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Render Board / Cement Board Calculator

Calculate boards, screws, jointing tape, and mesh for cement board substrates - external render systems, tile backer, and rainscreen applications. Supports multiple walls with opening deductions.

Render Board / Cement Board Calculator

Boards, fixings, jointing tape, and mesh for any cement board substrate

General-purpose cement board for external render substrates and wet areas. Fix at 150 mm perimeter / 300 mm field.
Common: 6, 9, 12, 15, 25 mm
- enter dimensions
Around board edges - typically 150 mm, min 15 mm from edge
Internal area - typically 300 mm centres
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Alkali-resistant mesh tape is embedded into the basecoat over all board joints.
Typically 50-150 mm wide
10%
10% standard; 15% for rooms with many cuts or irregular shapes

Wall / Area Sections

Job Costing (optional)

Stainless steel or hot-dip galvanised
~5-8 m² per litre depending on board porosity

Results

Net Board Area
-
m² (less openings)
Boards to Order
-
inc. waste
Screws
-
total fixings
Jointing Tape
-
linear metres
Primer / Base Coat
-
litres (approx)
Board Type
-
-

Area Breakdown

Area Gross Openings Net Boards Screws Tape (lm)

Board Type Guide

  • Standard cement board - General-purpose substrate for external render and wet rooms. Brands include Marmox, Hardiebacker, Proctorboard.
  • Aquapanel Outdoor - Knauf's proprietary glass-fibre reinforced cement board. Specifically designed for external render systems and ETICS. Has integrated mesh facing.
  • Render board (K-Rend / Weber) - Expanded polystyrene composite boards providing insulation and render substrate in one. Used in ETICS / EWI systems.
  • Fibre cement board - Higher density, suitable for tile substrates and external soffits. Brands include Eternit, James Hardie, Cedral.
  • Tile backer board - For internal wet areas and shower enclosures. Lightweight and waterproof. Brands include Wedi, Jackoboard, Marmox.

Fixing Guidance

  • Always use stainless steel or hot-dip galvanised screws - standard zinc will corrode behind render
  • Perimeter fixings: 150 mm centres, minimum 15 mm from board edge
  • Field fixings: 300 mm centres across the board face
  • Screw length: min. 35 mm penetration into the substrate (stud / batten / masonry)
  • For masonry: use stainless frame fixings, not woodscrews
  • Never fix closer than 50 mm to a corner of the board

Jointing & Mesh

  • All board-to-board joints must be taped with alkali-resistant (AR) fibreglass mesh tape - standard fibreglass degrades in cement
  • Embed the mesh into the first coat of render or adhesive while still wet
  • Leave a 2-3 mm gap at joints to allow for movement - do not butt boards tight
  • At corners, use corner bead or wrap the mesh around by 100 mm each side
  • For Aquapanel: use Aquapanel joint compound and tape, not standard render

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