Joist spacing for composite decking. By brand. By install pattern.

No. 24" o.c. is acceptable for some pressure-treated 5/4 wood decking but exceeds the maximum span for every major composite brand. Composite boards rely on a polymer matrix that deflects further between supports than continuous wood fibres do — at 24" o.c. you'll feel a noticeable bounce, and the manufacturer's warranty becomes void because you're outside the install spec. The maximum is 16" o.c. perpendicular for residential applications across Trex, Fiberon, Deckorators, and Eva-Last.

Geometry. When a deck board runs perpendicular to its joists, the unsupported span between joists is exactly the joist on-center spacing — 16 inches. When the board is rotated 45° (diagonal), the effective unsupported span increases by √2 (about 1.41×), so a 16" o.c. joist field becomes a 22.6" effective span under the diagonal board — outside every composite brand's load tolerance. Tightening to 12" o.c. brings the effective diagonal span back to ~17", inside the 16" baseline.

12 inches on-center, every brand. Trex, Fiberon, Eva-Last, and Deckorators all specify 12" o.c. or tighter for stair stringer spacing under composite stair treads. Some installations go as tight as 9" o.c. for a more rigid feel underfoot. Stair treads see concentrated point loads (a foot landing on a single board) that demand tighter support than a main deck field. Stair tread overhang is typically capped at 3/4 inch past the riser.

Yes, if the boards are running perpendicular to the joists and the framing is structurally sound. Three checks first: (1) joists are straight and crowned the same direction; (2) no rot, insect damage, or split joists — you have to trust the frame; (3) tape every joist top with G-Tape before laying boards so the new screw holes don't channel water into the existing wood. If you want diagonal install on the new boards, you'll need to add sister joists to bring spacing down to 12" o.c. before you start. Any brand TUDS stocks works at 16" o.c. perpendicular: Trex, Fiberon, Deckorators, Eva-Last.

Two separate rules apply: (1) Board cantilever (decking past the rim joist): typically capped at ½ inch to 4 inches depending on the specific composite product line — check the install guide for your board. (2) Joist cantilever (joist extending past the support beam): governed by Saskatchewan code and lumber spec, generally capped at one-quarter to one-third of the back span. The City of Saskatoon caps cantilever at 25% of the total span. A 12-foot back span allows up to 3 feet of cantilever for code, but typical residential builds keep it to 16-24 inches.

Eva-Last writes the spec in millimetres (the company is South African and ships globally) — 450mm centre-to-centre maximum for the 140mm Apex PLUS, Infinity IS, and Pioneer profiles. That converts to roughly 17.7 inches, which means North American framers using imperial 16" o.c. lumber stay comfortably inside Eva-Last spec. Trex writes 16 inch o.c. flat across all five lines. The functional rule for both brands is the same — frame at 16" o.c. perpendicular and you're inside spec. Eva-Last's metric spec gives the same answer as Trex's imperial spec.

Saskatchewan follows the National Building Code with provincial amendments. For residential decks: joist spacing is governed by joist size + species + span, not a fixed o.c. rule. A 2x8 SPF joist at 16" o.c. typically spans up to 12 feet between supports; at 12" o.c. up to 13–14 feet. The City of Saskatoon also requires footings to extend below 1.2 metre / 4 foot frost line, and caps cantilever at 25% of total span. Composite manufacturer spec (16" perpendicular / 12" diagonal / 12" stair) is more restrictive than code on the o.c. dimension — your frame has to satisfy both. Always pull a permit for a deck attached to a residence in Saskatoon, Regina, and most Saskatchewan municipalities. [VERIFY current Saskatoon and Regina deck construction guides at time of build.]