Decking guide
Decking Joist Spacing Explained
Get the frame right before the first board goes down. This practical UK guide covers timber, composite and diagonal decking layouts.
Start with the frame
Why joist spacing matters
Joists support the decking boards and transfer weight into the rest of the structure. Spacing them too far apart can leave the surface feeling springy, encourage boards to flex and put extra strain on the fixings.
Typical starting points
Recommended joist centres
Measure from the centre of one joist to the centre of the next.
Timber decking
A common domestic starting point when boards run at 90 degrees to the joists.
Composite decking
Requirements differ between products. Follow the exact board manufacturer’s guide.
Boards laid diagonally
Angled boards usually need closer support because they cross each joist differently.
Important: These are practical starting points, not a replacement for structural design or the decking manufacturer’s installation instructions.
Plan before cutting
Set out the whole frame first
Mark the overall deck area with string lines, check the corners are square and decide where every joist, support and board joint will sit before fixing permanently.
The frame should also allow water to drain and air to move around the timber. Avoid trapping the joists directly against wet soil or creating pockets where rainwater can sit.
- Measure the full length and width
- Check the frame is square
- Mark joist centres clearly
- Plan support beneath every board joint
- Allow for drainage and ventilation
Keep it consistent
How to lay out the joists
Mark each joist position along both sides of the frame. Fit joist hangers or suitable structural fixings, cut the joists accurately and check your spacing repeatedly as the frame comes together.
Add blocking or noggins where needed to reduce twisting, strengthen the frame and support board ends. The exact arrangement depends on the deck size and design.
- Measure centre to centre
- Use exterior-grade structural fixings
- Treat freshly cut timber ends
- Support board ends and butt joints
- Recheck level before laying boards
Simple working order
Build the deck frame in five stages
A little extra time spent setting out the frame makes the decking boards quicker to install and gives a cleaner finished result.
Measure
Confirm the deck size, finished height and access before ordering.
Prepare
Clear the site and create a firm, well-drained base for the supports.
Square
Build the outer frame and check both diagonals before tightening.
Space
Mark and fit the internal joists at the required centres.
Check
Confirm level, support, fixings and board-joint positions before boarding.
Avoid the usual problems
What poor spacing can cause
Bouncy boards
Insufficient support allows boards to flex noticeably under foot.
Loose fixings
Repeated movement can place extra strain on screws and clips.
Uneven joints
Unsupported board ends can move, lift or sit at different heights.
Shorter lifespan
Trapped moisture and a weak frame can lead to early deterioration.
Before you order
Decking materials checklist
Boards are only one part of the job. Work through the full frame, surface and finishing materials before sending your list.
- Decking boards or composite boards
- Treated timber joists and outer-frame timber
- Posts, supports or suitable deck feet
- Joist hangers and structural fixings
- Decking screws or manufacturer-approved clips
- Weed membrane and gravel where required
- End-grain timber treatment
- Fascia boards, trims and balustrade materials
Plan the whole decking job
Boards, framing timber, quantities and pricing.
Common questions
Decking joist spacing FAQ
What is the standard joist spacing for timber decking?
Around 400mm centres is a common domestic starting point when timber boards are laid at 90 degrees to the joists. Board dimensions, grade and system design can change the permitted spacing.
Does composite decking need closer joist spacing?
Sometimes. Composite requirements vary by brand and board profile, commonly falling somewhere between 300mm and 400mm centres. Always use the exact manufacturer’s installation guide.
What spacing should I use for diagonal decking boards?
Diagonal boards normally need closer joists. Some timber guidance recommends reducing the normal joist centres by roughly 30%, which often brings a 400mm layout close to 300mm.
Can decking joists sit directly on soil?
Keeping structural timber away from direct, persistently wet soil will generally improve its lifespan. Use a properly prepared, drained support system suited to the deck design.
Do I need noggins between decking joists?
Blocking or noggins can help prevent joists twisting and provide support around board joints or larger frames. Their position should follow the frame design rather than one fixed spacing for every deck.
When should I use a professional deck installer?
Get professional structural advice for raised decks, roof terraces, balconies, unusual ground conditions, balustrades or any deck where a failure could cause a serious fall or damage.
Need the full decking job priced?
Send us your measurements and materials list. We can help with boards, treated timber, fixings, treatments and local delivery across Essex.
