finish and then it does not matter what is underneath.
- Rushed cure. Seal put into service before it reached hardness, so it never achieved its rated performance and started failing early.
- Wrong seal. A single-component wall sealer used on a shower floor.
Note that only one of those is a problem with the microcement itself and it is the one about movement - which is really a problem with the substrate.
Questions worth asking whoever quotes your bathroom
- Which waterproofing system are you using and is it included in this price?
- How are internal corners and pipe penetrations detailed?
- Will the membrane be tested or inspected before the finish goes on?
- Which seal, how many coats and is there anti-slip in the shower zone?
- How long before the room can be used and what happens if that date slips?
An answer of "you don't need a membrane, microcement is waterproof" tells you everything you need to know and you should get another quote.
Our own wet room build-up is here and we are happy to talk through a specification with an architect or a contractor before anyone commits to anything - send us the drawings.