Do I need to waterproof before tiling a bathroom?

Yes. Tiles and grout are not waterproof. BS 5385 and manufacturers like BAL, Mapei and Weber all recommend a tanking system behind tiles in wet areas. This is especially critical in showers, wet rooms and around baths. Without tanking, water can penetrate grout joints and damage the substrate, causing mould, rot or structural issues.

How many coats of liquid tanking membrane do I need?

You need a minimum of two coats of liquid tanking membrane, applied at right angles to each other. Each coat should be approximately 0.5-1.0 kg/m² depending on the product. Allow the first coat to dry (typically 2-4 hours) before applying the second. Some manufacturers recommend three coats in high-risk areas.

What is tanking tape used for?

Tanking tape (also called joint 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. Standard rolls are 10m long and 120mm wide.

Do I need pre-formed corners for tanking?

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 typical shower cubicle has 4-8 internal corners. External corners are used at niches, shelves or half-height walls.

How much does bathroom tanking cost in the UK?

Tanking materials for a standard shower area (approximately 5-8 m²) typically cost between £80 and £200 depending on the system used. Liquid membranes (e.g. BAL Tank-it, Mapei Mapegum) cost around £20-35 per 5kg tub. Sheet membranes like Schluter DITRA cost more but offer decoupling benefits. Labour typically adds £150-300.

Tile Waterproofing Calculator — Tanking Membrane & Accessories

Calculate tanking membrane, joint tape, pre-formed corners, drain collars and primer needed for waterproofing wet areas before tiling.

Wall Area to Tank

Total wall area in the wet zone

Floor Area to Tank

Shower tray area or wet room floor

Membrane Type

Liquid is most common for standard showers

Internal Corners

Wall-floor and wall-wall junctions

External Corners

Niches, half-height walls, shelves

Joint Seams

Board joints requiring tape reinforcement

Drain Collars

One per floor drain point

Membrane Price (optional)

Price per unit of membrane

Tape Roll Price (optional)

Price per 10m roll of tanking tape

Corner Piece Price (optional)

Price per pre-formed corner

Drain Collar Price (optional)

Price per drain collar

How We Calculate This

This calculator estimates the waterproofing materials needed to tank a wet area before tiling. It covers liquid or sheet membrane, primer, joint tape, pre-formed corners and drain collars.

The formula

Membrane = Total area × Coverage rate × Coats × (1 + Waste %)

For liquid tanking at 1 kg/m² per coat with 2 coats on 8 m²: 8 × 1.0 × 2 × 1.10 = 17.6 kg. Primer at 0.2 L/m² on the same area: 8 × 0.2 × 1.10 = 1.76 litres.

Key components

Primer: Applied to the substrate before membrane at approximately 0.2 L/m² (5 m²/L) to improve adhesion.
Tanking tape: Approximately 0.5m per joint/corner seam, embedded in wet membrane.
Pre-formed corners: One per internal or external 90° junction.
Drain collar: One per floor waste/drain point to seal around the outlet.

Based on manufacturer guidelines from BAL, Mapei, Weber and Schluter for UK wet-room and shower waterproofing systems.

Frequently Asked Questions

Related Calculators

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Calculate the correct fall gradient for a wet room or shower tray.

Bathroom Extraction Rate Calculator

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.