A lot has happened since four enterprising Mobil executives found a way to meld recycled grocery bags and wood flour to create the first wood-plastic composite decking. The development of what became Trex in 1996 marked the beginning of an industry that's giving wood decking a run for its money.
Consumers increasingly see composites as a way of getting nearly the look of wood with less required maintenance. Trex has been joined by dozens of other companies that annually churn out millions of board feet of wood-plastic and all-plastic decking.
Deck builders have plenty of options, although choosing the right material might involve some trial-and-error and on-the-job problem solving. Realistic expectations about the performance of synthetic decking and a basic understanding of how the material is manufactured can help builders make the right choice.
Composite Chemistry Wood-plastic composites are made from one of several polymers — polyethylene, polyvinyl chloride (PVC), and polypropylene are the most common varieties — plus finely ground wood flour and a range of additives that stabilize the plastic and protect it from UV damage.
Synthetic decking comes in a range of colors and textures, giving homeowners and deck builders design flexibility. Hidden fastening systems and color-matched fasteners help give a finished look.Photo: TimberTech Composite decking isn't bulletproof, but it does have several advantages over most wood species. Installed correctly, it's less likely to check or crack and there's no evidence that termites will attack it. Although it does need regular cleaning, wood-plastic composites do not have to be stained or treated with a preservative.
A class-action lawsuit settled by Trex in 2004, however, was a reminder that it's easy to over-sell these attributes. The wood component also can make the boards susceptible to mold, mildew, and decay. “It's not a no-maintenance product but it's definitely low-maintenance,” says Nicole Stark, a research chemical engineer at the Forest Products Laboratory in Madison, Wis.
How composite decking performs depends on a variety of factors, including how it's manufactured; the type of plastic, additives, and stabilizers used; the ratio of wood flour to plastic; and even how the decking is installed.
Start with the plastic. As much as three-quarters of all wood-plastic composites are made with polyethylene, a soft plastic that goes into plastic bags and a variety of other products. A smaller number of manufacturers use polypropylene, a much harder plastic, or PVC.
Although they're all in the plastics family, these polymers have different characteristics. According to Stark, the strongest and stiffest of the three is PVC, followed by polypropylene, high-density polyethylene, and then low-density polyethylene. All things being equal, a plank made from PVC or polypropylene would be noticeably stiffer than one produced with low-density polyethylene. This helps explain why polypropylene decking can span up to 24 inches, while a polyethylene-based deck board will span just 16 inches.
The amount of wood flour is another wild card. The industry calls these materials “filled plastics” because the wood doesn't add any strength, and in fact, makes the plastic a little weaker. But it does make the material stiffer. Wood flour also decreases “creep,” the tendency for materials to deflect over time under a load, and it lowers the tendency to expand and contract with changes in temperature.
Finally, plastics weather differently. Stark says polypropylene, for example, is more susceptible to weathering and surface oxidation than polyethylene. PVC also is susceptible to weathering, but is easier to stabilize.
The M Word As many deck owners have discovered, wood-plastic composites can support mold and mildew growth, just like the wood they're designed to replace. That's part of the tradeoff involved in using wood flour in the formulation.
“Solid wood absorbs water pretty readily,” Stark says. “In composites, depending on the surface, how they're manufactured, and how much wood is used, wood particles can themselves absorb moisture.” She suggests examining composites for wood particles at the surface. The more apparent they are, the more likely the board is to absorb water. Lower proportions of wood flour mean most of the particles will be encapsulated by plastic and safe from water. When the wood flour component reaches 50% to 60%, water absorption becomes more likely.
Manufacturing may also have a bearing on a board's potential for water absorption, Stark says. Planks extruded under higher pressure tend to have a more polymer-rich surface, which provides some protection.
With weatherability and color fading in mind, some manufacturers have moved toward a “co-extrusion” process in which an inner layer of wood-plastic composite is capped with a layer of plastic that keeps moisture out and reduces the risk of stains. CorrectDeck's CX line is one such co-extruded product. Martin Grohman, president of the company that manufactures CorrectDeck, says the cap layer of polypropylene is a way of addressing the three chief complaints that consumers had about wood-plastic composites: color fade, mold and mildew growth, and staining. It also contains higher concentrations of UV and anti-microbial agents, which could not economically be added to the entire board. CX decking costs about 25% more than the company's standard line.
Away With Wood For a summary of the downsides of mixing wood flour with plastic you need go no further than David Cook of EPS Plastic Lumber, makers of Bear Board. Cook calls wood the Achilles' heel of composite decking, not only because its inherent porosity allows in water, but also because it just as easily soaks up grease, sap, and other stains.
Bear Board is made from high-density polyethylene gathered mostly from postindustrial recycling. According to Cook, the all-plastic formulation 5/4 planks are stiff enough for 16-inch on-center framing in residential applications with none of the water or stain absorption problems seen with wood-plastic composites, and virtually no color fading.
Other all-plastic deck options include PVC, and polystyrene. A newer player in the all-plastic genre by way of cellular PVC is Azek Deck, with which manufacturer Azek Building Products capitalized on the acquisition of Procell Decking Systems and its unique technology.
“When consumers first bought into composites, they really thought they were getting a lot,” says Azek president Ralph Bruno. “They did get attributes like not having to do water sealing, no splinters, and a long usable life, but they also thought they were getting more in the way of stain- and scratch-resistance and better color.” It is products in the all-plastic decking category that have the potential to offer these benefits, he says.
Of course, not everyone will like the look of all-plastic planks. Some brands have an embossed surface that mimics wood grain, while others have an unnatural surface sheen that doesn't look much like the wood it's trying to imitate. And because there are no wood particles in the formulation, thermal expansion is a more pronounced problem. Cook says that a 10-foot board can expand and contract more than ½ inch in a 100-degree temperature swing. That's going to mean careful installation and some inevitable gaps between plank ends, at least at some times of the year.
Bruno says Azek Deck's Procell technology allows for installation of fasteners just ½ inch in from the edge, which helps with stability, and he adds that the material's inclusion of waste agrifiber helps reduce expansion and contraction.
The Price of Performance Wood-plastic composites lag far behind wood lumber in terms of installed square footage, but they're quickly gaining ground. Between 1997 and 2004, according to a 2006 article in Forest Products Journal, composites grew from 2% of the market to 15% while lumber dropped from 96% to 79%. At the time, composites were running about twice the price of 5/4 ACQ-treated decking. Because both pressure-treated and synthetic decking require the same structural framing, however, authors Paul Smith and Michael Wolcott estimated that composites only added 15% to 20% in total costs to a deck when compared with an all pressure-treated deck. And because of lower maintenance requirements, the payback on composites could come in as little as two to five years.
Washington, D.C.-area, deck builder Clemens Jellema says that pressure-treated decks are typically priced around $25 per square foot. A composite deck with a vinyl rail system would cost $40 to $45 per square foot, or between $32 and $34 when figured on the deck without a railing. He says the cost of labor is roughly the same, although some composites with hidden fastening systems go down quickly, reducing labor costs.
Though the wood component in composite decking will allow for some water absorption, many plastic-based deck boards and railings will weather well, even in coastal climates.Photo: Azek Building Products
If the up-charge to homeowners for using composite is relatively small, it's not difficult to sell — at least in an affluent market like Jellema's. Composite and plastic decking comprise 40% to 50% of his jobs now, and he expects that will grow to 60% to 65% in the future. Three-quarters of the way through 2007, Jellema had yet to install a single pressure-treated deck.
This article was adapted from PROFESSIONAL DECK BUILDER magazine, with additions by REMODELING to address updates in the synthetic decking marketplace.
Keeping It Green
Synthetic decking may be appealing to homeowners who want to invest in sustainable building and eco-friendly products. The materials have kept millions of tons of discarded plastic and waste wood out of municipal landfills, turning low-value refuse into a useful end product.
But be careful not to over-sell this feature. While some synthetic decking is made from all, or nearly all, recycled material, that's not universally true. Moreover, some plastics are inherently more difficult to recycle at the end of their usable life, or are more hazardous to produce in the first place. The Healthy Building Network, found online at www.healthybuilding.net, is one source of information on this challenging topic. It rated the environmental merits of 55 brands of plastic lumber from 44 manufacturers and ranked them in five broad categories, from “most environmentally preferable” to “not environmentally preferable — avoid.” Preference was given to products with a minimum of 50% post-consumer content and those made solely with high- and low-density polyethylene, an easy-to-recycle plastic.
The organization was concerned that wood-plastic composites are difficult to recycle because they mix synthetic and biological materials, and said that products made with a mix of consumer plastics are likely to contain more contaminants.
PVC and polystyrene should be avoided altogether, it said, because of chemical hazards associated with their manufacture and disposal. A summary of findings and a product-by-product ranking is available on the Healthy Building Network Web site.
There is, of course, more than one way to rank products on an eco-friendly scale. The benchmarks used by the Healthy Building Network may seem a little tough to some consumers, to say nothing of decking manufacturers whose products got panned. But the list and accompanying discussion can be a starting point for weighing the merits of company marketing claims.