Tile Backer Board Calculator — Boards, Screws & Tape

Backer boards (like HardieBacker, Marmox, or wedi) are essential in wet areas — showers, around baths, and behind basins. They're waterproof, rigid, and won't degrade if water gets behind the tiles. For dry areas, plasterboard is fine. In shower enclosures and wet rooms, backer board is a building regulations requirement.

6mm backer board is suitable for walls over existing plasterboard or solid walls. 12mm is used as a direct replacement for plasterboard on timber stud walls. 20mm+ is used for floor applications or where extra rigidity is needed. For shower walls, 12mm is the standard choice. Always check the manufacturer's recommendation for your specific application.

Spacing depends on the board type. Foam/XPS boards fixed to timber studs (e.g. Marmox Multiboard) take a stainless screw and washer every 300mm — Marmox specifies about 12 fixings per 1200×600mm board (roughly 16-17 per m², so ~16 per 1200×800mm board). Cement boards (e.g. HardieBacker) are fixed at 200mm centres into each stud and around the perimeter — roughly 24-30 screws per 1200×800mm board (~25 per m²). Use stainless steel or coated backer-board screws (plasterboard screws corrode), keep fixings about 15mm from edges and 50mm from corners, and countersink the heads flush. This calculator lets you pick 200mm or 300mm centres.

Yes — all joints between backer boards must be taped with waterproof joint tape (usually a self-adhesive alkali-resistant mesh) and covered with waterproof tile adhesive or tanking compound. This creates a continuous waterproof surface. Internal corners should also be taped. Some systems use pre-formed corner pieces for a more reliable seal.

It depends on the board. Cement and fibre-cement boards usually take a coat of acrylic primer (e.g. BAL Prime APD or Mapei Eco Prim Grip) to improve the adhesive bond, but many waterproof XPS boards (such as Marmox Multiboard) are ready to tile straight away — always check the board and adhesive manufacturer's instructions. BAL Prime APD covers 100-200 ml/m² undiluted (about 5-10 m² per litre); this calculator estimates at the 0.15 L/m² mid-rate. Allow the primer to dry (typically 15-30 minutes to a few hours) before tiling.