Does Composite Decking Shrink and Expand?
Yes. All composite decking expands when heated and contracts when cooled. This is called thermal expansion, and it is completely normal.
The amount of movement is small. Over a 16 foot board, composite decking typically expands and contracts by about 3/16 of an inch (roughly 5 mm) across a full seasonal temperature range. That is significantly less than natural wood, which can move 6% to 8% across its width.
However, even a small movement causes problems if you do not plan for it. Boards installed too tightly will buckle in summer heat. Boards installed with too much gap will leave unsightly openings in winter. Getting the spacing right is the most important part of a successful composite wood decking installation.
Table of Contents
How Composite Decking Moves Differently Than Wood
Understanding the direction of movement helps you plan your installation correctly.
- Composite decking expands and contracts primarily along its length. Width changes are minimal. This means the ends of the boards move the most.
- Natural wood behaves the opposite way. Wood expands mostly across its width (6% to 8%) and very little along its length (about 2%). This is why wood decking often develops gaps between boards over time, while composite decking develops gaps at the butt joints.
Knowing this difference is essential. If you have experience with wood deck installation, do not assume the same spacing rules apply to composite. The gap priorities are reversed.
How Much Gap to Leave: Spacing Guide
Three types of gaps matter when installing composite decking. Each serves a different purpose.
End-to-End Gaps (Butt Joints)
This is the gap between the ends of two boards that meet in a line. It is the most important gap because the composite expands the most along its length.
Standard recommendation: Leave 1/4 inch (6 mm) between board ends when installing at moderate temperatures (15 to 25°C / 60 to 77°F).
Side-to-Side Gaps (Between Boards)
This is the gap between the long edges of adjacent boards. It allows water drainage and airflow, which prevents mold and debris buildup.
Standard recommendation: Leave 5 to 8 mm (about 1/4 inch) between board edges.
Wall and Obstruction Gaps
Where boards meet a wall, post, or fixed structure, extra clearance is needed because the board cannot push past the obstruction.
Standard recommendation: Leave 1/2 inch (12 mm) between the board end and any fixed structure.
Temperature Adjustment Table
The temperature at the time of installation determines how much extra gap you should leave. In cold weather, the boards are in their contracted state and need more room to expand. In hot weather, the boards are already expanded and need less gap.
| Installation Temperature | End to End Gap | Wall Gap |
|---|---|---|
| Below 4°C (40°F) | 3/8 inch (10 mm) | 1/2 inch (12 mm) |
| 4 to 15°C (40 to 60°F) | 5/16 inch (8 mm) | 1/2 inch (12 mm) |
| 15 to 25°C (60 to 77°F) | 1/4 inch (6 mm) | 3/8 inch (10 mm) |
| Above 25°C (77°F) | 3/16 inch (5 mm) | 1/4 inch (6 mm) |
These are general guidelines. Always follow the specific recommendations in your product’s installation manual. Different brands and board compositions may have slightly different requirements.
Capped vs Uncapped: Which Expands More?
Not all composite decking responds to temperature the same way.
Capped composite boards are wrapped in a polymer shell through coextrusion. This shell helps stabilize the board dimensionally. Capped boards generally show less thermal movement and better resistance to moisture absorption, which also contributes to dimensional changes.
Uncapped composite boards expose the core material on all surfaces. The wood fiber content in uncapped boards can absorb moisture, which causes additional swelling beyond thermal expansion alone. Uncapped boards tend to show more total dimensional change than capped products.
If minimizing expansion and contraction is a priority, capped composite is the better choice. EVODEK composite decking uses capped co-extrusion technology across its product line. The polymer shell reduces thermal movement and eliminates moisture absorption at the surface, providing improved dimensional stability compared to uncapped alternatives.
Learn More: What is the Difference between Capped and Uncapped Composite Decking?
Installation Tips to Manage Expansion
Install in Moderate Temperatures
The ideal installation temperature is 15 to 25°C (60 to 77°F). In this range, the boards are close to their midpoint size, making it easier to set correct gaps for both summer expansion and winter contraction.
If you must install in extreme heat or cold, adjust your gaps according to the temperature table above.
Install Early in the Day During Summer
On hot days, composite boards absorb heat and become more flexible. This makes cutting and aligning more difficult. Start installation early in the morning when the boards and air are cooler. If temperatures climb above 35°C (95°F), stop work and resume the next morning.
Use a Center Clip or Fastener
For boards longer than 12 feet, install a fixed fastener at the center point of the board. This forces the board to expand equally in both directions from the center rather than shifting entirely to one end. The result is more predictable and evenly distributed movement.
Use Hidden Clip Fastening Systems
Grooved composite boards paired with hidden clip systems handle expansion better than face screwing. Clips allow the board to slide slightly as it expands and contracts, reducing stress on the fasteners. Face screws lock the board in place, which can cause the board to warp or the screw to back out over time.
Never Butt Boards Tightly Together
This is the single most common installation mistake. If the board ends touch with no gap, they have nowhere to go when they expand. The result is buckling, warping, or boards lifting off the joists. Always leave the recommended gap at every butt joint, even if it looks too wide at the time of installation.
What Happens If Expansion Gaps Are Wrong?
Gaps Too Small
Boards push against each other as they expand. This causes buckling (boards lifting upward in an arch) or lateral displacement (boards pushing sideways off the joists). Fixing buckled boards usually requires removing and reinstalling the affected sections with correct spacing.
Gaps Too Large
Boards contract in cold weather, and the gaps become visually noticeable. Excessively large gaps can also allow debris to fall through to the substructure and may create a tripping hazard if the gap exceeds 3/8 inch.
How to Fix Buckling
If your composite deck has already buckled due to insufficient expansion gaps, follow these steps. First, remove the fasteners on the buckled boards. Then, allow the boards to return to a flat position naturally. Next, reinstall with correct spacing using a gap spacer tool. Finally, install a center clip on any board longer than 12 feet to control future movement.
Summary
Composite decking expands along its length and contracts when temperatures drop. The movement is predictable and manageable with correct installation.
Leave 1/4 inch (6 mm) end-to-end gaps at moderate temperatures. Increase gaps in cold weather and decrease them in hot weather. Always leave 1/2 inch clearance at walls and fixed structures in cold conditions. Use hidden clip systems to allow controlled movement. And never install boards with the ends touching.
Proper gap planning at installation eliminates expansion problems for the life of the deck. For help estimating materials and planning your layout, use the EVODEK decking calculator or request free samples to evaluate the product before you build.
FAQ
What gap should I leave between composite deck boards?
Leave 5 to 8 mm (about 1/4 inch) between the long edges of adjacent boards for drainage and airflow. At butt joints (end to end), leave 1/4 inch (6 mm) at moderate temperatures. Increase to 3/8 inch (10 mm) if installing in cold weather below 4°C (40°F).
Does composite decking expand more than wood?
No. Composite decking expands less than wood overall. However, the direction is different. Composite expands along its length. Wood expands across its width. Both materials require proper gap planning, but the gap priorities are in different locations.
What temperature should I install composite decking?
The ideal range is 15 to 25°C (60 to 77°F). At these temperatures, the boards are near their midpoint size, making gap calculations simpler. You can install outside this range, but you must adjust the expansion gaps accordingly using the manufacturer’s guidelines.
Can composite decking buckle from heat?
Yes, if the expansion gaps were too small during installation. When boards expand with no room to move, they push against each other and buckle upward. This is an installation error, not a product defect. Correct spacing prevents buckling entirely.
How do I fix buckled composite decking?
Remove the fasteners on the affected boards. Let the boards flatten naturally. Reinstall with the correct expansion gaps using spacer tools. Add a center fastener on boards longer than 12 feet to distribute movement evenly.
Do I need expansion gaps at the sides of composite boards?
Yes. Leave 5 to 8 mm between the long edges of boards. This gap allows water to drain through the deck surface and lets air circulate, which prevents mold and moisture buildup. Do not fill these gaps with caulk or sealant.
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