How to work out decking boards
Boards = rows × boards per row (deck length ÷ board length) × (1 + waste)
Joists = deck length ÷ joist spacing + 1
The most common UK softwood deck board is 144 mm wide, 28 to 32 mm thick, sold in 2.4, 3.6 and 4.8 m lengths. Leave a 5 mm gap for drainage and movement.
Worked example
A 4.8 m × 3.6 m deck with 144 mm boards and 5 mm gaps needs 25 rows. Using 4.8 m boards, that is 25 boards, or 28 boards with 10% waste. At 400 mm spacing it needs 13 joists 3.6 m long, and about 650 screws.
Tips
- Choose board lengths that match your deck to avoid joins; joins must land on a joist.
- Composite boards usually need joists at 300 to 400 mm: check the maker's guide.
- Use treated timber rated for ground contact (Use Class 4) for joists near the ground.
- A raised deck may need planning permission and must meet Building Regulations: check before you build.
Frequently asked questions
How many deck boards do I need for a 4.8 m by 3.6 m deck?
About 25 boards of 4.8 m with 144 mm boards and 5 mm gaps, or 28 with 10% waste.
What spacing should deck joists have?
400 mm centres is standard for softwood 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 13 joists needs about 650.
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Last updated October 2026.