Deck substructure calculator: joists, bearers and posts
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Deck length | 4 m |
| Deck width | 3 m |
| Joist centres | 400 mm |
| Largest joist span for your timber | 1.5 m |
| Largest bearer span between posts | 2 m |
| Joists hung from rim boards | Yes |
| Rows of noggins | 0 |
| Timber length you can buy | 4.8 m |
| Allowance per piece | 100 mm |
| Height above which decking needs planning permission | 300 mm |
You need 11 joists, 3 bearer lines and 9 posts
- Within the guidance Joist run between bearers against the largest joist span you entered: 1.5 m UK-wide: Your timber supplier's or TRADA span tables (figure you entered). Check with the source
- Within the guidance Bearer run between posts against the largest bearer span you entered: 2 m UK-wide: Your timber supplier's or TRADA span tables (figure you entered). Check with the source
- Out of scope Joist, bearer and post sizes Sizing timber is out of scope: use your timber supplier's or TRADA's span tables, or ask a structural engineer. UK-wide: TRADA span tables or a structural engineer. Check with the source
| Result | Value |
|---|---|
| Joists | 11 |
| Length of each joist | 3 m |
| Bearer lines | 3 |
| Posts per bearer line | 3 |
| Posts | 9 |
| Joist hangers | 22 |
| Timber lengths to buy | 16 |
| Length | Cut into | Offcut |
|---|---|---|
| 1 (4.8 m) | Rim board 1: 4.1 m | 0.7 m |
| 2 (4.8 m) | Rim board 2: 4.1 m | 0.7 m |
| 3 (4.8 m) | Bearer 1: 4.1 m | 0.7 m |
| 4 (4.8 m) | Bearer 2: 4.1 m | 0.7 m |
| 5 (4.8 m) | Bearer 3: 4.1 m | 0.7 m |
| 6 (4.8 m) | Joist 1: 3.1 m | 1.7 m |
| 7 (4.8 m) | Joist 2: 3.1 m | 1.7 m |
| 8 (4.8 m) | Joist 3: 3.1 m | 1.7 m |
| 9 (4.8 m) | Joist 4: 3.1 m | 1.7 m |
| 10 (4.8 m) | Joist 5: 3.1 m | 1.7 m |
| 11 (4.8 m) | Joist 6: 3.1 m | 1.7 m |
| 12 (4.8 m) | Joist 7: 3.1 m | 1.7 m |
| 13 (4.8 m) | Joist 8: 3.1 m | 1.7 m |
| 14 (4.8 m) | Joist 9: 3.1 m | 1.7 m |
| 15 (4.8 m) | Joist 10: 3.1 m | 1.7 m |
| 16 (4.8 m) | Joist 11: 3.1 m | 1.7 m |
Show the working
- Joists: 4 m ÷ 400 mm, rounded up, + 1 = 11, each 3 m long
- Joist hangers: 2 × 11 joists = 22
- Bearer lines: 3 m ÷ 1.5 m, rounded up, + 1 = 3, so joists run 1.5 m between bearers
- Posts per line: 4 m ÷ 2 m, rounded up, + 1 = 3, so bearers run 2 m between posts
- Posts: 3 lines × 3 = 9
- Timber: joists, rim boards and bearers with 100 mm per piece, from 4.8 m lengths = 16 lengths (0 joins)
How it works
- Joists = deck length ÷ joist centres, rounded up, + 1, each as long as the deck width
- Bearer lines = deck width ÷ bearer spacing, rounded up, + 1 (bearer spacing = your figure, or the largest joist span you entered)
- Posts per line = deck length ÷ post spacing, rounded up, + 1 (post spacing = your figure, or the largest bearer span you entered)
- Hangers = 2 per joist; noggins = rows × (joists − 1); 2 rim boards along the deck length
- Checks: the distance joists and bearers actually run between supports against the largest spans you entered
Sizing joists, bearers and posts is out of scope: use your timber supplier's or TRADA's span tables, or ask a structural engineer.
Use timber treated for ground contact where it touches the ground (BS 8417 treatment classes).
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Joist centres | 400 mm | Our assumption Not yet checked against the source | Our choice: change it to suit your job Check the board maker's maximum span. |
| Timber length you can buy | 4.8 m | Common UK size Not yet checked against the source | From your product Lengths of 2.4, 3.6 and 4.8 m are common. |
| Allowance per piece | 0.1 m | Our assumption Not yet checked against the source | Our choice: change it to suit your job Allowance per piece for squaring ends and mitres (brief v2 FL2). |
| Coach bolts per post | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job From the designer's or timber supplier's fixing detail. |
| Height above which decking needs planning permission | 300 mm | Regulation or HSE figure Planning Portal: decking more than 300 mm above the ground Not yet checked against the source | planningportal.co.uk (checked 19 October 2026) brief v2 F2 (raised deck > 300 mm → planning note). Researcher to confirm and check Wales. |
Regulations and standards
- TRADA span tables (not reproduced: use your timber supplier's or TRADA's figures)
- Planning Portal: decking more than 300 mm above the ground
The planning note is for England (Planning Portal). Wales, Scotland and Northern Ireland have their own permitted development rules.