Tile Waterproofing Calculator — Tanking Membrane & Accessories

Yes. Tiles and grout are not waterproof. BS 5385-4:2015 (the code of practice for tiling in wet and damp conditions) states that wet rooms and power showers must be tanked, and that an impervious grout and adhesive are not a substitute for a tanked installation. Manufacturers including BAL, Mapei and Weber all recommend a tanking system behind tiles in wet areas. Without tanking, water can penetrate grout joints and damage the substrate, causing mould, rot or structural issues.

You need a minimum of two coats of liquid tanking membrane, applied at right angles to each other. Each coat is typically 0.5-0.7 kg/m², giving roughly 1.0-1.4 kg/m² total: Mapei Mapegum WPS quotes about 1.2 kg/m² over two coats, BAL Tank-it works out at about 1.37 kg/m² of powder at its 1mm dry build, and Mapelastic AquaDefense around 1.0 kg/m². Allow the first coat to dry (typically 2-4 hours) before applying the second. Always confirm the figure on your product's technical data sheet, as it varies by brand.

Tanking tape (also called joint tape, scrim tape or sealing tape) is used to reinforce joints where walls meet floors, wall-to-wall corners, and around pipe penetrations. The tape is embedded into wet tanking membrane to create a flexible, waterproof seal at these vulnerable movement points. Roll lengths vary by brand: BAL scrim tape comes in 20 m rolls, while many ready-mixed tanking kits ship with 10 m. Width is typically around 100-120 mm.

Pre-formed internal and external corners are strongly recommended. They provide a reliable waterproof seal at 90-degree junctions that is difficult to achieve with tape alone. A single shower cubicle typically has around 4 internal corners (the bottom wall-floor junctions plus vertical wall-wall corners); larger wet rooms or recesses have more. The pre-formed piece seals the corner point itself while the tanking tape laps the linear seam runs that meet it, so a junction usually needs both. External corners are used at niches, shelves or half-height walls.

As an indicative guide (2026 prices, which vary by supplier and region), tanking materials for a standard shower area of roughly 5-8 m² cost in the region of £80-£200 depending on the system. Liquid and powder kits such as BAL Tank-it or Mapei Mapegum typically retail around £30-£50 per kit, and sheet membranes like Schluter DITRA cost more but add decoupling benefits. Labour might add £150-£300. These are market-rate estimates, not fixed prices — always get a current quote from your supplier or tiler.