Gap width between decking boards: getting it right

Why a gap is needed at all

A deck without gaps holds water. Rain sits on the surface instead of draining through, and wood that stays wet is wood that rots, greys unevenly, and grows algae in the shade. The gap is what lets water fall through to the ground or the substructure below rather than pooling on top of the boards.

The second reason is movement. Wood expands when it takes on moisture and shrinks as it dries out, and it does this across its width more than its length. A board laid tight against its neighbour in spring will have nowhere to go when it swells in a wet autumn, and boards under that kind of pressure cup, bow, or lift their fixings.

The gap also matters for the everyday business of a terrace: leaves, sand, and grit need somewhere to fall through rather than sitting in the joints and holding damp against the wood from above as well as below.

What gap to use for solid timber boards

For pine, spruce or thermally treated timber, a gap of 5 to 8 millimetres is the usual range. Where you land in that range depends on how dry the timber is when you lay it and what time of year it is.

Timber bought in summer, especially if it has been stored outdoors or under a roof rather than kiln-dried, tends to be relatively dry and will swell once it is exposed to autumn and winter weather. In that case lean towards the wider end, 7 to 8 millimetres, to leave room for that swelling.

Laying in fresh, damper timber in autumn or spring is the opposite case: the board is closer to the moisture it will settle at, so 5 to 6 millimetres is usually enough. Thermally treated timber moves less than untreated pine or spruce, so it can sit at the narrower end of the range regardless of season.

Composite boards follow their own logic

Composite decking, the wood-plastic mix sold under various brand names, moves with temperature rather than moisture. It expands in summer heat and contracts in cold, and that movement is driven by air temperature more than rainfall.

Most composite manufacturers specify a gap for their own product, typically in the region of 4 to 6 millimetres, and it is worth checking the installation sheet that comes with the boards rather than assuming it matches timber. Composite also needs a consistent gap at the ends of boards, not just along the sides, because end-grain expansion is where composite decking tends to show problems first.

If you are mixing composite decking with a timber substructure, remember that the substructure itself still moves with moisture even if the boards on top do not, so the fixings need a little tolerance too.

Keeping the gap even across the whole terrace

A gap that is right in principle but uneven in practice looks worse than a slightly wrong gap that is consistent. The eye catches unevenness in a joint line far faster than it catches the actual millimetre count.

The reliable way to do this is with spacers: small offcuts of wood, plastic decking spacers bought for the purpose, or even old nails or screws of the right diameter laid flat between boards as you fix each one down. Set the spacer, push the next board against it, fix it, then move the spacer along.

Check square regularly against the joists as you go, not just at the start, since a small error repeated across twenty boards becomes a visible taper by the end of the terrace.

• Use two spacers per board, one near each end, not one in the middle
• Match the spacer thickness to timber type and season, not a single number used throughout
• Recheck the gap every four or five boards against a straightedge or the house wall
• Keep spacers dry and clean of resin so they release easily
• For composite, use the manufacturer's stated gap rather than a general timber rule

The mistake most people make doing it themselves

The most common error is laying boards tight, with no gap or a barely visible one, because it looks neatest on the day. It rarely stays looking neat. Within a season the boards swell against each other, and since they have nowhere else to go, they push upward and cup, or force fixings to work loose.

The second common error runs the other way: gaps that drift wider or narrower along the terrace because spacers were used loosely, by eye, rather than consistently. Both mistakes come from treating the gap as a finishing detail instead of a structural one that needs deciding before the first board goes down.

When to stop and call someone

Spacing boards correctly is genuinely something most people can do themselves; it is a matter of patience and a consistent spacer, not special skill. Where it gets harder is on larger or irregularly shaped terraces, where getting the gap right while also keeping the whole layout square and level takes more planning.

If the substructure itself needs work, if boards have already cupped from being laid too tight, or if you would simply rather not spend a weekend on it, our decking and terrace work starts from 90 € per job, with a fixed quote given once we have seen the site or photos. Smaller fixes, like re-laying a section that has gone wrong, are charged as odd jobs from 39 € per hour.