A good-looking deck starts below the surface. We can get obsessed with the finish, colour, pattern, or edge trims, but the real strength of any deck lies in its bones: the joist layout.
Get that wrong, and it doesn’t matter what boards you use. You’ll end up with bounce, squeaks, and gaps that open up after the first winter.
Joist spacing is one of those details that looks simple on paper, 300, 400, or 600mm centres, yet each figure carries a world of difference in performance and lifespan.
Let’s break down what those numbers mean, how to apply them for timber and composite decking, and why getting it right will save you money and headaches later on.
What Joists Do In A Deck Structure
A deck is basically an outdoor floor. The joists are the skeleton, the part that carries the weight, resists sagging, and transfers the load to the ground or subframe.
They don’t just “hold up” the boards; they define how the whole platform behaves over time.
Tight joist spacing (say 300mm centres) provides a rigid, spring-free feel underfoot. Wider spacing (600mm) can make things feel spongy even if the deck is technically safe.
Every footstep, barbecue, or garden table load is transmitted through those joists. The closer they are, the less each piece of timber or composite board has to carry.
The UK’s Framing Reality: No Single Rulebook, Just Best Practice
There isn’t a “decking code” in the UK Building Regulations. Instead, spacing and load guidance come from general structural standards, like:
- BS 5268-2 for timber structures
- Eurocode 5 (EN 1995) for wood-based construction
- TRADA and NHBC guidelines for domestic platforms
Manufacturers fill the gaps with their own data, especially for composite boards, which have different flex characteristics. In practice, manufacturer instructions always override generic standards.
The usual domestic deflection limit for decks is L/200, meaning the board shouldn’t deflect more than 1/200th of its span under load. Joist spacing is what controls that.
The Common Spacing Options: 300mm, 400mm, And 600mm Centres
Here’s how these spacings play out in real-world decks across the UK.
300mm Centres: For Thin, Flexible or High-Traffic Decks
This is the tightest spacing you’ll normally see, often used for composite decking under 25mm thick or lightweight softwood planks in busy areas. Commercial decks in pubs or schools sometimes use this to avoid bounce.
It’s also used for diagonal board layouts because the effective span increases. The trade-off? You’ll use more joists and screws, but the feel underfoot is far superior.
One common mistake is forgetting that tighter spacing restricts airflow, so make sure the subframe has enough ventilation to avoid damp buildup.
400mm Centres: The UK Standard For Domestic Decks
If you’ve ever built or bought a garden deck, this is probably how it’s used for. 400mm on centre (measured from joist centre to joist centre) suits most 28–32mm timber boards and mid-range composite products.
It balances material use and performance neatly, stiff enough to walk on comfortably, flexible enough to cope with small movement from temperature or moisture. When boards are laid diagonally, tighten to 300–350mm centres to maintain the same rigidity.
600mm Centres: For Structural Joists, Not Deck Boards
Many DIYers assume 600mm spacing saves timber. In reality, it’s rarely appropriate to deck boards directly.
You’ll see it on support joists or bearers under the main frame, but never as a surface layout. At 600mm, even thick planks will bend under load and trap water.
Unless you’re using 50mm hardwood boards or a double-joist system, this is asking for trouble.
Material Matters: Timber Vs Composite
Softwood Decking
Traditional tanalised timber is forgiving, but its strength depends on the grade, C16 vs C24 makes a big difference. Softer grades bend more, so keep them closer together.
Moisture also plays a role. Decks in shaded or damp areas benefit from tighter spacing (300–350mm) for better stability. Adequate airflow under the deck prevents rot; aim for at least 150mm clearance between ground and joists, per NHBC recommendations.
Composite Decking
Composites look stiff but often flex more than timber because of their internal makeup (plastic and wood fibre blend). That’s why most manufacturers specify 300–400mm centres.
Another catch: expansion and contraction. Composites expand across their length, so uneven joist spacing exaggerates the issue, creating lifted corners or popped fasteners.
Always check the installation guide, spacing errors are one of the main reasons composite warranties get rejected.
Using joist tape or flashing between joist tops and boards also helps stop trapped moisture, which can rot the frame even if the boards themselves are weatherproof.
UK Standards And What They Mean In Practice
While no single code dictates spacing, several documents steer the approach:
- BS 5268-2 gives strength classes for timber beams, handy when choosing joist size for spans.
- Eurocode 5 (EN 1995) sets out deflection limits, helping determine maximum spacing between supports.
- TRADA span tables help identify safe distances for specific timber sizes (e.g., 47x150mm C24 joists at 400mm centres).
For domestic decks up to 600mm above ground, follow these benchmarks:
- Joist size: 47x150mm (typical)
- Joist centres: 400mm
- Support beams: 600mm–1m apart
- Post spacing: 1.8–2m
Always check manufacturer guidance first, especially for composites.
Fixing Or Reinforcing Poor Spacing
If your existing deck feels bouncy, it’s not always doomed. You can often fix it by adding extra support.
- Add noggins or bridging between joists to stiffen the frame.
- Insert additional joists mid-span where the deflection is worst.
- For composite boards, it might mean a partial dismantle, frustrating, but necessary if the spacing’s too wide.
It’s worth checking that poor spacing isn’t a symptom of deeper problems. Sagging joists or uneven footings might point to structural movement.
If you notice cracks or dipping beyond the deck, it’s smart to read up on how to identify if a building is structurally unstable before proceeding.
Often Overlooked Factors In UK Deck Installs
A few extra details make or break the job.
Decking over patios or membranes:
Water drainage is key. Make sure joist spacing and layout allows water to escape freely and don’t compress waterproof layers with fixings.
Joist direction:
Always run joists perpendicular to the house if possible, it helps water drain away from the wall. Align them with slope direction for better runoff.
Fixings and connectors:
Use stainless steel or galvanised hangers. Corrosion in fixings will shorten the deck’s life faster than poor spacing.
And since all this involves saws, drills and heavy timber, don’t forget safety prep. Worth a look at these workplace health and safety tips, they cover the basics for staying safe on the tools from PPE to safe cutting setups.
Digital Tools For Smarter Training
Building confidence before cutting timber saves more than just offcuts. The Tradefox simulation app lets tradespeople, apprentices and even experienced joiners practice deck layouts, spacing and safety sequences in a risk free environment.
It’s a great way to test spacing setups virtually before committing to real materials, especially if you’re balancing different joist centres for composite and timber projects.
Final Thoughts
There’s no magic number for joist spacing that suits every deck. The sweet spot depends on what’s sitting on top, timber, composite, diagonal pattern or commercial foot traffic.
But if you’re unsure, closer is safer. You’ll never regret overbuilding a deck that will withstand winters and barbecues.
Decks that last don’t happen by accident. They start with good spacing, straight joists and a builder who takes the time to plan before cutting.
