Dozens of manufacturers together churn out millions of board feet
of wood-plastic composites a year. In addition, a growing number
make all-plastic decking — and for something completely
different, there's powder-coated aluminum (see "An Aluminum
Alternative," page 3).
With so many options, there's bound to be some trial and error when
selecting and learning to work with a new material. One source of
help is decking manufacturers, many of which provide detailed
installation instructions on their Web sites (Figure 1). A basic
understanding of how the materials are manufactured can also help
deck builders choose the right products.
Figure 1. For the most part, composites can be
screwed down just like wood. However, tighter joist spacing may be
required, special screws are available, and you generally need to
consider the material's movement along its length, not its width.
Most manufacturers have installation guidelines on their Web
Wood-plastic composites are made from one of several polymers
— polyethylene, polyvinyl chloride, and polypropylene are the
three most common — plus finely ground wood flour and a
variety of additives that stabilize the plastic and protect it from
This medley of materials gives composites several advantages over
most species of wood: Installed correctly, they're much less likely
to check or crack, and there's no evidence that termites will
attack it. Also, it doesn't have to be stained or treated with a
preservative — that alone is enough to gather in homeowners
weary of trying to keep wood decking looking new.
Composite decking isn't bulletproof, however. The wood component
can make these boards susceptible to mold, mildew, and under the
right conditions, decay. And it still needs regular cleaning. "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 a particular brand of composite decking performs depends on a
variety of factors, including what it's made of — the type of
plastic, the kinds of additives and stabilizers, and the proportion
of wood flour to plastic in the mix — and how it's
manufactured and installed. And here's the rub: Each manufacturer
has its own secret recipe and method of production.
Plastic ingredients. As much as three-quarters of all wood-plastic
composite decking is made with polyethylene (recycled or virgin), a
soft plastic that's used for plastic bags and a variety of other
products. A smaller number of manufacturers use polypropylene,
which is a much harder plastic, or polyvinyl chloride (PVC).
Some manufacturers use only recycled plastics in their decking, but
this is not universally the case. Though having post-consumer
plastic as an ingredient makes the decking more attractive from a
green-building point of view, an obstacle is that recycled plastic
may be a mixture of several types. As a consequence, its properties
are less predictable than those of virgin plastics. Moreover, since
some longwearing types of plastics — polypropylene, for
instance — are tough to get as a recycled product,
manufacturers may be forced to use a blend of virgin and recycled
While they're all in the plastics family, these polymers have
different characteristics. The strongest and stiffest of the three
is PVC, according to Stark, followed by polypropylene, high-density
polyethylene, and then low-density polyethylene.
All other things being equal, a plank made from PVC or
polypropylene will be noticeably stiffer than one produced with
low-density polyethylene. Decking made from polypropylene can span
up to 24 inches while a polyethylene-based deck board will span
only 16 inches.
The plastics weather differently, too. Polypropylene, for example,
is more susceptible to weathering and surface oxidation than
polyethylene, says Stark. PVC, though also susceptible to
weathering, is easier to stabilize than either polypropylene or
The bottom line? The type of plastic used to make the decking is
important, but it's not the whole story.
Wood flour. The amount of wood flour added to the brew is another
wild card. It can come from either hardwood or softwood species,
although some manufacturers avoid woods high in tannins, such as
oak, because of the higher risk of staining. From a structural
standpoint, the species of wood used probably doesn't make much
Because wood flour doesn't add any strength — in fact, it
makes the plastic a little weaker — the industry calls
decking composites "filled plastics." Wood flour does make the
material stiffer, however. If that seems counterintuitive, think of
steel cable: It's very strong but quite flexible. Or glass: It's
very stiff but relatively weak.
Wood flour decreases "creep," the tendency for materials to deflect
over time under a load, and lowers the coefficient of expansion,
meaning a composite board shrinks and expands less with changes in
temperature than an all-plastic one does. Wood flour also gives the
product a more woodlike appearance and feel. In some decking, wood
flour is the only recycled content, with no use of recycled
As many deck owners have discovered, wood-plastic composites can
support the growth of mold and mildew — just like the wood
they're designed to replace. That's part of the trade-off when
using wood flour.
"Generally speaking, for fungal decay and attack by mold fungi,
wood needs about 20 percent moisture content," says Stark. "Solid
wood absorbs water pretty readily. In wood-plastic composites,
depending on the surface or how they're manufactured or how much
wood is in there, wood particles can themselves absorb
She suggests taking a close look for wood particles at the surface
of the composite. The more readily apparent they are, the more
likely the board is to absorb water, thus increasing the risk of
both mold growth and eventual decay (Figure 2).
Figure 2. Because composite decking contains
wood, some brands may support mold growth. Depending on the
proportion of wood flour to plastic resin and how the decking is
manufactured, particles of mold-feeding wood may be exposed on the
Lower proportions of wood flour mean most of the particles will be
encapsulated by plastic and safe from water. When the percentage of
wood flour reaches 50 percent to 60 percent, some particles
inevitably will touch each other, making water absorption more
Some manufacturers might not disclose the ratio of wood flour to
plastic in their decking; in those cases, the best bet may be
Stark's eyeball test. A large number of prominent wood particles on
the surface doesn't mean the decking should be avoided, but it is
an indication that the planks will be more likely to absorb
How a board is manufactured may also affect water absorption.
Planks extruded under higher pressure tend to have a more
polymer-rich surface, says Stark, which provides some protection
from water. "Moisture really is the key to improved durability,"
she says. "You have to control moisture in wood-plastic composites.
Any way you can improve that, you'll have improved durability, not
only with decay and mold but also with weathering and color."
Regular cleaning can reduce the risk of mold. Contaminated surfaces
may be brought back to life with a commercial deck cleaner and
brightener, and some composites may even benefit from a water
sealer and preservative or stain. "People think it's
no-maintenance," says Washington, D.C.-area deck builder Clemens
Jellema, who regularly uses composites from three different
manufacturers. "But I tell them there's no such thing as no
maintenance. You still have to clean it."
The other inherent problem with wood flour is it fades in sunlight.
Ultraviolet radiation attacks the lignin in wood; therefore
wood-plastic composites made with a high proportion of wood flour
will fade and weather much like solid wood planks. A number of
stains specifically formulated for wood-plastic composites are
available, and more are under development.
To combat fading, some manufacturers are using a "co-extrusion"
process: An inner layer of wood-plastic composite is capped with a
layer of plastic that keeps moisture out and reduces the risk of
stains. Correct Building Products' CorrectDeck CX line, which
incorporates an anti-microbial agent called Microban in the top
layer, is manufactured this way.
Martin Grohman, president of Correct Building Products, says the
cap of polypropylene addresses three chief complaints consumers
have about wood-plastic composites: color fade, mold and mildew
growth, and the difficulty of removing stains. Adding a higher
concentration of UV-blockers and anti-microbial agents to just the
cap is more economical than adding them to the entire board. Even
so, CX decking costs about 25 percent more than the company's
For a summary of the downsides of mixing wood flour with plastic,
you'll have to go no further than David Cook of EPS Plastic Lumber,
the maker of Bear Board. Cook, the vice president of sales and
marketing for the company, calls wood the "Achilles heel" of
composite decking, not only because its porosity allows in water
but also because it just as easily soaks up grease, sap, and other
Bear Board is made from high-density polyethylene gathered mostly
from post-industrial recycling. According to Cook, the all-plastic
formulation 5/4 planks are stiff enough for 16-inch on-center
framing in residential applications (12 inches on-center in
commercial work), with none of the water or stain absorption
problems of wood-plastic composites and virtually no color
All-plastic decking has its own downsides, however: Some has a
shiny look that not everyone likes, and because there are no wood
particles in the formulation, thermal expansion is a more
pronounced problem. Cook says EPS's manufacturing process gives
Bear Board more of a matte finish than some all-plastic planks, but
there's not much he or anyone else can do about plastic's thermal
A 10-foot board can expand and contract more than 1/2 inch in a
100-degree temperature swing. That means even with careful
installation it's inevitable that there will be some gaps between
plank ends, at least at certain times of the year. Because
high-density polyethylene moves around so much, the company
recommends using hidden fasteners and boards with grooved edges,
rather than face-nailing or screwing, for lengths over 12 feet.
Also, an intermediate crosspiece can be introduced on long runs to
keep deck boards shorter (Figure 3).
Figure 3. One way to deal with the greater
expansion and contraction of all-plastic decking is to avoid long
lengths. Adding planks perpendicular to the main run not only adds
interest, it allows for expansion joints.
There are a number of other all-plastic deck options, including PVC
(Deck Lok from Royal Outdoor Products), polystyrene (Eon from CPI
Plastics Group), and cellular PVC (Procell, made by Procell Decking
Like high-density polyethylene, these plastics are less prone to
staining than most wood-plastic composites, but also like
polyethylene, they are more prone to thermal expansion. And while
some brands have an embossed surface that realistically mimics wood
grain, others have an unnatural surface sheen that doesn't look
much like the wood it's trying to imitate.
PVC carries with it an added environmental burden: its manufacture
produces some dangerous by-products, and it releases dangerous
toxins when burned. That may not affect its in-
service performance, but homeowners with strong eco-sensibilities
may prefer to stay away from it, nonetheless (see "Keeping It
Higher Costs vs. Improved
Wood-plastic composites lag far behind wood in terms of installed
square footage but are quickly gaining ground. Between 1997 and
2004, according to a March 2006 article in Forest Products Journal
("Opportunities for Wood/Natural Fiber-Plastic Composites in
Residential and Industrial Applications"), composites grew from 2
percent of the market to 15 percent while wood dropped from 96
percent to 79 percent. "Countering the long-held tenet that
building products are purchased solely on price rather than
performance benefits, WPCs have succeeded in the market with a
substantially higher material cost compared to the competition,"
wrote authors Paul Smith and Michael Wolcott.
At the time that article was written, composites were running about
twice the price of 5/4 ACQ-treated decking. Because both
pressure-treated and composite decking require the same structural
framing, however, the authors estimated that composite decking
added only 15 percent to 20 percent in total cost when compared
with a deck made entirely from pressure-treated wood. And because
of lower maintenance requirements, the payback on composites for a
homeowner could come in as little as two to five years.
Pressure-treated decks are priced at about $25 per square foot in
the Washington, D.C., area, Jellema says. A composite deck with
vinyl rail would cost between $40 and $45 a square foot, or between
$32 and $34 a square foot 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 (Figure
Figure 4. Depending on the product, composite
decking may install more quickly than other options. Deck builders
report that some hidden fastener systems, such as CorrectDeck's,
actually speed installation, as well as provide a fastener-free
If the upcharge to homeowners for using a composite is relatively
small, it's not hard to sell — at least in an affluent market
such as Jellema's. Composite and all-plastic decking together now
comprise 40 percent to 50 percent of his jobs, and he expects that
to grow to 60 percent to 65 percent in the future. Three-quarters
of the way through 2007, Jellema had yet to install a single
Scott Gibson is a writer in East Waterboro,
Maine, who specializes in construction topics.