Waterproofing wet rooms — walls and floors

Membranes, tanking, and why tiles alone aren't waterproof

Tiles are not waterproof

This is the most important thing to understand about wet room tiling: tiles and grout slow water down but don't stop it. Water molecules are smaller than the pores in cement grout. Over months and years, water migrates through grout joints and reaches the substrate behind. Without a proper waterproofing membrane, this moisture causes mould, structural damage, and eventual tile failure.

Where waterproofing is needed

• Shower enclosures — walls and floor, full height. The most critical area.
• Around bathtubs — the wall above the bath to at least 300 mm above the shower head height.
• Wet room floors — the entire floor when there's no shower tray.
• Behind sinks — the splash zone on the wall behind basins.
• Steam rooms — all surfaces: walls, floor, and ceiling.

Types of waterproofing

Liquid-applied membranes are painted or rolled onto the substrate. They cure to form a seamless, flexible waterproof layer. Easy to apply, they conform to complex shapes and corners. Most require two coats with a reinforcing tape at joints and corners. This is the most common choice for domestic bathrooms.

Sheet membranes are pre-formed waterproof sheets bonded to the substrate with adhesive. They provide a consistent, guaranteed thickness. Used for larger commercial wet areas and where a faster workflow is needed. More expensive but more reliable.

Cementitious waterproofing is a cement-based coating that forms a rigid waterproof layer. Suitable for tanking (sealing concrete against ground water) but not ideal for tile substrates as it doesn't accommodate movement.

Critical details

• Corners and joints — where walls meet floors and where walls meet other walls are the highest-risk areas. Always reinforce with waterproof tape embedded in the membrane.
• Pipe penetrations — every pipe through the membrane is a potential leak. Use pre-formed sealing gaskets or extra membrane layers around pipes.
• Floor falls — wet room floors must slope toward the drain at 1-2%. This fall should be built into the screed before waterproofing.