Loft Boarding Calculator — How Many Loft Boards Do I Need?

This calculator determines the number of loft boards needed by dividing the total loft area (including a 10% wastage allowance) by the coverage area of each board.

The formula

Boards = (Loft length × Loft width × 1.10) ÷ Board area

Board sizes

• Standard board: 2400 × 600 × 18mm = 1.44 m² coverage
• Small board: 1220 × 320 × 18mm = 0.39 m² coverage

Battens (timber-raising method)

One way to raise the boarding above the insulation is to lay 75 × 50mm (3" × 2") timber raising battens along the top of each joist, parallel to the joists, then board on top. The calculator estimates one continuous batten per joist run: the number of joists depends on your joist spacing (400mm or 600mm centres), and each batten spans the loft. Battens are supplied in 2.4m lengths, so the total batten length is divided into 2.4m pieces and rounded up.

This batten figure is a trade-convention estimate for the timber method only. The modern best-practice method, proprietary loft legs / stilts (see FAQ), is counted per board or per grid spacing instead of by linear metre. It avoids the thermal bridging that continuous timber battens can create, so the batten quantity above does not apply if you use loft legs.

Screws

About eight screws per standard board is a sensible default fixing rate: roughly two at each short end, plus single fixings into the joists or battens crossing the board. The exact number depends on joist spacing, so you can adjust the screws-per-board value (4–16) under Advanced options. Use 4 × 40mm coarse-thread chipboard screws and pre-drill near edges to stop the chipboard splitting.

Important notes

Always raise boards above the insulation using battens or loft legs, never compress it. The current UK recommendation (Approved Document L / Energy Saving Trust) is 270mm of loft insulation measured from the ceiling. This is typically laid as ~100mm between the joists plus ~170mm across the top. The boards need to clear the full depth: their underside should sit about 170mm above the joist tops, which gives the recommended 270mm of insulation in total. Maintain ventilation gaps at the eaves, and do not push boards right to the eaves edge.