How Much Decking Do I Need?
Quick Answer
You need approximately 18 boards plus 7 joists for a 4m × 3m deck
Based on: standard 3.6m × 145mm × 28mm treated softwood decking boards with 5mm gaps
How We Calculated This
Decking board quantity is based on the deck width divided by the board coverage (board width plus gap):
- Deck area: 4m × 3m = 12m²
- Board coverage: 145mm + 5mm gap = 150mm per board
- Boards across 3m width: 3,000 ÷ 150 = 20 boards
- Board length: 3.6m (trimmed to 4m deck length — need 4.2m or two boards butted)
- Practical answer with standard 3.6m boards: ~18 boards (allowing for efficient cutting)
Joist Calculation
Joists run perpendicular to the decking boards, typically at 400mm centres:
- Deck width: 3m = 3,000mm
- Joist spacing: 400mm centres
- Number of joists: (3,000 ÷ 400) + 1 = 8.5, round to 7–8 joists
- Joist length: 4m each (or 2 × 2.4m with a bearer underneath)
Common Decking Board Sizes (UK)
- Standard softwood: 3.6m × 145mm × 28mm
- Premium softwood: 4.8m × 145mm × 32mm
- Composite: 3.6m × 150mm × 25mm
- Hardwood (balau): 3.35m × 145mm × 21mm
Subframe Materials
As well as decking boards, you will need:
- Joists: 47mm × 100mm or 47mm × 150mm treated softwood
- Bearers/beams: doubled-up joists or 100mm × 100mm posts
- Deck screws: 60–75mm stainless steel or coated, ~8 per board
- Joist hangers or coach bolts: for frame connections
- Weed membrane: to suppress growth beneath the deck
Wastage Allowance
For a simple rectangular deck, allow 5% wastage. For L-shaped, curved, or angled decks, allow 10–15%. If your deck length exceeds available board lengths, you will need to butt-join boards over a joist — add extra boards accordingly.
Typical UK Costs
- Treated softwood boards (3.6m): £6–£12 each
- Composite boards (3.6m): £15–£30 each
- Hardwood boards (3.35m): £20–£40 each
- Joists (3.6m): £8–£15 each
- Deck screws (200 pack): £10–£20
Last updated: April 2026