Timber Cladding Calculator

The effective cover of each board equals the board width minus the overlap. For 150mm shiplap with 25mm overlap: effective cover = 125mm. Linear metres of board per m² of wall = 1000 ÷ effective cover (mm) = 1000 ÷ 125 = 8 linear metres per m². Multiply by the wall area (e.g. 25 m² → 200 linear metres), then divide by your board length (e.g. 3.6m) and round up to get the number of boards (200 ÷ 3.6 → 56). Finally add 10% for wastage on cuts and ends (56 → 62 boards).

Shiplap has a uniform thickness with a rebated overlap of about 25mm — boards slot together for a flat appearance. Featheredge (weatherboard) tapers from thick at the bottom to thin at the top, with a 40-50mm overlap (use 50mm for boards 150mm and wider). Featheredge is more traditional and sheds water well, while shiplap gives a cleaner, more modern finish. For interior cladding, see our Tongue and Groove Calculator.

Yes, a breather membrane is essential behind timber cladding. It allows water vapour to escape from the building while preventing wind-driven rain from penetrating. Fix the membrane to the wall, then install battens over it to create a ventilated cavity of at least 25mm behind the cladding. Without this cavity, moisture will be trapped and cause rot.

Battens should be fixed vertically at 600mm centres for horizontal cladding (or horizontally at 600mm for vertical cladding). Use 25×50mm or 38×50mm treated softwood battens. The battens create the ventilation cavity behind the cladding and provide the fixing points for the boards. Use stainless steel or galvanised fixings.

Use stainless steel ring-shank nails or screws — never plain steel, which will rust and stain the timber. Annular ring-shank nails provide excellent holding power. For hardwood cladding, pre-drill to prevent splitting. For overlapping profiles such as featheredge, fix each board with a single nail per batten placed just above the top edge of the board below, so the fixing never passes through two boards (TDCA guidance; nailing through both boards restrains movement and splits them). Secret-nail through the tongue on T&G profiles. For flat boards wider than 100mm, use 2 fixings per board at each batten.