Decking Board Spacing and Joist Centres Explained

Timber decking boards are typically spaced with a 3-5mm gap between them, on joists centred roughly 400-450mm apart; composite decking usually needs closer joist centres, commonly 300-400mm, and a slightly wider board gap of around 5-6mm — always check the specific manufacturer's spec sheet, since requirements vary between products more than many installers assume.

Why board gaps matter

The small gap between boards isn't just cosmetic — it allows water to drain through rather than pooling on the surface, lets air circulate to help boards dry after rain, and gives timber room to expand slightly when it absorbs moisture without boards buckling against each other. Too tight a gap and you lose drainage and expansion room; too wide a gap and the deck becomes uncomfortable underfoot, more prone to catching narrow items (like chair legs or dropped items), and can look poorly finished.

Why joist spacing matters

Joist centres (the distance between the structural timbers the boards are screwed or clipped onto) determine how much unsupported span each board has to bear weight across. Space joists too far apart and boards can flex, bounce, or sag over time — this is a common cause of the warping and bowing covered in our guide to why decking warps. Composite boards generally need closer joist spacing than timber of a similar thickness, because composite can be more prone to long-term creep (gradual sagging under sustained load) than solid timber.

Common spacing mistakes

• Using timber joist spacing for composite boards. This is one of the most common installation errors and a major cause of premature sagging — always check the specific composite product's spec, don't assume timber rules apply.
• Inconsistent board gaps across the deck. Uneven spacing looks unprofessional and can create uneven drainage, with some gaps too tight to drain properly and others too wide.
• Ignoring the fall/slope requirement. Decking should be built with a very slight fall (often around 1:80 to 1:100) away from the house to help water run off — flat decking, even with correct board gaps, can still hold water on the surface in heavy rain.
• Not accounting for board expansion at the ends. Gaps are needed not just between boards along their length but also where boards meet a wall, post or perpendicular board — skipping this causes buckling as timber and especially composite expand seasonally.

Does board direction matter?

Boards are usually run perpendicular to the joists for the shortest, strongest span, though a herringbone, diagonal or other decorative pattern is possible with the right substructure design underneath — these patterns typically need additional noggins (cross-bracing between joists) to properly support boards running at an angle, which adds cost and complexity but can look striking. Discuss pattern preferences with your installer before the substructure is built, since retrofitting a different board direction afterward usually isn't practical.

Getting this right when planning new decking

See how much decking you need for general area planning, and if you're deciding between materials in the first place, our comparison of composite vs timber decking covers the practical differences beyond spacing alone. Getting spacing and joist centres right from the start is far cheaper than correcting it after installation — a deck built to the wrong spec for its material is genuinely at higher risk of the warping, sagging and premature wear covered elsewhere in this guide.

Common mistakes

• Assuming all composite products use the same spacing requirements — they don't, always check the specific brand's installation guide.
• Skipping the fall/slope for drainage because it's a small, easy-to-miss detail during construction.
• Under-speccing joist size or spacing to save on materials, only for it to cost more in repairs later.

Cantilever and overhang limits

Decking boards can overhang the outer joist slightly (often up to around 50mm) for a cleaner finished edge, but this overhang shouldn't be stretched too far — an excessive unsupported overhang is a common cause of edge boards feeling springy or eventually cracking under repeated weight near the edge, such as from furniture legs or foot traffic along the perimeter. Composite manufacturers are generally more specific about maximum permitted overhang than is typical for timber, since composite's flexural properties differ from solid wood — check the product's installation guide rather than assuming a "standard" figure applies.

Fixing methods and their effect on spacing

Timber decking is most commonly face-fixed with decking screws driven straight through the board into the joist, which gives the installer some flexibility to fine-tune gap consistency board by board. Composite decking, in contrast, typically uses a hidden clip system that locks into a groove along each board's edge — this generally produces a more visually consistent gap automatically, but leaves less room to adjust individual boards to correct for a slightly out-of-true joist, which is another reason getting the substructure genuinely square and level before boards go down matters more with composite than with timber.