How to Waterproof a Wetroom or Bathroom Before Tiling
Water getting behind tiles is one of the most common — and most expensive — failures in bathrooms and wetrooms. Tiles and grout are not waterproof, so the wall and floor behind them need a proper tanking system before you tile.
Why bathrooms and wetrooms fail
Constant moisture, steam and standing water work through grout lines and into plaster, boards and the structure behind them — causing damp, mould, blown tiles, rot and costly remedial work. The fix is a continuous waterproof layer behind the finish.
What the standards say
BS 5385-4 is clear that tiles and grout are not a substitute for a tanked installation — substrates in wet areas should be protected with a tanking membrane before tiling. NHBC guidance (updated November 2024) makes tanking mandatory for wetrooms and power showers (flow rates of 12 litres per minute or more); lower-risk standard showers may use a water-resistant board with waterproof adhesive and grout. If in doubt, tank it.
The solution: a liquid-applied waterproof membrane
Poseidon is a flexible, liquid-applied waterproof membrane that cures to a seamless, watertight barrier. Applied by brush, roller or spray over concrete, cement and tile backer boards, it forms the impervious layer the standards call for — ready to tile directly onto.
Don’t forget the junctions
Most leaks start at corners, wall-to-floor junctions and around pipes. Reinforce these first: seal movement joints and penetrations with The GOAT or Copagro flexible sealant so the whole enclosure is watertight, not just the open areas.
How it is applied
- Prepare a clean, sound, dust-free substrate
- Seal internal corners, wall-to-floor junctions and penetrations
- Apply Poseidon in a uniform layer (typically two coats) to the required film thickness
- Allow to cure, then tile directly onto the membrane
Benefits
- Seamless, flexible and crack-bridging
- Meets BS 5385-4 / NHBC tanking guidance
- Brush, roller or spray applied
- Bonds to common bathroom substrates
Frequently asked questions
Do I need to tank the whole bathroom or just the shower? Wetrooms and power showers should be fully tanked; in standard bathrooms, tank at least the wet zones (shower/bath surrounds and floor). Tanking more is never wrong.
Can I tile straight onto it? Yes — once cured, Poseidon is designed to be tiled directly with a suitable adhesive.
Is it the same as damp proofing? No — see Damp Proofing vs Waterproofing for the difference.
Browse the full range of liquid waterproof membranes, or ask us to spec your job.