Waterproofing Coating (Around Bathtub, Around Washbasin, Under Shower Tray)

The shower gets the most water, especially a walk-in or level-access shower with a floor drain or linear channel. Here the whole shower floor is waterproofed. The walls are waterproofed at least as high as the water splashes, which in practice usually means above the shower head.

Why waterproof under the bath when the bath holds the water anyway? Water runs down the tiles and gets through the joint between the bath and the wall. From there it collects under the bath, where nobody notices it. That is why the floor under the bath, the wall behind it and a strip of wall above its rim all get a coating. The same applies to a shower tray. Water runs down the walls and the silicone joint along the tray edge wears out with time, so the walls around the tray and the floor beneath it need protection too.

Around the washbasin and toilet there is less water, but a strip of coating on the wall behind the basin is good insurance. The floor is usually waterproofed across the whole bathroom and turned up the walls around the edges. If the bathroom floods, the water then stays in the room and does not run into the walls or the next room.

In a block of flats, waterproofing also protects the flat below. If your renovation involves the risers or other shared pipework, speak to the building management or owners' association before you start.

• Liquid waterproofing membrane (dispersion-based): ready-mixed in a tub and applied with a roller or brush. It is easy to work with, so most DIYers and tilers use it in ordinary bathrooms.
• Two-component cementitious coating: mixed from a powder and a liquid. It is tougher and suits areas that stay wet for long periods, such as a walk-in shower without a tray or a spa area.
• Waterproofing sheets and membranes as part of a system: bonded to the surface. They have a consistent thickness and you do not wait between coats. They work well where the surface is still slightly damp or where the job needs to move on quickly.

Every system comes with accessories: sealing tape for corners and wall-floor joints, preformed corner pieces, and sealing collars for pipe penetrations and the floor drain. Ideally, take all the materials from one system by one manufacturer, including the primer and the tile adhesive. Then you know they are made to work together. Waterproofing products for use under tiles are tested to the European standard EN 14891, and this is usually stated on the packaging.

The amount of work, and so the final cost, depends mainly on the following:

• Condition of the surface: flat, firm, dry plaster or screed only needs priming. Uneven areas, old adhesive residue or crumbling plaster mean extra work.
• Type of surface: plasterboard, aerated concrete or anhydrite (calcium sulphate) screed each need a suitable primer. An anhydrite screed must also be dry enough, and this should be checked with a moisture meter.
• Layout of the room: every corner, recess, boxed-in installation wall and pipe penetration is another detail that needs tape or a collar.
• Type of shower: a shower with a tray is simpler. A walk-in shower with a channel drain needs a precise connection to the drain and a carefully formed fall.
• Area to be coated: waterproofing only around the shower and bath is less work than the whole floor and the walls to full height.

A handy homeowner can apply a liquid membrane. What matters is patience and sticking exactly to the manufacturer's drying times. If you are not sure about the connection to a floor channel, leave that detail to a tiler.

1. Remove the old tiles, adhesive residue and any loose plaster.
2. Fill larger dips and uneven areas, and let the repairs dry.
3. Vacuum the surface and clean off dust, grease and dirt.
4. Apply the primer recommended for your type of surface and let it dry.
5. Embed sealing tape into the first coat in the corners, at external angles and along the wall-floor joint.
6. Fit collars over the pipe penetrations and a sealing collar around the floor drain or channel.
7. Apply the first coat over the whole area and let it dry as the instructions say.
8. Apply the second coat, ideally at right angles to the first, so no thin spots remain.
9. Check that the layer is continuous, with no holes, bubbles or see-through patches.
10. Wait until it has fully cured, then fix the tiles with a flexible tile adhesive.

• No tape in the corners: corners are where the surfaces move, and the coating alone cracks there.
• One thin coat: manufacturers usually specify two coats and a minimum thickness that a single coat does not reach.
• Skipping the primer: the coating then bonds poorly and can peel off, tiles and all.
• Waterproofing the floor only: shower water runs down the walls, and without a coating turned up the walls it gets into the masonry.
• Coating not taken right up to the drain: in walk-in showers this is a common cause of leaks.
• Tiling onto a coating that is still wet: the membrane has not cured, and the adhesive can damage it.

Look over the coating before the tiles cover it. Afterwards, finding a fault is much harder. Go through this list:

• The layer is continuous and evenly coloured, with no see-through patches
• Sealing tape is embedded in the corners and along the wall-floor joint
• Pipe penetrations have collars, and the drain or channel is connected with a sealing collar
• In the shower, the wall coating reaches above the shower head
• The walls around the shower tray and the joint between tray and wall are covered
• The floor is also coated under the bath, and the coating is turned up the walls
• The coating has no bubbles, cracks or peeling spots
• The tiler knows which system was used and fixes the tiles with the adhesive the manufacturer recommends for it