Joist spacing for composite decking

The short answer, brand by brand

For a house deck with the boards running square across the joists, every composite and PVC maker we could read allows 16" on centre. Turn the boards to an angle and every guide tightens that, to 12" for the common 1-inch boards. Stairs and commercial decks tighten again, and that is where the brands stop agreeing.

Read from each maker's installation guide on 18 October 2026. Brand guides change and product lines get renamed; the guide for the exact board you buy is the one that counts.

Why composite has its own rule

The code's spacing table, IRC 2021 R507.7, is for wood decking only. For plastic composite boards the same section points elsewhere: the spacing comes from R507.2, which requires each board or its packaging to carry a label showing its allowable load and maximum span, tested to ASTM D7032. And R507.2.2.5 says composite boards, treads and guards are installed to the code and the manufacturer's instructions.

So for composite the install guide is not advice. It is the document the code defers to, and the one an inspector can ask to see. For the wood side of the question — what 12, 16 and 24 inch centres cost and buy you — read deck joist spacing. This page stays with what the board makers require.

Diagonal and angled boards

The reason is geometry, not caution. A board laid square to the joists bridges exactly the joist spacing. Lay it at 45° and it crosses each bay on the slant, bridging about 1.41 times as far: 16" centres become roughly 22.6" of unsupported board, and 12" centres bring it back to about 17".

Trex states it as a rule by angle, measured between board and joist. At 60° the maximum spacing is 2" less than its chart, at 45° it is 4" less, and at 30° it is half. On a board rated 16" that is 12" at 45° and 8" at 30° — and at 30° a board crosses a bay at twice the spacing, so halving it puts the unsupported length back where it started.

The others give one number for any angle: TimberTech 12" for all angled installations, adding that more severe angles may need closer spacing; Fiberon 12" for diagonals up to 45°; MoistureShield 12" for its 1-inch planks at an angle. A herringbone is an angled layout too, and Trex is plain that any special pattern means adjusting the framing to support it.

Stairs: where the brands disagree

The code's wood table excludes stairways outright, so stair stringer spacing for composite treads is entirely the maker's call. They do not agree. Trex wants its 1×6 boards on 12" stringers and Enhance on 9". TimberTech's composite chart runs from 8" to 10" depending on the line. Fiberon's composite collections want 12" on a house and 8" on a commercial job, while its PVC boards go to 24" — but only with a treated 2×4 set on edge under the middle of every tread. MoistureShield asks for at least five stringers on any stair.

A few more rules ride along. Fiberon does not approve grooved boards as treads; use square-edge boards. TimberTech wants a riser board on every step, a tread at least two full boards deep, and a stringer in the middle of the stair's width. Lay the flight out with the deck stair calculator, then set the stringer count from your board's guide, not from the width alone.

Commercial decks and heavy loads

TimberTech and Fiberon both set commercial work at 12" square, and Fiberon at 8" for diagonals. Trex works from load ratings instead. Its guide notes that the building codes call for a 40 psf rating on most walking surfaces and that some commercial work needs 100 psf or more — then publishes what each board earns at each spacing:

Read it carefully. Some Trex boards carry a rating at 24", and Trex still recommends 16" for a stiffer surface. A hot tub or a big planter is a load no decking chart covers: Trex sends you to a building engineer or inspector, Fiberon to the hot tub maker's own framing requirements, and the joists under it are sized separately with the joist span calculator.

Picture frames, breaker boards and butt joints

A border board running around the edge sits at right angles to the field boards along two sides, so on those sides it runs along the joists, not across them. TimberTech's drawing for a picture-frame deck is 16" centres plus additional support under the frame itself (12" for commercial). Trex draws breaker boards — a cross board that stops the field boards mid-run — framed on a single joist and on a doubled one.

Board ends are the other place framing matters. Trex says an end must always rest on a framing member, with no more than ¾" of overhang. TimberTech's PVC guide caps overhang at ½" past any framing or fascia, requires staggered joints so they never all break on the same joist, and requires a doubled joist or secure double blocking at butt joints when its CONCEALoc clips are used. Fiberon recommends 2× blocking at butt joints wherever temperature swings are large. MoistureShield allows 1½" past the rim for a solid-bottom board and only ¼" for a scalloped one.

Where the joints go is a layout decision you make before the frame is built — the patterns are in how to stagger deck boards.

Blocking, crown and a flat plane

Composite and PVC do not hide a bad frame the way a thick wood board can. TimberTech says its boards conform to the contour of the joists they are fastened to, and tells you to run a string line across the joist tops as they go in, shim or plane to bring them into one plane, and set aside any badly crowned stock. MoistureShield says the same of new and existing joists alike: the boards follow any contour left in the frame. Trex asks for joists level and plumb, and for every board to span at least three joists.

One condition tightens spacing well below any table. TimberTech asks for joists at 6" to 8" on centre, and top-down fasteners only, for PVC boards within 6' of Low-E doors and windows that come down to within 30" of the deck — reflected sun heats the boards there.

Fascia and risers are not framing

Fiberon and MoistureShield both state that fascia is a non-structural cover for the rim joist and stringers, never a walking surface. It screws to the rim, so joist spacing does not govern it; the guides set their own fastener pattern instead. Fiberon: three screws down the face at most 12" apart (16" with its dedicated fascia screws), risers fastened at every stringer with stringers no more than 12" apart. Trex: two fascia screws on a 2×8 rim and three on a 2×10 or deeper, the outer ones 1" from the rim's edges; deck boards used as fascia get two screws every 12" plus construction adhesive. MoistureShield: three screws, 12" to 16" apart.

Two makers also say where it sits. MoistureShield puts fascia under the board overhang, never flush with the deck surface, and Fiberon runs the perimeter board over the top of the fascia so ice cannot build up behind it in cold climates. The deck square footage calculator gives the exposed edge of any deck shape, which is the fascia run to order.

Before you frame

Seven questions to answer on paper first. Open each for the detail.

Then size the joists themselves for the span and the spacing you settled on, with the deck joist span chart, and compare what the board choice does to the budget in composite decking vs wood.