Composite decking joist spacing and board spans

Most composite boards call for joists at 16 in. on center, with 12 in. or less for diagonal layouts, stairs, and commercial loads.

For most 1 in. composite deck boards laid square to the joists on a residential deck, the published maximum is 16 in. on center. Diagonal layouts, stair treads, and commercial loads all push that number down to 12 in. or less, and a few thicker 2×6 profiles can go to 24 in. The rule that overrides everything here is simple: follow the current installation guide for the exact board you are buying, because spans differ by brand, profile, and angle.

Most composite decking calls for joists at 16 inches on center.

Here is what the manufacturers publish for boards laid at 90 degrees to the joists, along with their angled and stair figures. All numbers are maximum center-to-center spacing.

Envision also notes that if you want a stiffer feel underfoot, you can reduce joist spacing by 4 in. TimberTech says the same thing in different words: 16 in. is the limit, and 12 in. gives a more rigid deck. Every manufacturer above requires each board to span at least three joists.

Diagonal boards need tighter joist spacing.

When a board crosses the joists at an angle, the distance it travels between supports gets longer even though the joists have not moved. That is why every guide shortens the allowable spacing for angled layouts.

• Trex reduces the chart value by 2 in. at 60 degrees, by 4 in. at 45 degrees, and cuts it in half at 30 degrees.
• TimberTech lists 14 in. at 30 degrees and 12 in. at 45 degrees for PRO and EDGE boards, and says steeper angles may need closer spacing.
• Envision lists 12 in. for 1×6 boards and 16 in. for the 2×6 Evergrain board at any angle up to 45 degrees.

Picture-frame borders and inlays also need extra framing. Plan blocking wherever a border board or a change in board direction lands, and check the decking line’s guide for where it wants that support.

Stairs are where spans get tightest.

Stair treads carry concentrated foot traffic over short spans, so manufacturers publish their own stringer spacing. Trex calls for 12 in. stringer spacing for Transcend and Signature 1×6 treads and 9 in. for Select 1×6. Envision lists 12 in. for its 1×6 boards and 8 in. for Ridge Premium. Trex also asks for stairs at least 36 in. wide, four stringers at 36 in., and five stringers on anything wider. If you are mixing tread and deck products, frame the stairs to the tightest number of the boards you are using.

Commercial and heavy loads usually mean 12 inches or less.

Trex’s span chart is tied to load. For 1 in. boards it lists 16 in. at a 100 psf load and 12 in. at a 200 psf load, the column covering commercial decks, boardwalks, and marinas. TimberTech treats 12 in. as the standard for commercial projects. For hot tubs, heavy planters, or outdoor kitchens on a deck, Trex tells builders to consult a local building engineer or inspector, and that is good advice for any brand.

On the code side, IRC R507.2.2 requires plastic composite deck boards and stair treads to comply with ASTM D7032, carry a label showing their allowable load and maximum span, and be installed per the code and the manufacturer’s instructions. In other words, the code sends you back to the manufacturer’s numbers. Confirm the requirements with your local building department, since local amendments vary.

Good framing makes the spacing work.

Correct spacing on a crooked or twisting frame still produces a wavy deck. These are the framing steps the manufacturers call out:

1. Keep joists level and in plane. Trex and TimberTech both require a flat, level frame before boards go down.
2. Add rows of blocking. TimberTech recommends solid wood blocking in rows no more than 4 to 6 ft apart. Trex fastens through the rim into each block with 3 in. framing screws and keeps the top of each block flush with the joist.
3. Support every butt joint. Envision calls for a sister joist at each butt joint. Trex allows a butt joint clip over a single joist or a 2×4 nailer beside the joist.
4. Protect the wood. Joist tape on the tops of joists, rims, and ledgers helps keep water out of fastener holes. Painting joist tops black hides them through the board gaps.
5. Gap boards correctly. Envision calls for at least 1/8 in. side to side. Trex calls for at least 3/16 in. and never more than 1/2 in.

Leave room for air under the deck.

Composite boards perform best when moisture can leave from below. Envision says its decking must never be installed directly on solid surfaces such as concrete or membranes, or anywhere drainage or ventilation is inadequate. Trex says all decks need air circulation and suggests opening the space under the deck or widening board gaps to 3/8 in. to improve airflow. For low sleeper systems, Trex allows a minimum height of 1-1/2 in. and recommends 3-1/2 in. where debris can build up. On low, ground-level decks along the Gulf Coast, plan for that clearance early, because it is hard to add later.

Steel framing is an option for long spans and coastal sites.

If wood framing has warped on a past project, or the deck is high with long spans, steel is worth pricing. Fortress Evolution steel framing uses 2×6 joists with a ledger, rim joist, and blocking pre-spaced at 12 in. and 16 in. on center, so the frame matches common decking spans. Fortress lists a 25-year limited warranty, a powder-coated finish, a Class A fire rating (WUI), and compatibility with wood, composite, and PVC decking. Trex notes that on metal framing you should follow the framing maker’s double-joist and end-span requirements and use two screws per joist. As with wood, follow Fortress’s current installation guide and span tables.

Frequently asked questions.

Can I put composite decking on 24 in. joists?

Only with a board rated for it, such as the Envision Evergrain 2×6 or the Trex 2×6 at 24 in. Standard 1 in. boards need 16 in. or less. On an old wood deck framed at 24 in., Trex suggests adding a joist between each existing joist.

Is 12 in. spacing worth it on a home deck?

It costs more lumber and labor, but it gives a stiffer feel and is required for most diagonal layouts. It is also the commercial standard for TimberTech and the 200 psf figure in Trex’s chart.

Does the angle rule apply to stairs?

No. Stairs have their own stringer spacing, which is often tighter than the deck spacing. Use the stair figure from the tread manufacturer’s guide.

Do you supply framing as well as decking?

Yes. We supply decking, railing, and steel framing, and we can put the whole package on one quote.

Send us your deck dimensions, framing plan, board choice, and layout direction, and we will build a takeoff around the correct spacing. Request a quote, or browse our decking and Fortress lines.