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Hollow Composites

Though generally lighter than their solid counterparts, hollow composites are engineered to be stiffer, which allows for wider on-center spacing — typically 20 to 24 inches for residential applications and 16 inches for commercial work.

Hollow decking materials can’t bend the way the solid composites can, however, and they usually require wood skirting or proprietary trim pieces to hide the cut ends of boards (Figure 3). As a rule, hollow boards don’t expand and contract as much as solid composites, but most manufacturers still recommend end-gapping to allow for thermal expansion.


Hollow decking products tend to be lighter and stiffer than solid composites, allowing for wider spacing. For example, CrossTimbers 5/4x6 radius-edge decking can span 24 inches in residential installations. Installers generally use skirting or trim pieces to hide the cut ends.

One benefit to these products is that their hollow profile offers a convenient space for running wires and cables for lighting, electrical devices, and speakers.

Some hollow products are tongue-and-groove; these are typically installed with a slight gap about the thickness of a credit card, to allow for expansion. Hollow T&G products may have weep holes or fit loosely enough to allow water to drain through the decking and prevent surface ponding. Even so, sloping the decking surface is also recommended — or sometimes required.

The Wood Look Sells

How realistically a synthetic product mimics wood decking is a key issue for many homeowners. Jack Hanson, president of Woodpile Construction in Meridian, Idaho, tells his clients, “There’s nothing more beautiful than a wooden deck — for the first six months. Then it’s a maintenance issue from there on out.” Hanson believes that composite products hold up much better than traditional wood decking, though he concedes that occasionally a composite deck plank will fade differently from the rest of the batch.

George Drummond, owner of Casa Deck of Virginia Beach, Va., says that products with deeply embossed wood grains or finishes that emulate the look of tropical hardwoods are the most popular with his customers. But finding a deck board that will continue to look good for many years can be tricky, he says. “A lot of the exotic-style deck boards have a sheen on the surface that scratches easily,” he notes. “We install them using drop cloths to protect the surface from our tools, but as soon as the homeowner drags a chair across the deck, there’s a problem.”

Kim Katwijk, a deck builder from Olympia, Wash., set up an experiment in which nearly 600 customers were asked to rate nine different products. His findings mirrored Drummond’s experience: The most popular boards were those with a deep embossed wood grain that approximated the look of a traditional wood plank — products like CrossTimbers, Tamko’s EverGrain line, and TimberTech.

Code Approval

With any decking product, there’s more to consider than performance and good looks. Frank Woeste, professor emeritus of wood science at Virginia Tech, says, “It appears that less than half of all composite and plastic decking manufacturers have a code report.”

For a plastic or wood-plastic composite decking product to be approved under the IRC, it must have an ICC Evaluation Service (ICC-ES) report indicating that it has passed a series of tests outlined in ICC-ES document AC 174, which is based directly on ASTM standard D 7032. The standard addresses span, fastener holding ability, bending strength, elasticity, UV and freeze-thaw resistance, fire performance, termite resistance, and the effects of temperature, moisture, and fungal decay.

You can find out if a product is code listed by asking the manufacturer or checking on the ICC-ES Web site ( There’s also a list of code-approved composite decking products at (click on “Composite Decking updated”), the Web site of a Minnesota chapter of the International Code Council. Under the 2007 IRC update, which references ASTM D 7032, all composite decking manufacturers must include their code listing on their product packaging. As this version of the IRC becomes more widely adopted, finding out whether a product is listed should get easier.


In areas where wildfires are an issue — Southern California, for instance — additional fire code regulations may apply.

Nearly all composite decking products have fire testing data available from the manufacturer. The most prevalent data is the flame-spread index (ASTM E84), a numerical scale from 0 to 200, with 0 being the best possible score. A Class A product has a flame-spread index between 0 and 25, Class B between 26 and 75, and Class C between 76 and 200.

Heat-release rate. Another good measure of flammability is a building product’s heat-release rate: The higher the rate, the greater the intensity of fire the product will produce if ignited. “Without copious amounts of flame retardants, construction plastics have heat-release rates much higher than wood,” says Vytenis Babrauskas, author of the Ignition Handbook (Fire Science Publishers, 2003). “Though it is possible to add enough flame retardants so that this behavior is controlled, the majority of manufacturers do not do that.”

California fire codes. As of January 1, 2008, the California Building Code has required all newly constructed decks to meet a stringent fire-performance standard called State Fire Marshal (SFM) standard 12-7A-4.

“To be code approved,” says Stephen Quarles, a scientist at the University of California, Berkeley, “the evidence of flaming or glowing must stop and the structure must still be intact after 40 minutes with the burner off. If the product has a Class C flame spread, then the walls must be covered with noncombustible materials like fiber cement or traditional three-coat stucco.”

Manufacturers must submit test data to the California State Fire Marshal to be eligible for code approval. You can find a list of products that pass 12-7A-4 at If a product you want to use isn’t listed, you must present the manufacturer’s fire-performance documentation to the authority with jurisdiction for approval.

Reid Shalvoy is a freelance writer based in Burlington, Vt.