How to waterproof a bathroom before tiling

Step-by-step procedure

1. 1

   Strip and inspect

   Remove old finishes back to sound screed or concrete. Check for cracks, honeycombing and loose sand-cement. Confirm all plumbing rough-in is complete and pressure tested — no penetration may be cut after waterproofing.

2. 2

   Correct the falls

   Verify a minimum 1:100 fall towards the floor waste, and 1:80 inside a shower area. Re-screed flat or ponding areas; a membrane cannot compensate for standing water.

3. 3

   Form fillets and repair

   Cut out cracks, fill with repair mortar, and form a 25 mm cove fillet at every wall-floor junction so the membrane is never asked to turn a sharp 90° angle.

4. 4

   Prime the substrate

   Dampen or prime per the membrane data sheet. Cementitious membranes usually need a saturated surface-dry substrate; acrylic ones need a fully dry, primed one.

5. 5

   Band the movement points

   Bed reinforcing tape into a wet first pass of membrane at all internal corners, wall-floor junctions, drain flanges and pipe penetrations. This detail is where nearly all wet-area leaks begin.

6. 6

   Apply two coats

   Brush the first coat over the full floor and up walls to 1.8 m in showers and 150 mm elsewhere. Once touch-dry, apply the second coat at right angles to the first to eliminate pinholes. Respect the total wet film thickness in the data sheet.

7. 7

   Flood test

   After full cure, plug the drain, flood the floor to 25 mm and hold for 24 hours. Check the ceiling and walls below. Only a passed flood test releases the area for tiling.

8. 8

   Tile with a compatible adhesive

   Use a polymer-modified C2 adhesive over the membrane. Never mechanically fix through the membrane; if a fixing is unavoidable, seal it with membrane and tape.

Why the flood test is not optional

Rectifying a leaking wet area after tiling costs many times the price of the membrane. A documented 24-hour flood test with photographs is the cheapest insurance on a project.