Render Carrier Board Calculator — Timber Frame Sheathing

Timber frame construction moves with moisture and temperature changes, which causes traditional sand:cement render to crack. Render carrier boards (such as Knauf Aquapanel, Cembrit, or Masterboard) provide a rigid, dimensionally stable substrate that doesn't move with the frame. The render is applied to the board rather than directly to the sheathing, preventing cracking.

For Knauf Aquapanel Exterior the board is fixed to vertical timber battens (or studs) at a maximum of 600mm centres, with screws at least 15mm in from the board edge and spaced at a maximum of 250mm centres down each batten line (BBA Certificate 09/4633). On a 900mm × 2400mm board this works out at roughly 25 screws per board (around 11–12 screws per m²). Use the system-specific stainless steel screws — the 40mm SN40 for 12.5mm board onto timber — and leave a 3–5mm gap between boards for movement.

Yes — all render carrier board systems require a layer of alkali-resistant fibreglass mesh embedded in the basecoat. The mesh reinforces the render, bridges board joints, and prevents cracking. Standard mesh is 160 g/m² with a minimum 100mm overlap at joints. Some systems use a double mesh layer at ground floor level for impact resistance.

Standard Knauf Aquapanel Outdoor boards are 900mm × 2400mm (2.16 m²). Cement particle boards and fibre cement boards typically come in 1200mm × 2400mm (2.88 m²). Larger formats are available for some products but less common. Choose the largest practical size to minimise joints, as each joint is a potential weakness.

Most render carrier boards are suitable for use below DPC (damp proof course) level, but check the specific product data sheet. Knauf Aquapanel is fully waterproof and can be used below grade. Cement particle boards are also suitable. However, fibre cement boards vary — some absorb moisture and are not recommended below DPC. Always provide drainage and a starter track at the base.