How it’s worked out
- Lineal metres per m² = 1 ÷ (board width + gap, in metres). For 86 mm + 5 mm that’s about 11.
- Boards: rows of boards across the deck depth, each row covering the deck length in your chosen board length, plus 10% for offcuts and defects.
- Joists run from the house outwards at your spacing: deck length ÷ spacing + 1.
- Screws: two per board at every joist crossing, plus 10%.
This covers the decking and joists. Bearers, posts and footings must be sized from span tables. See the full timber deck materials list, and check whether your deck needs approval. Adding steps? Use the stair calculator.
Questions
How many decking boards do I need?
Divide 1 by the board width plus gap (in metres) to get lineal metres per m², multiply by the deck area, and add 10%. For 86 mm boards with 5 mm gaps that’s about 11 m per m², so a 4 × 3 m deck needs about 145 lineal metres.
How many decking screws per square metre?
About 50 for 86 mm boards on joists at 450 mm centres: two screws at every joist crossing, plus 10% spare.
What joist spacing for decking?
450 mm centres is standard for 19 mm hardwood decking. Composite and thinner boards often need 400 mm or less. Follow the board manufacturer’s spacing.
Which way should decking boards run?
Usually across the joists, parallel to the house wall. Board joins must land on a joist, so buy lengths that suit your joist layout.
Estimates only. Check the product label and your measurements before you buy. More free tools: see them all.