How to work out decking boards
Boards = rows × boards per row (deck length ÷ board length) × (1 + waste)
Joists = deck length ÷ joist spacing + 1
Common Australian deck boards are 90 mm or 140 mm wide and 19 mm thick, in hardwoods such as merbau and spotted gum or in treated pine. Hardwood is often sold in random lengths by the lineal metre. Leave a 3 to 5 mm gap for drainage and movement; hardwood needs the bigger gap if it is fitted dry.
Worked example
A 4.8 m × 3.6 m deck with 90 mm boards and 4 mm gaps needs 39 rows. Using 4.8 m boards, that is 39 boards, or 43 boards with 10% waste (about 206 lineal metres). At 450 mm spacing it needs 12 joists 3.6 m long, and about 936 screws.
Tips
- Choose board lengths that match your deck to avoid joins; joins must land on a joist.
- 19 mm hardwood boards usually allow joists at up to 450 mm; composite boards often need 300 to 400 mm: check the maker's guide.
- Use H4 treated pine for posts in the ground and at least H3 for joists and bearers.
- Decks over a set height (often 1 m) usually need council approval and balustrades: check your state rules before you build.
Frequently asked questions
How many deck boards do I need for a 4.8 m by 3.6 m deck?
About 39 boards of 4.8 m with 90 mm boards and 4 mm gaps, or 43 with 10% waste.
What spacing should deck joists have?
Up to 450 mm centres for 19 mm hardwood boards; many composite boards need 300 to 400 mm.
How many screws do I need for decking?
Two screws per board at every joist. A 4.8 m by 3.6 m deck with 90 mm boards and 12 joists needs about 936.
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Last updated October 2026.