· by Michael Blair, Nashville's Deck
Composite decking is usually sold on the boards: the color, the grain, no stain to put on every other year. What decides whether those boards stay flat and tight is the frame underneath. Composite spans less distance between supports than the thick wood boards many older decks were built with, so a frame that carried wood for twenty years can be wrong for composite on the first day. This is what the board makers say the frame needs, how running the boards on a diagonal changes it, and what we do with an existing frame before it gets composite decking.
What joist spacing do composite deck boards need?
Composite deck boards need joists no more than 16 inches apart, measured center to center, when the boards run straight across the joists. Trex’s guide to installing deck joists gives 16 inches on center as the general limit for composite in standard use, and TimberTech’s guide to deck joist spacing says residential spacing should never exceed 16 inches on center. Both point to closer spacing when a deck has to feel stiffer or carry more. TimberTech names 12 inches on center for a more rigid feel and as the standard for commercial decks, and Trex says joists carrying heavier loads or spanning farther may need 12 inches or less.
Twenty-four inches on center is a spacing for thicker wood boards, not composite. Trex’s guide describes the 24-inch allowance as one for thicker wooden decking and does not give it for composite. A board that spans too far between joists gives underfoot, and that bounce is the first thing owners notice. Neither maker treats its own numbers as the last word; both tell builders to check the local building code, and on a permitted deck we build to what the inspector and the board maker both require.
Does running the boards on a diagonal change the spacing?
Yes, it brings the joists closer together. A board laid at an angle crosses each joist on a slant, so the distance it spans from one support to the next is longer than the joist spacing itself. Trex calls for 12-inch spacing when the decking runs diagonally across the joists. TimberTech sets 12 inches on center as the maximum for a diagonal install and adds that sharper angles may need closer spacing still.
TimberTech publishes spacing by angle for its PRO and EDGE boards only, listed below; other lines and other brands have their own figures. A herringbone or picture-frame pattern, a border, or a change in board direction partway across the deck each needs its framing laid out before the first board is cut, and we draw that into the plan for every new deck we build.
- Boards at 90 degrees to the joists: 16 inches on center
- Boards at 30 degrees: 14 inches on center
- Boards at 45 degrees: 12 inches on center
Can composite go on an old wood deck’s frame?
Often it can, once the frame has been checked and brought up to what composite needs. The frame is the part of an older deck that actually fails, which we wrote about in what a deck inspection usually turns up, so before any new boards go down we pull a few old ones and look at the joists, the ledger where the deck meets the house, the posts and the footings. Joists that are soft, split or pulling loose get replaced, and a frame we would not build on today does not get new boards.
If the joists are sound but 24 inches apart, Trex describes the fix as a simple one: add one new joist between each pair of existing joists, which brings the spacing down to 12 inches. On a frame already at 16 inches, the spacing suits boards running straight across, and the work is in blocking and fastening. Either way the old decking comes off first and the composite is fastened to the joists. That resurfacing work is part of our deck repair service.
Blocking, board gaps and the rest of the frame
Blocking is short lengths of lumber fastened between the joists in rows. Trex says blocking is required by code to keep joists from rolling or twisting; it staggers the blocks, coats the cut ends with wood preservative and fastens each block with two screws. TimberTech calls for solid wood blocking in rows no more than 4 to 6 feet apart, fastened with exterior-grade screws, and the AZEK and TimberTech installation guide says limiting joist movement this way helps reduce excessive gapping between boards.
Composite grows and shrinks as the temperature changes, and a deck in full sun sees a wide swing between a July afternoon and the night after it. The same installation guide calls for a side-to-side gap of 1/8 to 1/4 inch between boards and two screws per board at each joist. A frame that twists opens and closes those gaps unevenly, which is why we treat blocking as part of the deck surface rather than spare lumber.
If you are still choosing between materials, our comparison of composite and wood decking covers the boards; this post is about what holds them. We build and resurface composite decks across Nashville and Davidson County and the towns around it. For a frame check and a written price on new composite, ask us for a free estimate.
Sources
- Trex, How to Install Deck Joists (Trex Academy how-to guide) — Joists generally no more than 16 in. on center for composite in standard use; 12 in. for diagonal decking; 12 in. or less for heavier loads or longer spans; 24 in. allowance is for thicker wood boards; add one joist between each existing pair on a 24 in. frame; blocking required by code to stop joists rolling or twisting, staggered, two screws per block.
- TimberTech, Deck Floor Joist Spacing 101 — Residential spacing should never exceed 16 in. on center; 12 in. for a more rigid feel and the commercial standard; 12 in. maximum on a diagonal; PRO and EDGE spacing by angle (16 in. at 90 degrees, 14 in. at 30, 12 in. at 45); solid blocking in rows every 4 to 6 ft.
- AZEK and TimberTech installation guide (manufacturer PDF, hosted by Lowe’s) — Joist spacing never over 16 in. on center, 12 in. diagonal; blocking every 4 to 6 ft helps reduce joist movement and excessive gapping; boards expand and contract with temperature; 1/8 to 1/4 in. side-to-side gap; two screws per joist.
FAQ
Joist Spacing for Composite Decking, Explained — questions
Is 24 inch joist spacing OK for composite decking?
Not for composite boards running straight across the joists. Trex describes 24 inches on center as an allowance for thicker wood decking and gives 16 inches for composite, and TimberTech says residential spacing should never pass 16 inches. On an existing 24-inch frame, adding a joist between each pair brings the spacing to 12 inches.
What is the standard deck joist spacing?
For composite decking, 16 inches on center is the usual maximum when the boards run straight across the joists, and 12 inches when they run diagonally. Twelve inches also gives a stiffer deck and is TimberTech’s standard for commercial work. Wider spacing belongs to thicker wood boards. Local code and the board maker’s guide both apply.
Do you need blocking between deck joists?
Yes. Trex says blocking is required by code to stop joists from rolling or twisting, and TimberTech calls for solid wood blocking in rows every 4 to 6 feet. Keeping the joists still also helps keep the gaps between composite boards even, because a frame that twists opens and closes those gaps as it moves.
What is the gap between composite deck boards?
The AZEK and TimberTech installation guide calls for 1/8 to 1/4 inch from side to side between boards, with two screws per board at each joist. Composite expands and contracts as the temperature changes, and the gap gives it room to move. Other brands publish their own figures, and their guide is the one that applies.
Does composite decking expand and contract?
It does. Composite boards change size as the temperature changes, which is why the makers specify a side-to-side gap between boards and two screws at every joist. A frame that is spaced, blocked and fastened to the maker’s guide keeps that movement in the gaps, instead of in boards that bow, pull loose or open unevenly.