When used with care, plastic is proving to be a material that
can hold its own in a tough environment.
For coastal decks, looking good is always a desirable option, but
rugged is the rule. To withstand an incessant attack of sun, rain,
ice, and fungus, decking materials must be especially tough.
Season in and season out, we're asking deck boards on coastal homes
to do some very hard work, and no material handles the challenge
perfectly. Pressure-treated lumber may last for decades, but it
often develops splits, warps, checks, and splinters —
sometimes after only a few years. By contrast, naturally
rot-resistant redwood and cedar start out beautiful and age
gracefully, but they're soft enough that even normal foot traffic
can dent them. Rot-resistant tropical hardwoods are tough and
attractive, but they may come from endangered woodlands. Plus,
these hardwoods can be brutal on saw blades, and some, like the
Brazilian hardwood Ipe, are toxic enough to irritate a carpenter's
skin, eyes, and breathing passages.
Facing this lineup, contractors have good reason to take a look at
plastic and composite decking. Product literature promises a
consistent, attractive board that won't rot, swell, or warp.
Plastic lumber contains no toxic preservatives, and it needs no
painting or staining. Many brands have impeccable "green"
credentials: They're made from recycled material that would
otherwise pile up at the dump.
Plastic cuts like wood and screws like wood. But it isn't wood
— and to succeed with it, contractors have to understand how
it's different. What you can expect when you switch from wood to a
plastic or a wood-plastic composite will depend on the particular
material you select.
What's in That Stick?
Plastic lumber isn't all the same. Different brands may be made
with different plastics; some brands contain fiber from wood,
agricultural waste, or other natural sources and some don't
(although even the "all plastic" brands may use pigments, foaming
agents, lubricants, or other additives in the manufacturing
process). And though the same building codes apply to all —
and all must meet certain basic performance criteria — these
different ingredients result in very different performance
characteristics over the material's service life.
Types of plastic. Trash is the cheapest
source of plastic. Consequently, most plastic wood is made with
polyethylene or polypropylene from recycled packaging like
shrink-wrap films, milk bottles, and grocery bags. These
"thermoplastics" get soft when they get warm; melt and flow (or
even vaporize) when hot; and will burn if heated to the ignition
point. Plastic scrap is a friendly raw material that is relatively
easy to melt and re-extrude.
Some decking products are made with PVC (polyvinyl chloride), the
same polymer used for vinyl siding and plastic drainpipes. PVC is
stiffer and stronger than polypropylene or polyethylene, but it's
also more brittle. It's hard to ignite and it doesn't burn well,
but when it does burn, the fumes contain toxic compounds of
chlorine. PVC products contain mostly virgin material, so PVC is
not exactly "green" — in fact, because of its chlorine
content, it's on the enemies list of most environmental watch
Fiber content. A number of products
contain only plastic — either polyethylene or PVC — and
a handful are reinforced with fiberglass (an expensive option). But
the biggest slice of today's market belongs to composite products
that mix plastic with processed natural fiber (Figure 1). The fiber
source varies: It can be wood flour, longer wood fibers, rice
husks, or even paper-mill sludge. Usually the plastic portion is
polyethylene, less often polypropylene; only one major brand,
CertainTeed's Boardwalk, combines wood fiber with PVC.
Figure 1. Not all plastic and composite
decking products are the same, and performance characteristics can
vary as widely as the materials and configurations with which
they're made. In the absence of established performance standards,
contractors will be best served by using products from companies
that have made a name for themselves in the plastic or wood-plastic
Whatever their ingredients, plastic deck boards are billed as an
alternate material for treated-wood decking on decks and
boardwalks. But because they don't act quite like wood, they need
their own performance standard. In fact, plastic deck boards have
two standards, created by two different committees of the American
Society for Testing and Materials (ASTM). There's ASTM Standard
D6662, for boards containing 50% natural fiber or less, and
Standard D7032, for boards that contain more than 50% natural
Unfortunately, the two committees don't always agree. "Turf
battles" (a term used by both committee chairs) are waged politely
for the most part, but they're not just about procedure: The
wording of the rules could make an important difference in whether
or not products are accepted by state and local codes, how the
products are made, what they cost, and how they perform. There's
money on the table for manufacturers, and potential risks for
installers who don't understand what the standards do — or
don't — require.
Composite chemistry. Tom Nosker, a
professor at Rutgers University in New Jersey, serves on the
subcommittee that created ASTM Standard D6662. Nosker is not a fan
of wood-plastic composites: "I have never promoted taking
polyethylene and mixing it with wood, because I don't think it will
last very long," he notes.
According to Nosker's analysis, wood cellulose, a "polar" molecule
that attracts water, does not bond chemically with polyethylene or
similar "nonpolar" plastic polymers. Stress transfer between the
materials is limited, Nosker says, so adding wood fiber to plastic
lumber doesn't do much for its structural properties. And Nosker
claims that the affinity of the wood fiber for water causes
composite boards to soak up moisture, support fungal growth, and
suffer freeze/thaw damage.
Nosker helped create a test method for Standard D6662 that is
intended to expose this potential weakness. "I designed it so that
the products that I know failed out in the field would fail that
test," he explains. The test method calls for a specimen to be
tested for mechanical properties, weighed and labeled, then
submerged for 24 hours and re-weighed. If the sample has gained 1%
or more water by weight, it goes back in the water for another
24-hour soak — and it keeps going back until its 24-hour
water uptake drops below 1% of dry weight. When that happens (at
saturation), the piece is frozen for 24 hours, thawed out, and sent
back for another weigh-soak-weigh cycle. After three cycles of
soaking and freezing, the sample gets a second test of its
mechanical strength and stiffness. If the physical properties have
dropped by 10% or more, the product fails.
"It's a trivial experiment for an all-plastic material that doesn't
take up water," notes Nosker. But he says it can devastate
composite boards: "If water gets in and saturates some area, then,
when it freezes, the ice expands and creates a crack, and that
allows a lot of new paths for more water to go in and wet more
particles," Nosker explains. "Some of these boards will take up 20%
or 30% of their weight in water."
Where's the data? Robert Tichy of
Washington State University is chairman of the committee that wrote
ASTM Standard D7032. He notes that he's been hearing these claims
for years, but Nosker has never shown him any data. The D7032
standard has a much less extensive version of Nosker's test: The
samples are soaked and frozen three times, but they get just a
single 24-hour soak each time. According to Tichy, that's because
the time spent in repeat soakings is a waste — he says that
the boards max out at about 4% moisture content, and never suffer
the strength decline that Nosker alleges.
"I did 50 cycles of Nosker's procedure on samples from the three
products that make up about 80% of the market," notes Tichy. "After
the first few soaks, they took up almost no additional water, and
after a year, none of the samples had lost as much as 10% of their
structural strength." Tichy says that he also tested boards from a
New Jersey boardwalk that had been exposed on the beach for seven
years, comparing them with boards that had been stored inside a
warehouse. "There was no significant difference," reports
As for stress transfer, Tichy agrees that wood fiber and plastic
don't bond chemically. But he says that the mechanical interlock
between the materials achieves much the same effect. In fact, the
fiber content improves the strength because the fiber has a higher
bending stress than the plastic. "If you take half the plastic out
of a board and replace it with wood fiber," notes Tichy, "you can
double or triple the flexural strength and stiffness."
Standards and the code. At this point,
neither Nosker's nor Tichy's assertions are easy for outsiders to
verify. When grading standards for wood lumber were revised in the
1990s, there was an open process of stress-testing for all the wood
species, with data shared through the entire industry. But the
companies producing manufactured plastic or wood-plastic deck
boards are not currently releasing any data from the testing of
their products for freeze/thaw resistance, flexural stress
properties, or resistance to weathering. And neither D7032 nor
D6662 yet has the force of code: The International Code Council's
acceptance criteria document for deck boards, AC-174, has been
recently revised to incorporate the D7032 standard by reference,
but the revised AC-174 has not yet gained official acceptance
When the new AC-174 does take effect, even products that already
have code approval will have to requalify. Bob Tichy is looking
forward to it: "We hope it will make a difference," he says, "and
weed out any bad actors in the industry."
Plastic Lumber in the Real World
So, what can you expect when you handle plastic or composite boards
outside the laboratory? Characteristics relating to strength,
stiffness, durability, weathering, moisture intrusion, and
dimensional movement will vary from product to product, depending
on the material's makeup. They can all be significant, but they can
be handled if you know what you're doing.
Strong like … plastic? Pound for
pound, plastic and composites can't match the span capability of
wood. "With either an all-plastic material or the wood-plastic
composite materials, your strength is comparable to wood, but your
stiffness, or flexural [elastic] modulus, is quite a bit lower,"
explains U.S. Army Corps of Engineers researcher Richard Lampo, who
chairs the committee that created ASTM Standard D6662. "So your
design is not strength controlled but deflection controlled."
Joist spans will vary widely, depending on the thickness of the
material. While 1-inch and 5/4-inch decking is common, many
manufacturers offer a stiffer 2x material to improve deflection.
Use the manufacturer's recommended spacing as a minimum, and
consider using a closer spacing.
"You're not supposed to span more than 16 inches with it," noted
one contractor on the JLC-Online forums (www.jlconline.com)
about a 5/4-inch-thick product. "Often, we cut the span down to 12
inches." Another participant concurred that composite boards felt
"a little loose underfoot" at 16-inch spans. Joist spacing will
also have to be narrower when the decking runs diagonally, since
the decking itself is spanning a greater distance (Figure 2).
Figure 2. Each manufacturer publishes its own
guidelines for joist spans. This sample from Trex illustrates how
joist spacing will vary with the thickness of the materials and
with the orientation of the decking. In general, the strength of
Trex is not much different from that of wood, but this is not the
case with all the available products.
In addition, plastic or composite decking is more likely to deform
slowly under a sustained loading (the phenomenon known as "creep").
One forum contributor observed, "It's heavy, without a lot of
rigidity, so try to store it as flat as possible." Stacked on
uneven ground on site, or left unsupported, plastic deck boards
tend to gradually conform to the contours under them, like wet
noodles (Figure 3). At the usual light, transient loads of a deck,
creep won't tend to show up once installed, but it may be a problem
if heavy materials are stored on the deck, particularly in warm
left unsupported, plastic and wood-plastic lumber may droop,
particularly in hot weather. On the other hand, a slightly higher
elasticity can be used to advantage to create curved features, such
as this stairway built with Trex (middle).
Another vulnerable deck detail involves the overhang. When you
install the deck boards, minimize the overhang at the edge of the
deck; otherwise, the edges will sag, or even lift under load
between the first few joists. As the plastic softens in hot sun,
the condition is aggravated, particularly in high-traffic locations
such as beach walkways. If possible, keep the ends flush with the
edge of the last joist, or limit the overhang to an inch.
PVC is stiffer and stronger than polyethylene or polypropylene, but
it's also costlier. So, PVC-based products typically use a hollow
or channeled structure to achieve equivalent spans with less
material. Because of PVC's relative brittleness, however, those
products may be more prone to shear failure under high stress
— a high-heeled shoe may punch through a top surface, for
instance, or a fastener may crack through the board. As with the
poly products, it's wise to keep spans short and to take particular
care while nailing or screwing.
Sun, snow, rain, and dirt. On the coast,
the primary threat to plastics is sunlight: Ultraviolet rays can
break the bonds in the polymer's molecular chain. However, the
sun's effect on plastic deck boards is only skin deep. According to
Nosker, the sun penetrates only a few thousandths of an inch, and
while it degrades the surface layer, this layer of damaged plastic
blocks further penetration. Products exposed to sunlight may show
some surface fading or discoloration but will remain serviceable
Figure 4. In the long run, composite decking
will likely weather the sun, rain, and ice much better than
pressure-treated lumber. Ultraviolet light will fade the top
surface of composite materials (shown new at left), but the
material remains serviceable and does not usually continue to
degrade after initial weathering.
A more common complaint is staining from dirt, liquids, or even
from the material itself. Some composite boards use wood fibers
that contain "extractives" — pigments or resins that can
migrate and may change color. "When the extractives oxidize, it can
show up as small black dots that look like mildew on some
products," explains Tichy.
Aesthetic surface problems have not escaped the notice of
contractors. One recent post at a JLC-Online forum described a
delivery of composite boards from the lumberyard, already stained
with black splotches that wouldn't wash off. Another contractor
replied with the story of building a large deck, only to have the
customer's dogs stain it with footprints on the day it was
completed — and of "blotchy spots like bleach spots" showing
up soon thereafter. A third chimed in, "Watch out for Coke, catsup,
grease, and mustard stains. Barbecues make a mess."
According to Seattle-based home inspector and remodeling contractor
Mike O'Handley, such stains can be cleaned off using a garden
sprayer filled with a 50:50 solution of bleach and water. Spray it
on heavy, let it sit for 10 minutes or longer, then pressure-wash
at no more than 1,500 psi (Figure 5).
Figure 5. While composite decking is
susceptible to staining, most blemishes can be cleaned off —
first using a garden sprayer filled with a cleaning solution. After
letting this soak in, the deck is pressure-washed to restore a
"like new" appearance.
Several cleaning solutions containing sodium percarbonate (oxygen
bleach) tend to be even more effective than chlorine bleach, and
oxalic acid-based cleaners work best for heavy stains and rust
around fasteners. Contractor Joe Wood, a specialist in high-end
decks in San Diego, recommends Deck-Prep from Behr (www.behr.com) for Trex
decks: "Makes it look good as new," Wood reports.
"We had a load of TimberTech delivered with ground in dirt and mud
stains," reports exteriors contractor Jamie Hill. "We used Armor
All brand deck cleaner [www.armorallhomecare.com/products/ezspraydeckwash.html]
and it worked great. We now wash every deck with it when we are
done." Keep the material covered on the job site, he advises: "The
strangest things will stain it. The time messing with the tarps is
better spent than trying 10 different cleaners to get leaf stains
off a porch."
To avoid stains altogether, the best bet may be an all-plastic
material. Polyethylene's water-repellent chemistry blocks any
waterborne stain and resists most grease and oil formulas, too.
Remodeling contractor James Work has had good luck with U.S.
Plastic Lumber's Carefree Xteriors decking: "I installed a light
sand color with white railings where a walnut tree had previously
killed anything I put under it. Now I wash it down every once in a
while and blast it off once a year with a power washer. It looks
Dimensional movement. All-plastic
products have a higher coefficient of thermal expansion than
products containing wood. And even with composite products,
dimensional movement related to temperature changes is a problem.
In this regard, the product's uniformity is a drawback: Plastic
expands and contracts as much along the long dimension as it does
across the width.
The problem cropped up on a job for Mike O'Handley: "The challenge
came when trimming to fit around protrusions," he explains. "One
evening as I was getting ready to leave for the day, I measured,
marked, and cut the outside perimeter boards for a nice fit around
the balustrade supports. Boy, did I get a surprise the next
morning! I didn't get there until the bright morning sun had been
baking that nice dark plastic for about an hour. I picked up that
perimeter board to set it in place and found that it had expanded
lengthways more than half an inch and the slots I'd cut, which fit
so nicely the evening before, were nowhere near where they needed
to be to line up with the balusters!"
Most manufacturers recommend spacing board ends 1/8 to 1/4 inch
from each other, and up to 1/2 inch from solid objects (Figure 6).
However, the best spacing may depend on the outdoor temperature at
the time of installation. In hot weather, James Work cuts the
material "a fuzz longer," then "springs them into place —
like you might spring some baseboard into a corner." This way, says
Work, when everything shrinks up in colder weather, it won't open
up a huge gap. However, in cold weather, the material must be
gapped to avoid problems from the material expanding later.
Figure 6. Deck boards must be spaced from
solid objects or the material may expand and crack, as did this
extruded PVC used on a boardwalk in Cape Canaveral, Fla. (top).
Splits in decking used on the same boardwalk demonstrate one of the
reasons why hidden deck fasteners should be considered with all
decking materials, even plastics and composites.
Cost and Availability
Plastic lumber is a growth industry, with many new players and a
wide range of products. But a few established companies dominate:
At lumberyards, you're likely to find one or two brands of
composite board, and maybe one brand of all-plastic decking. The
more costly PVC-based brands may be available only by special order
— and the same is true of any specialty fasteners needed to
Even the most common varieties aren't cheap. Plastic lumber is
priced comparably with tropical hardwoods but not competitively
with pressure-treated wood. Factor in the learning curve, and many
contractors are inclined to stick with wood. "I'll install
synthetic decking if my customer asks for it; I don't suggest it,"
said one coastal builder. "But a lot of them do ask for it, because
it is advertised so much."
Contractors shouldn't be surprised to see more and more demand for
plastic-based materials from their customers. And as with any
unfamiliar material, contractors should expect some unexpected
twists and turns the first few times they use it. But when used
with care, plastic is proving to be a material that can hold its
own in a tough environment. ~Ted Cushman reports on the building industry from his home in
Great Barrington, Mass.