Tanking a Bathroom: How to Waterproof Walls and Floors

Tanking is the process of applying a waterproof membrane or slurry to bathroom walls and floors before tiling. It prevents water from penetrating behind tiles and into the substrate — which leads to damp, structural damage, mould, and ultimately tile failure. It is one of the most important and frequently skimped-on stages of any bathroom renovation.

All shower areas — walls and floor — should be tanked, regardless of whether a shower tray or wet room format is used. Walls behind a bath should also be tanked to the full height of any splashing. In wet rooms (walk-in showers with a drained floor), the entire floor and lower walls must be tanked to form a fully waterproof system. Above-bath areas without a shower are lower risk but still benefit from waterproofing.

The main tanking options are: liquid applied membranes (brush or roll-on) — such as Schluter Kerdi, BAL Tank-It, or Aquabits — quick to apply and compatible with most substrates; tanking boards — rigid cement boards such as Wedi or Schluter Ditra — that form both a substrate and a waterproof layer; and waterproof sheet membranes — applied under screed in wet room floors. Each has different suitability depending on the substrate and format.

Tile adhesive is not a waterproofer. Tiles are not waterproof. Grout is not waterproof. The only thing that keeps water out of your wall structure is a correctly applied tanking membrane behind the tiles.

— Tile Association

The most common mistakes in bathroom tanking are: insufficient coverage (particularly in corners and around pipe penetrations); failure to use corner tape or reinforcing fabric in internal corners (which are the most vulnerable points); applying tiles before the membrane is fully cured; and using the wrong membrane for the substrate (some membranes are not suitable for plasterboard or timber substrates without a primer). Always read the manufacturer's system instructions.