How this calculator works
The number of boards is the area to cover, plus an allowance for cutting, divided by the area of one board, rounded up. The boards lie across the joists, and each one is screwed to every joist it crosses: the length of the board divided by the joist spacing, rounded up, with two screws at each. For joists at 400 mm centres, 18 mm tongue and groove chipboard is used, and 22 mm for joists at 600 mm.
Worked example: a loft area 4 m by 3 m (12 m²) with 2400 × 600 mm boards, each covering 1.44 m², needs 10 boards with 10% extra for cutting (9 with none). That is 120 screws with joists at 400 mm, and the boards are 18 mm thick. With joists at 600 mm the boards are 22 mm thick and the screws are 80. A small 4 m² area takes 4 boards, and a larger 6 × 4 m area (24 m²) takes 19. With shorter boards of 1200 × 320 mm, covering 0.38 m², the 12 m² area needs 35.
Boards for common loft areas (2400 × 600 mm boards, 10% extra, joists at 400 mm)
The number of boards, and the screws.
| Area | Size | Boards | Screws |
|---|---|---|---|
| 4 m² | 2 × 2 m | 4 | 48 |
| 6 m² | 3 × 2 m | 5 | 60 |
| 9 m² | 3 × 3 m | 7 | 84 |
| 12 m² | 4 × 3 m | 10 | 120 |
| 16 m² | 4 × 4 m | 13 | 156 |
| 20 m² | 5 × 4 m | 16 | 192 |
| 24 m² | 6 × 4 m | 19 | 228 |
Tips before you buy
- Board only what you need for storage. A fully boarded loft needs a lot of insulation lifted above the joists.
- Lay the boards across the joists with the ends meeting on a joist, and stagger the joins in neighbouring rows.
- Screw down at each joist, with two screws a board: nails can work loose and creak.
- Leave the area near the eaves unboarded to allow air to flow and keep the roof ventilated.
- Keep wires and pipes clear: do not lay a board over a cable that could be crushed, and check for water tanks.
Tools & materials for this project
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Frequently asked questions
How many loft boards do I need?
For 12 m² with 2400 × 600 mm boards and 10% extra, 10 boards. Each board covers 1.44 m².
What thickness of loft board do I need?
18 mm tongue and groove chipboard for joists at 400 mm centres, and 22 mm for joists at 600 mm. The spacing of your joists decides.
How many screws do I need for loft boards?
Two for every joist a board crosses: 120 for 10 boards over joists at 400 mm.
Can I board over the insulation?
Not on the joists: the boards squash the insulation and make the loft colder. Build a raised platform on loft legs so that there is 270 mm of insulation under the boards.
How much weight can a loft hold?
Light storage only. Ceiling joists are made to carry the ceiling and some insulation, not furniture. If you want to store much, ask a builder or a structural engineer.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.