Most bathroom leaks are not caused by the tiles failing but by water finding a path beneath them. Grout and tile joints are not waterproof; over time, moisture passes through them and into the screed and slab, where it migrates sideways and downward. The common failure points are predictable. The floor-to-wall junction and internal corners flex slightly and crack if not reinforced. The drain and floor-trap surround is rarely detailed properly, so water bypasses the membrane at the very point where it pools. Pipe penetrations and concealed plumbing leak at threaded joints. Construction joints between old and new concrete open up. And in older bathrooms, no membrane was ever applied — only paint-on coatings on the visible surface. To diagnose before you strip a bathroom, look for patterns: a damp patch on the ceiling directly below the shower or WC points to floor or drain failure; staining on the outer face of a wet wall in the adjacent room points to wall splash-zone failure or a leaking concealed pipe; white crystalline deposits (efflorescence) confirm long-term water movement through cementitious material. A simple ponding (flood) test — plugging the drain, filling the floor with a few centimetres of water and watching the area below for 24 to 48 hours — is the standard way to confirm whether the floor membrane is intact before re-tiling. If leakage continues after a competent membrane has been applied and tested, the source is usually a pressurised concealed pipe rather than the membrane, and the plumbing must be opened up.

Surface preparation. The membrane is only as good as what it sticks to. The substrate must be structurally sound, clean, free of oil, grease, dust, laitance and curing compounds, and finished without sharp edges. Hack off loose plaster, fill large voids, and round off the sharp internal floor-to-wall angle into a cove (fillet) using a polymer-modified mortar so the membrane is not asked to turn a 90-degree corner. Saturate cementitious substrates to a damp, surface-dry condition (no standing water) where the product requires it; refer to the TDS. Priming. Apply the recommended primer or an SBR/latex bonding slurry where specified — this is especially important over dense or repaired concrete to promote adhesion. Detailing first. Before the field coat, treat the weak points: bed a reinforcing fabric or tape into a first application of membrane along all internal corners, the floor-wall junction, around the drain/floor trap and at every pipe penetration. This “band-aiding” step prevents the most common cause of failure. Field application. Apply the membrane in at least two coats, brushing or trowelling per the data sheet, taking the second coat at right angles to the first to avoid pinholes. Carry the membrane up the walls to the manufacturer’s recommended height — the full height in shower areas and a generous skirting band elsewhere. Maintain an even, continuous film without thin spots. Curing and testing. Allow each coat to set before the next, and allow full cure before flood testing and tiling — dry-to-touch is not the same as cured, and full cure typically takes a day or more depending on product, temperature and ventilation (refer to TDS). Carry out a 24-to-48-hour ponding test, then tile using a suitable polymer-modified tile adhesive — never sand-cement spot-fixing — so the membrane is not punctured.

The failures professionals see again and again are almost all avoidable. Skipping corner and drain reinforcement — relying on a flat coat to bridge a moving junction — is the single biggest cause of leaks. Applying membrane over a dusty, laitance-covered or oil-contaminated slab so it never truly bonds. Using too few coats or one thick coat that traps solvent/water and forms pinholes, instead of two thin cross-applied coats. Not turning the membrane high enough up the walls, especially in showers, so splash water gets behind it. Tiling before the membrane is fully cured and before a ponding test confirms integrity. Spot-fixing tiles with dabs of mortar that puncture the film. Ignoring the fall to the drain, so water ponds against junctions. Mixing incompatible primers, membranes and adhesives from different systems. And forgetting the concealed plumbing — a perfect membrane will not stop a leaking pressurised pipe. Following the TDS for coverage, coats, curing and over-coating windows eliminates most of these. When in doubt about substrate condition, movement, or a persistent leak, it is far cheaper to involve a professional applicator before tiling than to demolish afterwards.