Short answer: Tiles and grout are not watertight – the waterproofing lies underneath. Liquid membrane is brushed or rolled onto the primed substrate in two layers; all corners and connections first get a sealing tape into the fresh first layer, pipe penetrations and floor drains get a collar. Heights: in the shower up to 25 cm above the shower head, for a bathtub with shower function up to 2.25 m, otherwise 25 cm above the tap. The floor is waterproofed only once the wall tiles are in place. Tile adhesive of class C2 adheres perfectly to cured liquid membrane.
Why waterproof at all? Tiles are not watertight
The most widespread misconception in bathroom building is: the tiles keep the water out. They do not. The tile itself is watertight, but the grout is not – cement grout absorbs water and passes it on to the back. Added to that are hairline cracks caused by screed shrinkage and temperature changes, and the silicone joint at the edge of the bathtub, which will tear at some point.
What gets through ends up in the screed, plaster, plasterboard or timber structure. It stays there because it cannot dry out. The consequences show up years later: dark patches on the wall in the next room, a musty smell, swelling door frames, mould behind the pre-wall installation. A renovation then costs many times the waterproofing that was saved.
Waterproofing is therefore not an optional extra but is regulated in the standards as bonded waterproofing – the water-carrying layer lies beneath the covering and is firmly bonded to it.
Where in the bathroom does waterproofing have to be applied?
Not every wall in the bathroom needs waterproofing, but false economy in the wrong place is expensive. The standards sort by water exposure classes:
| Class | Situation | Waterproofing |
|---|---|---|
| W0-I | Guest WC, washbasin without shower, occasional splashes | usually no area waterproofing needed |
| W1-I | Bathroom in living area, bathtub, shower with moderate use | bonded waterproofing in the splash zone |
| W2-I | level-access shower, frequently used family bathroom | bonded waterproofing, floor over the full area |
| W3-I | commercial showers, swimming pool surrounds | tested system, often waterproofing slurry |
In practice, these dimensions apply for the specific heights:
Shower
On all walls of the shower zone at least 20 cm above the highest outlet – this is the standard base value. In practice, work is usually done up to 25 cm above the shower head, so that the dimension is still sufficient even after a later change of fitting height. At the sides at least 20 cm beyond the shower zone, the shower floor over the full area.
Bathtub
If the bathtub is also used for showering, the same height applies as for the shower – this usually results in around 2.25 m above finished floor level. If it is used only for bathing, 20 cm above the tap and 20 cm to the sides beyond the edge of the bathtub are sufficient.
Washbasin and other areas
Behind the washbasin 50 cm to the sides and 20 cm above the tap. If only the floor area is waterproofed, it must be turned up at least 5 cm at rising components. With a level-access shower, the floor is waterproofed over the full area.
Which area falls into which class and what follows from it under the standards is explained in detail in our guide Bathroom waterproofing: regulations, classes and systems.
What you need
- Liquid membrane, ready to use – calculate the quantity from the area
- Primer suited to the substrate, applied twice on strongly absorbent surfaces
- Sealing tape for all corners and edges
- Wall sealing collars for pipe penetrations, floor sealing collar for the drain
- Brush for corners and details, short-pile paint roller for surfaces, smoothing trowel or squeegee
- Folding rule, spirit level, pencil for marking the heights
- Tile adhesive of class C2 for the subsequent laying
- Sanitary silicone for the connection joints at the end
Step 1: Check the substrate
The waterproofing is only as good as what it lies on. Check four things before you open the first tin:
Evenness and alignment. Walls vertical, floor with the necessary slope to the drain. Unevenness is levelled beforehand, not concealed with the liquid membrane.
Scratch test. Draw a hard object across the substrate. If material trickles out, it is not load-bearing.
Tap test. Areas that sound hollow in plaster or screed have come loose and must be removed.
Moisture and cleanliness. The substrate must be dry, dust-free and free of release agents. Screed must be ready for covering – for cement screed usually below 2% residual moisture.
Step 2: Prime
The primer consolidates the surface, regulates the absorbency and improves the adhesion of the waterproofing. On strongly absorbent substrates such as aerated concrete, sand-lime brick or old plaster, the process is repeated until the surface no longer looks thirsty.
On non-absorbent substrates – concrete, old tiles – an adhesion primer with a quartz sand content is used. The primer must be completely dry before you continue; depending on product and room climate, that takes between one and several hours.
Step 3: Sealing tape and collars – the most important operation
Waterproofing almost never fails in the surface. It fails in the corners, at the connections and at the penetrations. That is why these places are done first, and more carefully than anything else.
Apply liquid membrane generously in the area of the corner and lay the sealing tape into the still-fresh compound. Press it down with the brush without air bubbles until the membrane comes through the fabric. Crucial: leave a small loop in the corner. The tape must not be stretched taut over the edge, otherwise it tears at the first settling movement. Then pull liquid membrane over the tape again.
This applies to all wall-floor corners, all internal wall corners, the connection to the bathtub or shower tray and all movement joints. Overlap tapes by at least 5 cm.
Pipe penetrations for mixer taps and wall connections get a wall sealing collar using the same method. The floor drain gets a floor sealing collar, which is embedded generously in the fresh waterproofing and worked over. With level-access showers, the drain is the most critical point in the whole bathroom.
Step 4: Waterproof the surfaces
Only now do the surfaces come next. The first layer is applied generously and fully covering – with the roller on large areas, with the brush at edges and details. Work right up to the sealing tape so that no defect remains.
Many liquid membranes change colour as they dry. That is the built-in check: as long as the surface still looks damp, it is not ready for the second layer. Once it has changed colour throughout, the second coat can follow – usually after one to two hours.
The second layer is applied at right angles to the first. This change of direction is not a ritual: it ensures that thin spots and roller marks of the first coat are reliably closed.
Maintain the total dry film thickness required by the manufacturer. It is the only value that decides watertightness, and it follows directly from the consumption per square metre.
Step 5: The right order – wall before floor
A point that regularly goes wrong in practice: you waterproof wall and floor, then tile the wall – and damage the finished floor waterproofing in the process. A single fallen tile shard is enough to puncture the membrane, and you cannot see it.
The safe order is: waterproof the wall surfaces, lay and grout the wall tiles, then waterproof the floor area and tile the floor. The transition is made via the sealing tape already laid, which was taken about 15 cm up above the screed – this creates the tray-like waterproofing that does not let water escape sideways.
Liquid membrane on old tiles
For a renovation in which the old tiles are to stay, liquid membrane is a practical solution – under three conditions.
The tiles must be firmly bonded. Tap the entire surface. Tiles that sound hollow are removed and the spot is levelled. A waterproofing on loose tiles tears along with them as soon as the tile comes away.
The surface must be degreased and roughened. Soap residues, care products and lime soap prevent any adhesion. Wash thoroughly with an alkaline cleaner, rinse, leave to dry. Additionally sand glossy glazes.
An adhesion primer for non-absorbent substrates is needed. A normal deep-penetration primer does not work on glaze; a bonding bridge with a quartz sand content is required.
Old silicone joints are removed completely beforehand – neither primer nor waterproofing adheres to silicone residues. How to do this without residue is explained in our guide Removing old silicone without residue.
Consider the build-up: waterproofing plus adhesive plus new tile together add around 15 to 20 mm. Doors, connections and drain heights must allow for this.
Does tile adhesive hold on liquid membrane?
Yes – that is exactly its purpose. Liquid membrane is designed as a substrate for the subsequent thin-bed laying. Two things are required:
First, the waterproofing must be completely dry, usually after about 24 hours. On a still-damp membrane the adhesive does not bond, and trapped moisture later finds no way out.
Second, a tile adhesive of class C2 belongs on it, that is a cementitious adhesive with increased tensile adhesion strength. The classification is printed on the bag. For large-format tiles, also look for S1 – a deformable adhesive that absorbs stresses.
Consumption depends on tile format and substrate; you will find the values in our overview of tile adhesive consumption.
Which substrate may go in the bathroom?
The waterproofing can only be as good as what it lies on. In the bathroom, the substrate often decides whether a build-up lasts permanently.
| Substrate | In wet areas | Note |
|---|---|---|
| Cement-bonded building board | first choice | moisture-resistant, dimensionally stable, can be coated directly |
| Sand-lime brick, clay brick, concrete | yes | joints flush, surface dust-free, prime |
| Cement plaster, cement screed | yes | sufficiently old, ready for covering, crack-free |
| Impregnated plasterboard (green) | conditionally | only outside the shower, not in the direct splash zone |
| Normal plasterboard | no | swells irreversibly when it gets damp |
| Anhydrite screed | conditionally | measure residual moisture, primer mandatory |
| Wood, chipboard, OSB | no | moves too much, waterproofing tears with it |
| Old tiles | yes | firmly bonded, degreased, adhesion primer required |
For a pre-wall installation or a new partition wall in the shower area, cement-bonded building board is almost always worthwhile – the extra cost compared with impregnated plasterboard is small, the risk considerably smaller.
Special case: level-access shower
The level-access shower is the most demanding situation in a private bathroom, because the water is not held back by a bathtub edge.
Slope. The floor needs an even slope of about 1.5 to 2% towards the drain – that is 1.5 to 2 cm per metre. Too little slope leads to standing water, too much makes tiles rock and is unpleasant when showering. The slope is created in the screed, not levelled out in the adhesive bed.
The drain. Point or channel drains are supplied with a bonding flange onto which the floor sealing collar is applied. This connection is the tightest spot in the bathroom – or the leakiest, if it has been done sloppily. Embed the collar generously in the fresh waterproofing, press it down without air bubbles and work over it completely.
Build-up height. Calculate backwards from the top edge of the drain: drain, screed to falls, waterproofing, adhesive bed, tile. In renovation work on existing buildings, the available height is often the limiting factor and decides the choice of drain system.
The transition to the rest of the bathroom. Without a threshold, water runs further than you think. That is why, with level-access showers, the entire floor is waterproofed, not just the shower zone.
Liquid membrane or waterproofing slurry?
| Liquid membrane | Waterproofing slurry | |
|---|---|---|
| Supply form | ready to use in a bucket | powder, must be mixed |
| Application | brush, roll | brush, spread, trowel |
| Drying until tiling | approx. 24 hours | 1–3 days |
| On old tiles | good with adhesion primer | only with bonding bridge |
| In contact with the ground, cellar | no | yes |
| Crack bridging | good | rigid none, flexible good |
| Typical use | bathroom, shower, interior | bathroom, balcony, cellar, terrace |
In the bathroom, liquid membrane is usually the quicker and simpler choice, especially for renovations on existing substrates. Where the work goes outdoors or soil is involved, there is no way around waterproofing slurry – the details are under Applying waterproofing slurry.
Finally: the silicone joints
After grouting the tiles, all movement and connection joints are closed with sanitary silicone: wall to floor, wall to wall, connection to bathtub and shower tray. These joints are maintenance joints – they should be renewed every few years and are expressly not part of the waterproofing.
When scraping out old joints, do not use a knife, but a broken-off folding rule or a plastic scraper – a knife cuts through the waterproofing underneath, and the damage goes unnoticed. With natural stone, natural stone silicone expressly belongs in the joint, otherwise dark edges form.
The most common mistakes
The required dry film thickness is not reached with a single coat. The surface looks watertight but is not in places.
The corner is where the greatest movement occurs. Without tape, or with a tape under tension, the waterproofing tears open exactly there.
Splash water runs down the wall and finds every top edge. Better 20 cm too high than 5 cm too low.
While tiling the wall, material falls down and damages the fresh floor waterproofing – invisible and fatal.
On a still-damp membrane the adhesive does not bond, and the residual moisture stays permanently trapped in the build-up.
Frequently asked questions
How long does liquid membrane need to dry?
Between both layers one to two hours depending on product and room climate, until ready for covering usually about 24 hours. Many products show progress through a colour change.
Can you paint liquid membrane onto tiles?
Yes, if the tiles are firmly bonded, thoroughly degreased and pre-treated with an adhesion primer for non-absorbent substrates. Old silicone joints must be removed completely beforehand.
How much liquid membrane do I need?
Depending on the product, about 1.0 to 1.5 kg per square metre for both layers together. For an average shower bathroom with around 10 m² of waterproofed area, that is about 12 to 15 kg. Allow 10 percent extra for corners and waste.
Is liquid membrane vapour-permeable?
No, it is designed as a water-impermeable layer. That is why the substrate must be sufficiently dry before application – trapped moisture later finds no way out.
Is liquid membrane only needed behind the shower?
For the wall surfaces, the defined splash zone is enough. With a level-access shower, however, the floor should be waterproofed over the full area, because water there cannot be confined to the shower zone.
What about plasterboard and wood-based boards?
In wet areas, impregnated or better still cement-bonded building boards belong. Normal plasterboard swells irreversibly when it gets damp – a waterproofing over it only delays the damage.
Does the washbasin also have to be waterproofed?
The area behind the washbasin gets waterproofing of about 50 cm to the sides and 25 cm above the tap. That is considerably less than in the shower, but not zero – splash water at the washbasin is one of the most common causes of swollen vanity units.
