All decking expands and contracts with the season, rain, and heat. But composite decking does so even more than many wood decking.
Composite decking will expand and contract on average 3/16” from temperature change.
This is why you must be mindful of expansion and contraction when building a composite deck.
Most composite decking brands require an expansion gap whenever butting end boards together or when the decking butt into a post or wall.
There are a few ways to minimize the amount of expansion. Allowing you to have smaller gaps in your decking. Either by the brand of composite decking or by installation method.
But before we get to solutions let’s answer a few questions about composite decking expanding and contracting.
Does composite decking expand or contract with heat?
Yes, heat is the number one reason why composite decking expands.
The reverse is also true. Cold will make the decking contract.
Expansion and contraction are largely limited to the length of the board. You still want gaps between boards, but this is for drainage, not expansion and contraction.
How much do composite decking boards expand?
The average composite decking will expand and contract 3/16” over a 16’ board.
This includes brands like Trex which suggest a ½” (12mm) with abutting boards to walls, when the temperature is less than 40°F (4.5°C). This is reduced to a 3/16” gap between mitre joints and end-to-end joints.
But this can vary by brand and decking material. Not all composite decking is wood based.
The worst offenders for expansion are pure PVC decking. PVC boards like TimberTech Azek, along with many other PVC brands.
Composite decking with the least expansion and contraction is Deckorators Mineral based decking. Which has virtually no expansion and contraction in the heat. You can butt Deckorators decking tight together in minus 4 (-20°C) or plus 86 (30°C). It will not affect the decking.
Which gives you a lot more options for mitre joints, decking inlays and the sort. Without having to design for board movement.
Does composite decking need an expansion gap?
Most composite decking will need an expansion gap at the end of each board. Often this will be a ¼” (6mm) when the boards butt together.
An expansion gap is also needed between the end of the decking and your picture frame board or transitions.
Expansion gaps should also be cut into boards when notching around posts.
And especially when the decking butts up to the house wall.
A lack of expansion joint will cause the boards to push up against each other in the heat. Causing the boards to buckle under pressure and even tear out from their fasteners.
Can composite decking be butted together?
Composite decking can have butt joints with the end of boards running into each other. But to avoid excess stress on the board, an expansion gap is needed.
An exception is Deckorators, Mineral based composite.
Personally, when I build composite decking, I integrate transition boards instead of butt joints. Transition boards look more intentional. Deleting ugly grime-gathering joints between the decking. It also allows for more intentional framing designed to accommodate the joints before starting to install the decking. Placing extra joists as required instead of adding blocking or sistering joists at every butt joint.
With many brands offering 20’ decking boards, the need for butt joints is even further reduced.
Do composite deck boards swell?
Quality composite decking has minimal swelling from water.
So, you only need to accommodate for temperature change, not water.
Some of the best composite deckings for water contact are MoistureShield and Deckorators Mineral-based composite. Both can be installed near, touching, or even in water with no impact on the board.
Making them ideal decking for docks.
But low water absorption also makes the decking good for around hot tubs or pools.
But there are many factors beyond water absorption for deciding the best decking for around your hot tub, pool or on a dock. Things like traction and resistance to fading from high sun exposure.