What Are Composite Deck Boards Made of?

Beautiful Deck
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Capped composite boards have a polymer coating that gives them a shiny appearance.

Having been on the market since only the early 1990's, composite decking boards do not have a long history. Nevertheless, the ones available today are significantly longer-lasting and more wear resistant than the originals. The composition hasn't changed much -- most manufacturers still make them from wood dust and recycled or virgin plastic. The difference is that modern boards are capped, which means they are coated with a plastic film which prevents deterioration from moisture. Composite boards look very similar to pure PVC ones, but they aren't the same. PVC boards last longer, but they are more expensive and bear less resemblance to real wood.

What's in a Composite Board?

The Trex company first introduced composite boards in the early 90s and marketed them as a green building product. The company used sawdust and wood chips recycled from sawmills, flooring factories and cabinet shops and combined these with grains of plastic obtained from recycled bottles and other throwaway items. Together, wood chips and plastic made up 95 percent of the material in the composite boards. The other 5 percent were dyes and binders.

Their basic formula is still in use by the many manufacturers who sell these boards today, although some substitute virgin plastic when recycled material isn't easily available. There is no appreciable difference in performance between these materials, because the plastic is ground into tiny particles before it is used. Manufacturers also differ in the size of the wood chips they use. Fine wood powder makes the most stable boards, but it's more expensive than coarser chips and sawdust, which often get used instead.

The Manufacturing Process

Composite board manufacturers use one of two different process to make deck boards. One is extrusion, whereby the wood/plastic pulp is heated until the plastic melts then forced through an opening with the cross-section of 2-by-6 decking board. In the other process, the raw material is poured into a mold. The wood grain pattern is etched onto the surface of each board before it hardens.

Capping Solves Early Problems

First-generation composite boards were far from perfect, and Trex had to settle a class-action lawsuit brought by consumers who complained about their propensity to grow mold. This was because the boards absorbed water, which also made them swell. These problems disappeared in 2009 when the process of co-extrusion -- or capping -- became mainstream. Capped boards are coated with a hard layer of PVC plastic, which waterproofs them and makes them stronger. Capped boards are similar in quality to pure PVC ones, but because they have cores made from recycled materials, they are less expensive and create a smaller carbon footprint.

A "Green" Building Material?

There's little doubt that the manufacture of composite boards is an environmentally friendly process. The wood chips and much of the plastic are recycled and would otherwise end up in landfill. Moreover, manufacturers typically obtain them from nearby sources, which reduces the energy needed to transport materials.

Environmentalists have noted that composite boards themselves cannot be recycled, and after their 20- to 30-year service life is over, they end up clogging landfills. In fact, composite boards can be turned into a host of everyday items, such as tool chests, shims and lobster rails. However, once composite material does find its way to the landfill, it decomposes very slowly and could remain there for centuries, slowly releasing its plastic granules into the soil.