Choosing the Best Composite Decking for Cold Climates

The best composite decking for cold climates resists moisture.

The best composite decking for cold climates resists moisture and stays stable through temperature variations.

One of my good friends recently got to buy some property out west, and he built his family dream home in the hills of the Rockies. When it came time to build a deck to take full advantage of those lovely views he’d purchased, he bought into the promise of a virtually maintenance free deck at a bargain price and built the deck from the cheapest composite he could find. Two weeks after installation the boards were curling up at the ends, and recently other issues have cropped up, like fading.

Many homeowners and deck building professionals are cautious about composite decking because they’ve been burned by a cheap product. This is especially true for those living in cold climates where the temperature extremes between winter and summer will exacerbate flaws in poorly-made decking. But not all composites have issues–well-made composites are extremely sturdy and will weather the cold better than wood will. When choosing the best composite decking for cold climates, it is important to buy a quality product. Knowing how composites are made will help you sort through the choices to find water-resistant composite decking that can stand up to the freeze and thaw cycle of cold climates.

How Composite Decking Is Made

Composite decking is exactly what it says: a composite material. In this case, it’s a combination of wood byproducts and recycled plastic. Composites started off as a way to reuse discarded plastics and avoid cutting new timber. Originally, this meant using discarded plastic bottles and sawdust from factories where wood was worked. These were blended together, heated until molten, and finally pushed through a mold into the desired shape in a process called extrusion.

While this process did result in eco-friendly composite decking, the initial results were inconsistent due to the different grades of plastic used and the sawdust’s lack of uniformity. This accounts for a lot of the horror stories associated with composite decking, with even composite boards produced from the same batch reacting in wildly different ways to stresses over time. To avoid composites with this issue, seek out manufacturers dedicated to making high-quality products.

The Plastics in Composite Decking

Let’s start with the plastic component of composites. There are three different plastics commonly used in composites:

  • Polyethylene is the most widely used thermoplastic in the world. It is the plastic used in soda, milk, and other bottles in a specific combination called PET, and it is the plastic that is most often used in composite decking as well. It does experience thermal expansion and contractions, meaning it can swell slightly in hot weather and shrink slightly in very cold weather. The extent of these dimensional changes has to do with the ratio of wood flour to plastic. Composite boards with a greater proportion of wood flour and a higher density won’t change much in dimension.
  • Polypropylene is new in the manufacturing of composite deck boards. It is somewhat more thermally stable than polyethylene. Its biggest disadvantage is that it breaks down more readily when exposed to UV light. When used in decking, polypropylene boards an extremely UV-resistant cap surrounding the core of the board to protect it from the sun’s rays.
  • Polyvinyl chloride, better known as PVC, is a plastic widely used in outdoor applications, from piping to fences. Currently, PVC is being used only on its own in decking, rather than in combination with wood flour. As a result of this all-plastic composition, PVC decking boards have thermal properties that are worse than composite, with greater expansion and contraction. PVC can also grow very brittle in cold weather. While that usually isn’t an issue with a fence, a deck gets stepped on, and brittleness combined with the constant impacts of footfalls is a cause for concern.

If you’re in the market for composite decking, seek out and read the specifics of what sort of plastic is used in the composite you’re considering. Avoid hollow polyethylene boards, since this design can exacerbate their issues with expansion and contraction, and look for composites made using high-density polyethylene in combination with another material.

The Wooden Element of Composite Decking

While the current industry standard for composite decking is wood flour—very fine sawdust—there are different types of wood flour used. Most composite decking uses wood flour from softwoods, although occasionally you’ll find a company that boasts of using hardwood flour. It’s difficult to determine whether or not this makes a difference in the final product’s strength in any climate. All that is really known for sure is that composites made with larger grains of wood flour tend to be stronger than those made with smaller particles. Composites made with bamboo flour do a better job resisting moisture intrusion than those made with wood flour, and also seem to have greater strength, both of which are important during the harsh winter weather.

One complaint about early composite decking was that it got slippery when wet. To mitigate this, manufacturers began adding a covering, or ‘capping’ to the boards, which also helped seal out water. This led to a new issue in which this capping material started peeling up from the board below, leaving the boards vulnerable to rotting and molding. There is a way to prevent a board’s capping from separating from the core of the board, however. To securely bond the two together, the cap needs to be applied to the deck board while it is being created or else the bond between these two parts of the board won’t last. This process is called co-extrusion, and involves a using a carefully controlled cooling process.

Yet another issue arises when manufacturers only cap the top side of the board. This protects the top of the board from the worse of the sun and rain, but leaves the rest unprotected. This is not sufficient protection for most composite boards, and it’s especially risky in cold climates where snow might melt during the day and refreeze at night, leading to water seeping slowly between and under the boards.

The Best Decking for Cold Climates (and Everyday Living)

The best composite decking for cold climates is also, in my opinion, just the best composite decking to use, period. The single biggest issue that composites in cold climates will have to deal with is moisture absorption, as water soaking into the wood fibers of a composite board and then freezing will cause the board to fail. That moisture resistance, in turn, is going to be determined by the density of the board, how well the cap is bonded to the board, and the composite’s ability to remain dimensionally stable as temperatures rise and fall.

Essentially, the best composites for cold climates will be the ones where the greatest care is taken during manufacturing. Boards that are made too quickly won’t be as dense and the bond between the cap and the board won’t be strong, all of which are important to how well the board stands up to any environment. A manufacturer that doesn’t fully cap their board, uses too little wood or bamboo, or chooses a type of plastic that doesn’t stay stable in changing temperatures also won’t produce a composite that can stand up to cold climates.

One manufacturer that doesn’t cut corners is Fortress Deck. Their Infinity composite deck boards are made with high-density polyethylene and 55 percent bamboo flour for superior strength, density, and water resistance. They’re also fully capped with a tough non-slip resin, and they’re co-extruded for greater integrity and moisture resistance. Attention to detail and quality is the hallmark of all Fortress products, from their decking to their other carefully-designed products such as railing and fencing.

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