Joist spacing for composite decking: why it is tighter than hardwood

Composite decking needs joists closer together than a hardwood deck, and the figure comes from the board maker, not from a span table for timber. Australian install guides give 450 mm between joist centres for a straight residential lay¹, or 400 mm for some ranges. Stairs, commercial decks and diagonal lays get a closer figure again. Timber can be framed wider. Timber Queensland's advice sheet on timber decks allows joists up to 500 mm apart under 19 mm hardwood boards and 650 mm apart under 25 mm boards⁸. A frame built to those figures has to change before composite goes on it.

How far apart are decking joists for composite boards?

The answer is whatever the guide for your boards says, because the figure changes with the board's thickness and profile. These are the figures in current Australian guides, so you know the range before you choose:

• Straight lay, residential: 450 mm centres is the common figure for a 23 mm board². One imported brand's Australian web page gives 400 mm as its maximum⁵, while its 2026 install guide lists 450 mm for one of its ranges and 400 mm for another⁶. The resin-board guide asks for 400 mm on any residential deck⁷.
• Stairs and landings: 400 mm¹, or 300 mm in the guide that groups steps and doorways with commercial use⁷. One guide spaces its stair stringers no more than 305 mm apart⁴.
• Commercial: 400 mm¹, 350 mm³ or 300 mm⁷ depending on the range.
• Boards at 45 degrees: take 100 mm off the straight figure³, or go to 300 mm residential and 240 mm commercial in the resin-board guide⁷.
• Boards at 30 degrees: halve the straight figure³.
• Thick marina boards: one maker allows 600 mm for a 32 mm solid board, with a note that heavy point loads can pull that back².

Every one of those is a maximum. Closer is always fine. The guides also want each board resting on at least three joists, so a short offcut spanning two joists near a wall is not supported enough⁷.

Why composite needs closer joists than hardwood

Composite is wood fibre bound in plastic, and under a load that stays in one place it keeps bending slowly. Engineers call that creep. One maker's engineering notes assume a creep factor of four¹. On those numbers the long-term sag under a heavy point load can reach four times the sag on the day, and the notes advise putting heavy loads over a support rather than mid-span. The same notes say the spacing is governed by how far one board deflects under a concentrated load. A heavy planter left in one spot through a summer is the case I'd plan for. Within one range, the thinner profiles also carry a tighter spacing than the thicker ones.

More joists also means more fixings along each board and a shorter free length between them for the board to lift or wave. I'd count that as part of the reason too.

What happens when the joists are too far apart

The first sign is bounce. One Whirlpool member, 5Oswald, laid a resin board over an old frame with joists at 450 mm centres when the maker recommended 400 mm, after checking with the maker that it should be fine⁹. In December 2016 they reported "a bit of bounce when you walk on it, but no big deal"⁹. Another member in the same thread, Typ0, described a hollow composite trial board in December 2018 that had "huge flex", and they ended up double-joisting the whole deck to halve the spacing they had built⁹. The extra timber was a cost they had not budgeted for, after the frame was built.

The lesson from both is that the spacing has to be decided before the frame goes in. A third member, thesandgirl, wrote in April 2020 that the extra cost and hassle of changing the joists for a board that needed 400 mm rather than 450 mm was enough to put them off that board⁹. Pick the board first, then order the joists.

Bounce is not always the joist spacing, though. A Bunnings Workshop member, banjo, built a deck in December 2023 with 240 x 45 joists at 450 mm centres and still felt spring underfoot¹⁰. Watching from underneath while their son jumped on the deck, banjo could see the joists flexing mid-span, and a bearer added under the middle of the longest joists fixed it. Spacing controls the board between joists. Joist size and bearer spacing control the joist itself, and a composite deck will show both.

Boards at 45 degrees, stairs and borders

A board laid diagonally crosses each joist at an angle. The length of board between one joist and the next is longer than on a straight lay, and the guides handle that by bringing the spacing down. The reduction is not small: 100 mm off in two guides³. Herringbone and other patterns need the framing adjusted to support the pattern.

Stair treads get a closer figure than the deck in each guide that gives one, and my guide to composite decking stairs covers the stringers. Where a picture-frame border runs parallel to the joists it needs blocking under it, which I cover in picture-frame borders on a composite deck.

How thick should deck joists be, and what timber?

Joist spacing is about the boards. Joist size is about the span between bearers, and that comes from span tables and whoever is signing off the frame. Timber Queensland's advice sheet on timber decks gives joist sizes for 450 mm centres and refers anything outside its tables to the residential timber framing standard, AS 1684⁸. The same sheet recommends joists 45 to 50 mm wide where the boards are fixed through the top⁸. One composite maker asks for 45 mm joists wherever top-fixed boards butt-join, so each end has its own timber to screw into². Treatment matters as much as size, and H3 or H4 treated pine under a composite deck covers which goes where. The treated pine framing page has the grades, and how to build a deck frame for composite decking puts the whole frame together.

On a steel or aluminium frame I'd keep the board maker's spacing. One imported brand gives the same maximum centres whether the frame is timber, steel or aluminium⁵. The fixing changes. One guide limits its metal screws to steel up to 2 mm thick and asks for every hole to be pre-drilled beyond that⁴. Steel vs timber deck frames covers the rest.

Do you screw deck boards to every joist?

Yes. Face-fixed boards take two screws at every joist they cross², and in a clip system each clip sits on its own joist. I wouldn't skip alternate joists to save fixings. It leaves a length of board free to lift, and the gaps between boards go uneven along that length. Where two board ends meet, each end needs its own joist, and the guides ask for a doubled or sister joist at every butt join.

Reusing an old timber-deck frame

A frame built for hardwood boards may be at wider centres than most composite guides allow. You can add a joist between each existing pair if the bearers are sound, but check the post bases and the bearer spans first. Check the level too, because one guide asks for a completely level frame before any board goes down⁴. Before you decide, read laying composite decking on an existing deck frame and how to install composite decking for the rest of the frame checks.

What I'd do

Choose the board and download its guide before any timber is ordered. Mark the joist centres on the bearers from one end and keep working from that end, so a small error does not drift across the deck. Use the guide's figure for a straight lay and the tighter figure at stairs and borders, and if the deck is on an angle, frame it for the angle from the start. Keep heavy planters, and anything else that will sit in one place for months, over a joist. The joist calculator will count joists at the centres you enter. The plan it draws is a concept plan, so take it to a builder or certifier before you build.