Decking Joist Spacing UK: 400mm or 600mm?
Quick Answer
400mm centres for a standard timber decking board. 600mm is too wide for typical boards
400mm is what our decking calculator uses by default, and it is the maximum support centre the Timber Decking and Cladding Association (TDCA) gives for a standard 145 x 27mm C16 board. 600mm centres are sometimes carried over from structural floor joist spacing, where they are perfectly normal, but on a typical decking board they cause noticeable bounce and flex. TDCA only rates decking boards to 600mm centres when the board itself is thicker and a higher structural grade, not the standard stock board.
Joist Centres by Board Type and Lay Pattern
| Board type / lay | Joist centres | Note |
|---|---|---|
| Standard softwood board, 32mm nominal (~28mm finished, C16) | 400mm | Default in our decking calculator; matches TDCA’s 400mm support-centre rating for a 145 x 27mm C16 board |
| 28mm finished softwood board | 350mm | Recommended centres for this board size per our UK decking board sizes reference |
| Composite decking | 300-400mm | Manufacturer-specific, some brands cap at 300mm; always follow the installation guide supplied |
| Diagonal or herringbone lay (45°) | 300mm | Roughly 25% tighter than the 400mm perpendicular base, boards cross more joists at an angle |
| Thick, high structural-grade board (~34mm finished, C24/D30) | up to 600mm | TDCA’s upper limit for graded structural boards only, not the standard stock board |
Figures from the TDCA Code of Practice: Raised Timber Deck Structures on New Homes (6th edition, May 2018), Table 5, cross-checked against manufacturer installation guides. The 400mm row matches the default in our decking calculator and our UK decking board sizes reference.
Worked Example: 400mm Centres on a 3.6m Deck
Our decking calculator counts joists along the run of the deck as the deck length divided by the joist spacing, rounded up, plus one extra joist for the far end:
- Deck length (direction the joists are spaced along): 3.6m (3,600mm)
- Joist spacing: 400mm centres
- Joists needed: 3,600 ÷ 400 = 9, plus 1 for the end = 10 joists
Worked Example: Diagonal Lay Needs More Joists
Lay the same 3.6m deck diagonally and the site’s standard reduction is 25% tighter centres: 400mm becomes 300mm.
- Joist spacing: 300mm centres
- Joists needed: 3,600 ÷ 300 = 12, plus 1 for the end = 13 joists
- That is 3 extra joists on the same run, 30% more timber in the subframe than a perpendicular lay at 400mm
TDCA’s own guidance is qualitative rather than a fixed percentage: “where diagonally boarded platforms are concerned, joist centres will need to be closer” to hit the same support-centre distances as a perpendicular lay. The 25%/300mm figure is this site’s standard reduction, used consistently across our decking calculator and board sizes reference.
Why 600mm Centres Are a Common Mistake
600mm joist centres are not a made-up number. They are completely normal for structural floor joists: our C16/C24 joist span tables list 400mm and 600mm as the two standard centres used in domestic floors, with deeper joists chosen to make the 600mm option work. The mistake is carrying that habit outside, onto a deck.
A decking board is thinner and less supported than a floor build-up (there is no chipboard or plywood deck skin tying it together), so it needs joists closer together to stay stiff underfoot. Our own step-by-step decking guide flags this directly: 600mm centres, common in floor joists, cause bounce and flex in decking, and its advice is to stick to 400mm maximum for timber and 300mm for composite. TDCA’s decking board support-centre table backs this up for the size of board most UK merchants actually stock, a 145 x 27mm (or 120 x 27mm) C16 board is rated to 400mm, not 600mm.
TDCA does list 600mm as an achievable centre, but only for a thicker, higher-grade board: 145 x 34mm finished at C24 or D30. That is a heavier-section board than the standard 32mm-nominal stock decking board most people buy, so 600mm should be treated as a manufacturer-specified upper limit for a specific board, not a default setting.
Typical Joist Sizes for a Decking Frame
For a standard garden deck (platform under 600mm off the ground), joist size and span work together: a deeper joist spans further between bearers, which is what lets you use wider centres in the first place. Our UK decking board sizes reference lists three common sizes:
- 47 x 100mm, low-level decks only, roughly 1.2m maximum clear span
- 47 x 150mm, the standard choice for domestic decks, roughly 2m maximum clear span
- 47 x 200mm, for larger spans
Bearers underneath the joists are usually a doubled-up 47 x 150mm C16 timber, or a 100 x 100mm post where the deck needs point support. All decking subframe timber should be pressure-treated softwood rated for outdoor, above-ground use.
Raised Decks Above 600mm: This Is a Different Job
Everything above applies to garden-level decking. TDCA defines a raised deck as one with any part of its platform more than 600mm above the ground, and treats it as a structural design problem, not a subframe-spacing question. Its own Code of Practice specifies significantly heavier joists for raised decks, starting at 170 x 44mm processed softwood at 300 to 600mm centres depending on board size and grade, engineered against a 3.0 kN/m² design load, not the light garden-decking sizes above. TDCA also states that Building Regulations require a safety parapet or balustrade at least 1,100mm high on any deck platform more than 600mm from the ground.
If your deck platform will sit higher than about 600mm anywhere along its length, treat the spacing figures on this page as background reading only. Get the frame designed by a structural engineer or a competent deck designer, and check with your local building control officer or your NHBC warranty provider before it is built or used. This is a safety-critical structure, not a DIY spacing calculation.
Figures verified against the TDCA (Timber Decking and Cladding Association) Code of Practice: Raised Timber Deck Structures on New Homes, 6th edition, May 2018 (tdca.org.uk), and cross-checked against this site’s own decking calculator, UK decking board sizes reference, joist span tables, and step-by-step decking guide. No span or spacing figures have been invented; where manufacturer guidance is the only source (composite decking centres), that is stated. Last updated: July 2026.