Should I use 6mm or 12mm cement board?

6mm cement board is suitable for wall applications as a tiling backer in bathrooms and showers — it's lighter and easier to cut and handle. 12mm board is recommended for floor applications (stronger, more rigid), external render substrates, and heavy-duty wall areas subject to impact. 18mm boards are used for structural flooring applications. For wetrooms, 12mm is the minimum recommended thickness.

What screw pattern should I use for cement board?

Fix cement boards at 200mm centres in both directions across the entire board area (not just the edges). Use special cement board screws with a countersunk head — standard drywall screws will corrode. Screws must be a minimum of 10mm from board edges and 25mm minimum penetration into the substrate. For floors, use 300mm centres with adhesive bedding.

Do I need waterproof tape on cement board joints?

Yes — all joints must be sealed with self-adhesive waterproof joint tape (alkali-resistant fibreglass mesh or butyl tape). This prevents water ingress at board joints, which is critical in wet areas. Apply the tape over the joint and embed it in a thin layer of flexible tile adhesive or waterproof tanking compound. Internal corners should also be taped.

Can I tile directly onto cement board?

Yes — cement board is designed as a tiling substrate. Use a flexible (S1 or S2 rated) tile adhesive to accommodate any slight movement. There is no need to apply PVA or prime the board before tiling (unlike plasterboard). However, for wetrooms, you should apply a liquid tanking membrane over the boards before tiling for full waterproofing.

What brands of cement board are available in the UK?

Common UK brands include Hardie Backer (James Hardie), No More Ply, Marmox Multiboard, Knauf Aquapanel, and Wedi board. Hardie Backer 500 (12mm) and Hardie Backer 250 (6mm) are the most widely stocked at Screwfix, B&Q, and builder's merchants. Wedi and Marmox boards are foam-core alternatives that are lighter but more expensive.

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Cement Board Calculator — 6mm vs 12mm Boards, Screws & Tape

Calculate cement backer board quantities, screw patterns, waterproof joint tape, and primer for tiling, wetrooms, and external render.

Total Area

Wall or floor area to be boarded

Board Thickness

6mm walls, 12mm floors/external, 18mm structural

Application Type

Determines whether primer is needed

Price Per Board (optional)

Per 1.2m × 2.4m sheet

How We Calculate This

This calculator estimates cement backer board quantities, fixings, waterproof tape, and primer needed for your project. It accounts for board size, screw pattern density, and joint sealing requirements.

The formula

Boards = (Area × Wastage factor) ÷ Board area (2.88 m²)

Screws are calculated on a grid pattern at your chosen centres. For 200mm centres on a standard 1.2m × 2.4m board: (7 × 13) = 91 screws per board. Waterproof tape is calculated from the total length of vertical and horizontal board joints, with each standard roll covering 45 metres.

Primer is only included for wet-area applications (bathrooms, wetrooms) at 0.15 L/m². Flexible adhesive is estimated at 3.5 kg/m² for board-to-substrate bonding.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.