How high do I need to tank a wet room?

As a minimum, tank the floor and all walls to a height of at least 2.1m (above the shower head). Many manufacturers and Building Control officers recommend tanking walls to full height, especially in shower zones. The floor must be fully tanked including under any permanent fixtures. Refer to BS 5385-4 for guidance on wet area waterproofing zones. Use our shower tray fall calculator (/tiling/shower-tray-fall-calculator/) to work out the correct floor gradient before tanking.

How many coats of tanking liquid do I need?

Two coats are required for proper waterproofing. Apply the first coat and allow it to dry (typically 2-4 hours depending on the product), then apply the second coat at right angles to the first. Each coat should be applied at approximately 1 litre per m². Some manufacturers recommend a primer coat on porous substrates before the first tanking coat.

Do I need reinforcement tape at all joints?

Yes. All internal corners (floor-to-wall and wall-to-wall junctions) must have reinforcement tape (also called scrim tape or tanking band) embedded in the tanking liquid. This prevents cracking at movement joints. Pre-formed internal and external corner pieces are also available and provide a neater, more reliable seal at corners.

Can I use liquid tanking over plywood floors?

Most liquid tanking systems are designed for use over cement-based substrates (sand-cement screed, concrete, cement board). For plywood or timber floors, you typically need a decoupling membrane (such as Schluter DITRA) or a specialist flexible tanking system. Check the tanking manufacturer's technical data for approved substrates. In most wet rooms, cement board (e.g. HardieBacker, Marmox) over the timber floor is the preferred approach.

What is the difference between liquid tanking and sheet membrane?

Liquid tanking is a brush-on waterproof coating applied in two coats — it is easy to apply around complex shapes and pipe penetrations. Sheet membrane (such as Schluter KERDI or Impey WaterGuard) is a pre-formed waterproof sheet bonded to the substrate — it provides a consistent thickness and is often faster for large flat areas. Both are acceptable under BS 5385-4. Liquid tanking is more common for DIY and smaller wet rooms.

Wet Room Tanking Calculator — Waterproofing Materials Needed

Calculate tanking liquid, reinforcement tape and corner pieces needed to waterproof a wet room. Covers floor and walls to your specified height.

Floor Area

Total wet room floor area

Wall Height to Tank

2.1m min (above shower head) or full height

Number of Walls

Number of walls to tank

Wall Lengths

Wall 1 Length

Wall 2 Length

Wall 3 Length

Wall 4 Length

Include wastage allowance

Recommended for application losses

Wastage Percentage

10% is standard for tanking products

Tanking Liquid Price/Litre (optional)

Price per litre of tanking liquid

Tape Price/Roll (optional)

Price per 10m roll of reinforcement tape

How We Calculate This

Our wet room tanking calculator works out the total area to be waterproofed (floor plus walls to your specified height), then calculates the tanking liquid, reinforcement tape and corner pieces you need.

The formula

Total tanking area = Floor area + (Sum of wall lengths × Tanking height)

Tanking liquid = Total area × 1 L/m² × 2 coats × Wastage factor

What you need

Tanking liquid: Applied at 1 litre per m² per coat, with 2 coats required. Common products include BAL Tank-it, Mapei Mapelastic, Weber Superflex and AquaSeal Tanking Liquid.
Reinforcement tape: Applied to all internal junctions (floor-wall and wall-wall corners) and embedded in the first coat of tanking liquid. Typically sold in 10m rolls.
Internal corner pieces: Pre-formed corner pieces for wall-wall vertical junctions and floor-wall corners provide additional waterproofing reliability.

Building Regulations

Wet room waterproofing must comply with Building Regulations Approved Document C (resistance to moisture) and follow BS 5385-4 (tiling in wet areas). The tanking system must form a continuous, sealed membrane across the floor and up the walls. All pipe penetrations, drain connections and corners must be sealed with compatible products from the same system.

Our coverage rates are based on manufacturer guidelines from leading UK tanking system suppliers. We recommend adding 10% wastage for application losses and brush-on variability.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.