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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.
Total wet room floor area
2.1m min (above shower head) or full height
Number of walls to tank
Wall Lengths
Longest dimension of the area
Longest dimension of the area
Longest dimension of the area
Longest dimension of the area
Recommended for application losses
10% is standard for tanking products
Price per litre of tanking liquid
Price per roll of reinforcement tape
Tanking liquid per m² per COAT. Default 0.5 → ~1 L/m² over 2 coats. Mapei Mapelastic AquaDefense ~0.5 kg/m²/coat (≈1 kg/m² total); Weber webersys protect 0.4 kg/m²/coat — always check your product PDS.
Number of tanking coats required (default 2)
Metres per roll of reinforcement tape (default 10)
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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 × Coverage per coat × Number of coats × Wastage factor
We default to 0.5 L/m² per coat × 2 coats = 1 L/m² total — the upper end of the realistic range. Most liquid membranes use roughly 0.7–1 L (or kg) per m² for the complete two-coat system: Mapei Mapelastic AquaDefense is about 1 kg/m² over two coats, and Weber webersys protect about 0.8 kg/m². Coverage varies by product, so set the per-coat figure from your chosen product’s Product Data Sheet using the advanced options.
What you need
- Tanking liquid: Roughly 0.7–1 litre (or kg) per m² for the complete two-coat system (about 0.4–0.5 per m² per coat), with 2 coats required. Common products include BAL Tank-it, Mapei Mapelastic AquaDefense, Weber webersys protect and Everbuild/Sika Aquaseal — always confirm the exact rate on the product’s data sheet.
- 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 5385-4:2015 (design and installation of tiling in wet/damp areas, clause 7 covers frequently-wetted conditions). 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 default coverage rate is taken from leading UK tanking-system Product Data Sheets, but consumption varies between products (roughly 0.7–1.25 L/m² total over two coats), so set the per-coat figure from your chosen product’s data sheet for an exact result. We recommend adding 10% wastage for application losses and brush-on variability.
Frequently Asked Questions
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 to work out the correct floor gradient before tanking.
Two coats are required for proper waterproofing, and most liquid membranes use roughly 0.7–1 litre (or kg) per m² for the COMPLETE two-coat system — not per coat. For example, Mapei Mapelastic AquaDefense is about 1 kg/m² over two coats (≈0.5 kg/m² per coat), and Weber webersys protect is about 0.8 kg/m² total (0.4 kg/m² per coat). Coverage varies by product, so always confirm the rate on your chosen product's Product Data Sheet. Apply the first coat, let it dry (typically 1–4 hours depending on the product), then apply the second coat at right angles to the first. Some manufacturers recommend a primer coat on porous substrates first.
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.
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.
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.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.