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Wall Cladding Calculator — Boards, Battens & Fixings
Calculate horizontal wall cladding boards, battens and fixings for external walls. Supports shiplap and featheredge profiles with opening deductions and wastage.
Standard storey height is 2.4m
Average length per wall
Doors and windows
Shiplap/T&G ≈ 25mm; featheredge 40mm (≤150mm boards), 50mm (>150mm) — NORclad/iwood
Default: 1.5 m² per opening
600mm maximum centres (TDCA); 400mm for green oak or diagonal boards
TDCA: 2 fixings at every batten crossing for boards ≥100mm; 1 for boards under 100mm
Default 3.6m — typical timber cladding board length
Adds 10% for cutting/offcut loss (iwood ×1.1 rule of thumb)
10% standard; 15% for walls with many openings
Safety notice
Structural elements must be designed or checked by a structural engineer before construction. Treat these figures as initial guidance for planning and budgeting, not a structural design.
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How We Calculate This
This calculator sizes a horizontal timber rainscreen. Boards run along the wall length in courses stacked up the height; each course covers its effective cover (board face width minus the overlap). It then sizes the vertical support battens and the fixings.
The formula
Courses = Wall height ÷ (Board width − Overlap)
Boards per course = Wall length ÷ Board length (rounded up)
Boards = Courses × Boards per course × (1 + wastage)
Battens = (Wall length ÷ Spacing) + 1 vertical, + 2 horizontal
Fixings = Boards × 2 × battens each board crosses
Effective cover assumes a 25mm lap for shiplap and 40–50mm for featheredge. The board count is conservative — it assumes no offcut reuse between courses — and a 10% wastage allowance (iwood ×1.1) is added by default. Fixings follow the TDCA rule of two per board at every batten crossing for boards 100mm and wider.
Frequently Asked Questions
For horizontal cladding, divide the wall height by the effective cover per course (board face width minus the overlap) to get the number of courses, then multiply by the boards per course (wall length ÷ board length, rounded up). For a 2.5m × 6m wall using 150mm shiplap boards (125mm effective cover) and 3.6m boards: 2.5 ÷ 0.125 = 20 courses × ceil(6 ÷ 3.6) = 2 boards per course = 40 boards before wastage, about 44 once a 10% cutting allowance (iwood ×1.1 rule of thumb) is added.
Shiplap and tongue-and-groove boards have a fixed rebate giving roughly a 25mm lap. Featheredge (weatherboard) is overlapped on site: use a minimum 40mm overlap for boards up to 150mm wide and 50mm for boards over 150mm, never less than 40mm so green timber can shrink up to 15mm as it dries without exposing a gap (NORclad / iwood guidance).
For horizontal boards, fix vertical battens at 600mm centres maximum. Use 400mm centres for high-moisture-content timber such as green oak, and no more than 400mm for diagonal boards. Use treated 25×50mm or 38×50mm battens and always include horizontal battens at the top and bottom of the wall (TDCA / BS 8605). See our Cladding Calculator for an area-based count with breather membrane and counter-battens.
The TDCA standard is two fixings per board at every batten crossing for boards 100mm and wider, placed at the quarter points and at least 20mm in from board ends; boards under 100mm wide take a single central fixing. A 3.6m board over 600mm-centre battens crosses about 7 battens, so needs roughly 14 fixings — not just 2 per board. Use stainless steel (A2/304, A4/316 near the coast), especially with cedar, Douglas fir and hardwoods, which stain black with ordinary steel.
Yes. A vapour-permeable breather membrane should be fitted against the wall behind the support battens, with the battens forming a drained and ventilated cavity (25mm minimum) between the membrane and the cladding. The membrane sheds wind-driven rain while letting moisture vapour escape. Approved Document C (resistance to moisture) requires external walls to resist moisture, and BS 8605 sets out the timber cladding detail.
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Last updated: June 2026
Verified against UK standards · estimates only, confirm with your supplier.