A leaking bathroom stains the ceiling below, damages the wall behind it and costs far more to fix than to prevent, because the tiles have to come off. Almost every leak starts at a corner, a pipe or the floor trap.
Step by Step
- Prepare the surface. Clean the slab and walls; repair cracks; remove loose material.
- Fillet the corners. Form a rounded cove where floor meets wall so the coating is not stretched across a sharp corner.
- Seal penetrations. Fix waterproofing collars around the floor trap and every pipe through the floor or wall.
- Apply the coating. Two or more coats of polymer-modified cementitious or liquid membrane, as the product specifies, over the floor and up the walls. Reinforce corners with mesh tape.
- Carry it up the walls. At least 300 mm everywhere; 1.8–2 m in the shower area.
- Pond test. Block the drain, hold water on the floor for 24–48 hours, check below.
- Protect the coating. A protective screed or the tile bed goes on carefully so the coating is not punctured.
- Tile with good grout. Use epoxy or polymer-modified grout in wet areas.
Sunken Slabs
Many houses have a sunken slab under bathrooms to hide the drain pipes. Water that reaches the filling has nowhere to go and seeps through.
- Waterproof the sunken slab and its sides before filling.
- Fill with lightweight, non-absorbent material.
- Consider a small weep pipe at the lowest point to drain any water that does get in.
- Waterproof again on top of the filling, under the tile bed.
Slope and Drainage
The finished floor should fall towards the trap — enough to clear water, not so much that it feels uneven. A flat floor holds water against the grout and eventually leaks.
Signs of a Failing Bathroom
- Damp patches on the ceiling below or the wall outside.
- Grout darkening or crumbling.
- Hollow-sounding tiles near the drain.
Fix early: regrouting and sealing can stop small leaks before they reach the slab.
The Short Version
Waterproof floor and walls before tiles, seal every pipe and the floor trap, keep the slope to the drain, and never skip the pond test. In sunken slabs, waterproof twice — under and over the filling.
