Composite Decking Joist Spacing

Building a deck includes the substructure of support posts, beams, and joists that most take for granted when standing or sitting on a deck. The choice of wood or composite decking and design or layout can affect the placement of joists. So, if you’re wondering if composite deck joist spacing is the same or not, we’re here to help!

The spacing between joists affects deck strength and is determined by the joist span and the direction the decking intersects the joists. Space joists at 16” OC if the decking intersects at 90°, 14” OC if intersecting at 60°,12” OC if it intersects at 45°, and 8” OC for decking intersecting at 30°.

In this guide, we’ll explore the necessity of joists, the type of joists to use, and their spacing when using composite decking. We’ll also discuss the maximum and minimum joist spacing for Trex decking, framing options, composite decking sag, spacing between deck boards, and if joists need to be perfectly level with each other. Our goal is to provide the information you need for your composite decking project.

Do You Need Joists for Composite Decking?

Composite decking is a blend of plastic and wood fibers but the term is often applied to any manufactured decking, including those made of 100% plastic. The wood-plastic blend, also known as WPC (wood-plastic composite) decking, is typically heavier than solid wood of the same dimensions, is more flexible, and is not as strong. The weight and lack of strength can affect the size and spacing of dimensional lumber used for the deck substructure.

Composite decking can be used to resurface an existing deck after the old decking is removed. They can be placed on a concrete pad but should have 1×4, 2×2, or 2x4s on the flat as ‘joists’ that are fastened directly to the concrete.

The deck boards are then fastened to the ‘joists’ which allow airflow and help level the decking and prevent moisture damage. Properly spaced joists provide a level support framework for the composite boards, otherwise, the boards will flex, bend, sag, or break.

What Joists Do You Use for Composite Decking?

The decision to use composite decking typically has little impact on substructure material choice. As with wood, composite decking requires a solid, level base. It can even be used to replace existing wooden decking without changing the joists, beams, or posts. Composite decking is often fastened to softwood or hardwood joists in most residential settings. However, there are composite joists that are becoming more common.

Composite joists are manufactured from a blend of virgin and recycled plastics and wood fiber, similar to composite decking. They are durable and more moisture, insects, mildew, and rot resistant than pressure-treated lumber, and can be used near the ground or for elevated platforms. It is even UV treated. Composite joists, unlike wooden joists, don’t require moisture shielding between them and the deck boards, so there is no additional work or costs once installed.

Joist Spacing for Composite Decking

Deck material and its thickness commonly determine the spacing of the joists. The decision to use composite decking can affect the joist spacing, but much depends on whether the decking will be perpendicular or diagonal to the joists. Additionally, the International Residential Building Code (IRC), which is the foundation for most local building codes, can also impact joist spacing. So, always check with your local building department.

To ensure the decking selected can support the allowable loads and spans as identified by the IRC – 2021, check that the packaging or decking label indicates compliance with ASTM D7032. All composite wood decking, stair treads, railing posts, and railings that contain a wood-plastic blend must comply with ASTM D7032 requirements.

Typically, joists are spaced 12”, 16”, and 24” on center (OC) apart. On center means the joist is centered at 12”, 16”, or 24”, so the spacing between joists is actually less. The closer the joists are together, the sturdier the deck. Additionally, the spacing between joists affects the distance the joists can span without support. In some situations, the spacing can be less, but if it is too narrow, it may be difficult to install blocking, joist hangers, or flashing.

Pro Note: The general rule of thumb is to space joists at 16” OC if the decking intersects at 90°, 14” OC if the intersection is at 60°, 12” OC if the boards intersect at 45°, and 8” OC if they cross at 30°.

12” on center

Joists placed at 12” centers mean they can span further and that decking running perpendicular to the joists spans only 10-1/2” without support. The narrower span makes for a more solid, stable, and firm feeling deck. Typically, 12” OC is only required for residential decks if laying 1” or 5/4” boards, whether solid wood or composite, 45° diagonally to the joists.

Spacing joists at 12” OC adds an additional joist every 4-foot section over the number required for 16” OC spacing. The additional cost of joists, fasteners, extra weight on the beam, extra leveling, and labor, is often heralded as the reasons to use joists at 16” OC for decking run perpendicular. There are other reasons 12” OC may be necessary, so if the drawings or local codes call for it, go with it.

16” on center

Joist spacing at 16” OC is commonly used for 1” and 5/4” thick decking laid perpendicular (90°) to the joists. Most major composite deck board manufacturers, such as Trex, TimberTech (AZEK), Dura-Life, Envision, and Fiberon, recommend 16” OC for their 1” or 5/4” decking. Joists at 16” OC are also used when diagonally laying 2” thick decking 45° to the joists, or some reinforced 1” or 5/4” composite or synthetic decking brands, like WearDecking.

24 inches

Placing joists at 24” OC means there are 3 supports every 4’, and deck boards 90° to the joists must span 22-1/2” without support. The distance can result in a springy, spongey feel, and cause boards to develop a mid-span sag, which is why it isn’t acceptable for most 1” and 5/4” wood or composite decking.

If using nominal 2” thick solid wood or plastic-wood composite decking, the 24” OC spacing is acceptable as the greater thickness counters the flex felt in the thinner boards. However, there are 1” and 5/4” synthetic or 100% PVC decking, like WearDecking Fiberglass which is reinforced on the top and bottom faces with fiberglass, that can span 24” OC.

What Is the Minimum and Maximum Joist Spacing for Trex Decking?

When selecting composite decking, the spacing between joists can vary from manufacturer to manufacturer. It can also vary with individual product lines within a Brand, much like it can for different wood species. Most manufacturers list maximum spacing, not minimum. However, if using short lengths, they must span a minimum of 3 joists to prevent flexing and stress on the fasteners. Additionally, decking supporting heavy loads like a hot tub, outdoor kitchen, and planters typically require narrower spacing between joists or even joists of greater depth.

According to the Trex installation guide, all lines of nominally 1” thick Trex decking can span joists at 16” OC if the decking intersects at 90°. The joist spacing changes to 14” OC if the intersection is at 60°, 12” OC if the boards cross the joists at 45°, and 8” OC, if they intersect at 30°.

For nominally 2” thick decking, the spacing of the joists is 24” OC if the decking intersects at 90°, 22” OC if the intersection is at 60°, 16” OC if the boards cross at 45° and 12” OC if they intersect at 30°.

Deck Framing Options for Trex

Trex composite decking can be installed on KDAT (kiln dried after treatment) lumber, wet/green pressure treated lumber, steel or aluminum framing, or composite joists. Remember to check with your local building department to ensure compliance with the codes based on material choices. Plus, always follow the manufacturer’s directions for installation and use appropriate fasteners.

Wet or green lumber is heavy with moisture and will shrink, check, and twist as it dries. KDAT has been control dried in a kiln after treatment so the moisture content is 19% or less, making it lighter and less likely to warp, cup, or twist.

Metal joists offer greater stability but are more expensive than wood, and won’t rot. Composite joists are made of similar materials to composite decking, so won’t rot, but like metal, they are more expensive than wood.

Does Composite Decking Sag?

Composite decking will sag over time due to gravity if the joist spacing is greater than recommended. They will also sag or buckle due to thermal expansion, which typically is caused by improper installation and spacing of joists or deck boards. Additionally, darker color decking can attract more heat, which can contribute to sagging or buckling.

Do Deck Joists Need to Be Perfectly Level?

Typically, there should be a slight slope from one edge of the deck to the opposing edge for drainage. If the slope runs in the direction of the decking, the joists should have a 1/8” slope. If the decking is perpendicular to the slope, the joists require a 1/4” slope or more to drain water off the boards.

Due to moisture content or milling, dimensional lumber used for joists or beams is not always the same exact dimensions. There will commonly be differences from one joist to another. Some may be crowned more, others will have different moisture content, so once the joists are in place and crowns are up, use a string line to check the elevation consistency across the joists.

The joist framework should be blocked and as flat as possible across the joists, not necessarily level. Use a string line stretched across the joists to identify any that are higher or lower than their neighbors. Those that are too low can be shimmed up, and those too high can be notched or planed, or the beam may be notched (within code allowances). Composite decking isn’t as rigid as wood, so uneven joists will transfer elevation differences more, making the deck look wavy and causing trip or stubbing hazards.

What Is the Spacing Between Trex Deck Boards?

The side-to-side gap and end-to-end gap between Trex deck boards may vary depending on joist composition and temperature. When using either wood deck joists or wood sleeper joists, leave a 1/4″ (6mm) gap between any structure and the starting board if the temperature is above 40°F, and 1/2″ (13mm) if it’s cooler.

The side-to-side gap should be 3/16” regardless of the temperature, and the end-to-end gap 1/8” when warmer than 40°F and 3/16°F if cooler. For steel joists, the gap between is 3/16” (5mm) and the end-to-end 1/8” (3mm) if the temperature is warmer than 40°F (4.5°C) and 3/16” if cooler. Using the recommended hidden fasteners helps set and maintain the recommended side-to-side gap too.

Conclusion

Joist spacing for wood-plastic composite decking is important as it helps prevent sag or bounce for a more solid, stable deck. The spacing is typically the same for composite decking as for that of solid wood decking of the same thickness. For nominally 1” or 5/4” thick decking, joists are spaced at 16” OC if perpendicular (90°), 14” OC if angled at 60°, 12” OC spacing when angled at 45°, and 8” OC if at a 30° angle.

If using nominally 2” thick composite decking, the joist spacing increases to 24” OC when perpendicular, 22” OC for 60° angles, 16” OC for 45°, and 12” OC when laid 30° to the joists. However, it is best to check the manufacturer’s label or literature for their recommended spacing and consult the local building department, as some areas require different spacing. Hopefully, we’ve provided you with the information you need for your composite decking project.