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

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.

It depends on how the board is fixed. On a stud wall, James Hardie's spec is to fasten 'a maximum of 8 inches (≈200mm) on centre at all supports' — i.e. along each stud line. On a floor (or any board bedded in mortar/thinset on a solid substrate), the spec is to fasten 'every 8 inches (≈200mm) on centre over the entire surface' — a full field grid. Keep screws about ⅜ inch (≈10mm) in from board edges and 2 inches (≈50mm) in from corners. A full-field 200mm grid on a 1200 × 800mm board works out to roughly 30–35 screws — this calculator uses the full-field figure so it covers floor-tile and bedded solid-wall installs and never under-orders. Use purpose-made cement board screws with a countersunk head (standard drywall screws corrode) and a minimum 25mm penetration into the substrate. Knauf AQUAPANEL allows a slightly wider 250mm maximum screw spacing.

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.

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); a tile primer is optional and only adds extra adhesion on dusty boards. Priming is genuinely needed only before a self-levelling compound. For wetrooms, apply a liquid tanking membrane over the boards before tiling for full waterproofing — that is a waterproofing layer, not a tiling primer.

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) — both 1200 × 800mm (0.96 m²) — are the most widely stocked at Screwfix, B&Q, and builder's merchants; No More Ply comes in 1200 × 800mm and a 1200 × 600mm 6mm board. Knauf AQUAPANEL Indoor is 1200 × 900mm (1.08 m²), with a 2400 × 1200mm large-format sheet. Note that 1.2 × 2.4m is a plasterboard size, not a standard cement backer board size. Wedi and Marmox boards are foam-core alternatives that are lighter but more expensive.