Yes, composite decking can expand and contract in response to temperature changes like most solids, but it’s very slight. A materials scientist would say that composite decking has a low coefficient of thermal expansion.
While I’m no scientist, I can assure you that this physical property of composite decking is well-understood and easy to manage during deck building. In fact, this trait fits perfectly with the other outstanding properties of quality composite decking, including board-to-board consistency, low maintenance, and superior durability. Plus, composite decking is basically immune to moisture-related expansion and contraction, unlike traditional wood.
Read on, and I’ll tell you how much composite decking expansion and contraction to expect and how to use spacing between the deck boards to accommodate it.
What Causes Composite Decking to Expand or Contract?
Temperature changes cause expansion and contraction in most materials. This is due to an increase or decrease in molecular motion. In some materials, such as wood, water can also affect a board’s dimensions.
Causes of Expansion
Heat: As the temperature rises, the molecules in composite decking—or most any substance—become agitated and bounce against each other, expanding the space they occupy. This causes the deck board to “grow” slightly.
Excess moisture: Moisture doesn’t affect composite decking much at all, especially when it includes a PVC shell, or cap, which is impervious to water. As a result, composite decks easily withstand rain, snow, and changes in humidity. Wooden decking is porous and absorbs moisture, making it expand.
Causes of Contraction
Cold: When the temperature drops, composite decking cools and shrinks slightly. Again, the shrinkage is minimal.
Low moisture: Lack of moisture has essentially no effect on composite decking, but natural wood releases moisture easily in a dry environment. The resulting shrinkage of the wood fibers can cause the boards to splinter.
How Much Composite Decking Boards Expand
Different brands and product lines of composite decking have different expansion and contraction responses. Most are rated at 0.1% to 0.3% of the length of the boards. (The width of the board also expands, but it’s accommodated by standard longitudinal drainage spacing.)
To account for end-to-end expansion and contraction, manufacturers of composite boards recommend that you leave a gap between all butt joints and miter joints. The size of the recommended gap varies depending on the temperature at decking installation. Trex, for example, calls for a 1/8″ gap in butt joints of its Signature line of decking when it’s installed at 40°F or higher and a 3/16″ gap below 40°F.
Most decks using wood-plastic composites (WPC), like a Trex deck, will require similar gapping. Refer to the manufacturer’s specifications and guidelines. Mineral-based composite decking boards expand and contract less than WPC boards and can be installed with few, if any, expansion gaps.
Another way to minimize butt joints and the required gaps is to use the longest available boards, usually 16 or 20 feet. Deck designs that use perpendicular transition boards, also known as breaker boards, and picture framing can also minimize the need for end-to-end joints and gaps. That reduces the decking area where dirt and debris can accumulate, and with less debris buildup, there’s less chance of mold and mildew growth.
Because composites absorb so little moisture, they’re ideal for use around pools and hot tubs. Even so, the pressure-treated wood framing below the deck can expand and contract with moisture. I recommend applying flashing tape over the joists to protect your deck substructure from water and moisture.
Is the Expansion and Contraction of Composite Decking a Concern?
Homeowners should be aware of how composite decking performs, but don’t let it keep you from enjoying your outdoor space. Just make sure that your DIY decking project accounts for it. Or ask your deck builder how they plan to handle it.
How to Correctly Space Decking Boards
Most decks are built with side-to-side spacing of the boards to ensure proper drainage and airflow and to allow tree litter, dirt, and debris to pass through instead of accumulating on the deck. If you use hidden fasteners, this side-to-side spacing is set for you.
But when board ends meet in a butt or miter joint, you must leave appropriate expansion gaps to handle temperature fluctuations. If not given enough spacing during installation, the boards might make contact when it gets hot and expand, loosening their fasteners and warping the boards. Plus, to ensure warranty coverage, you must follow the manufacturer’s installation process to the letter.
Here’s how to get the spacing right:
1. Measure correctly
Accurate measurements and precise cuts are essential. For best results, measure the full length of the space where the board is to go, and then subtract the amount of spacing you want to leave.
2. Use a jig
Using a tape measure to set or verify board spacing is time-consuming and inaccurate. Instead, use the world’s simplest jig, a gauge or spacer. You can buy different sized gauges, or you can use a nail, a coin, pencil, or almost any solid object of the correct thickness. Use different thicknesses to vary the spacing as the situation requires.
3. Factor in the weather
Board length grows in hot weather and shrinks in cold weather. Thus, in cold weather, you might need to space the joints looser, while in high heat, the spacing could be a little tighter. Even in fair weather conditions, direct sun can cause the boards to absorb more heat than you realize.
Remember to factor in expansion/contraction spacing not only when installing composite decking but also when you add composite fascia and railings, as well as built-in features, such as benches and pergolas. Composite cladding (siding) should also be spaced per the manufacturer’s installation guide.
How Does Composite Decking Compare to Other Decking Materials?
In addition to its thermal and moisture resilience, composite decking materials rank favorably in many other ways compared to wooden boards, PVC decking, and aluminum. For example, they are more durable, need less upkeep, and exhibit exceptional board-to-board consistency compared to wood.
