The Deck Handbook: Part 5
The Pros and Cons of Composite Decking
Ask anyone who has chosen composite decking why artificial wood was their choice and you are overwhelmingly going to get one, two, or all of three of these answers:
Composites require little or no maintenance,
Composites are more environmentally friendly than wood, and
Composites are longer lasting than wood.
Is any of this actually true? Do composites really require less maintenance? Are they more eco-friendly? And, do they really last longer than wood decks?
Mildew build-up on a composite deck
Normal use of the deck wears off the protective plastic coating that surrounds the wood particles in the composite, leaving them exposed to mold and mildew.
Wood in its natural state contains chemicals that fight off fungus. But, the wood flour used in composites has had most or all of these protective chemicals removed, so it is very vulnerable to mold and mildew.
This is a Trex composite, but most non-capstock composite decking materials have this problem to some degree.
After over thirty years of experience with these products, we can confidently say that none of these beliefs is actually true. Composites require nearly as much maintenance as wood; they are not any more environmentally friendly than wood, and may even be more harmful, and they are not necessarily longer lasting than wood.
So, how did these myths come to be? Are the composite manufacturers just lying to us?
Not really. (Well, maybe just a little.) They were fostered by composite manufacturers in the heady early years of the product when claims of "maintenance free" and "lifetime decks" were tossed about freely. The manufacturers relied on predictions about how the material should behave but failed to do the long-term research to substantiate the truth of the claims.
Now that a little time has passed and some research has been done, these expansive claims are just not holding up. In fact, since 2004, seven major wood-plastic composite manufacturers have faced class-action lawsuits related to a host of material problems, including fading and color changes, slippery surfaces, shrinkage, swelling, deterioration and massive mold buildup.
Keep in mind that all composite decking materials, including capstock composites, are still largely experimental. Unlike wood — with which we have worked for about a million years and the characteristics of which we know very well — composites are a new and imperfectly understood material.
Scientists can approximate how a particular chemical stew of plastic, filler, and additives will react to prolonged exposure to the elements over time, but without actual long-term exposure tests, they never know for sure. And, no manufacturer tests its material for 25 years before putting it on the market.
The result has been a lot of failures and several major composite decking companies going bankrupt or quitting the business, some after massive recalls of defective and dangerous composite decking products and wholesale defaults on their warranties.
Yet, even after claims of "maintenance free" and "lifetime deck" have been thoroughly debunked by research and experience, we still see them widely repeated, even by those who should know better. As late as July 2012 the online edition of Popular Mechanics reported that
"[t]he appeal of composite decking is that it's virtually maintenance-free. It never needs to be sanded, scraped, refinished, or stained …"
Composite decking companies themselves are much more circumspect. None now claims composite decks to be maintenance free. The most they will venture is that their composite decks require "less maintenance" than wood decks. Certainteed, for example, allows only that its EverNew® 20 decking offers "weatherable low-maintenance design…". The term "weatherable", translated means "if you don't seal and stain it, it will turn gray and ugly over time", something Certainteed is not likely to come right out and say ( hence the mysterious "weatherable").
Retailers and deck contractors, however, are often much more effusive. Home Depot, for example, claims that Trex Enhance "never needs staining", although Trex itself makes no such claim.
No deck is actually maintenance free. Routine cleaning and maintenance are required for both wood and composite decks. Composite manufacturers now admit that there are no "maintenance free" decking materials of any kind. Every decking material needs at least a periodic washing. Mold, mildew and dirt attack every deck - plastic, metal, wood or composite — with evenhanded tenacity, and have to be taken care of at regular intervals. All composite manufacturers now recommend a periodic maintenance schedule, specific procedures, and even products by brand name for cleaning and maintaining their composite materials.
Never Needs Sealing?
So, the question remains: Are composite decks actually lower maintenance than wood decks? The maintenance chores that irk most deck owners, and the ones they want to get rid of, are periodic deep cleaning, sealing and staining. Many homeowners feel that the premium cost of composite decks is definitely worth it if these burdensome tasks can be eliminated or much reduced. But, do you, in fact, gain a significant reduction in maintenance if you build a composite deck? The answer is sometimes "yes", but more often "no".
Deck wood is essentially raw lumber. It may be treated to retard rotting and reduce insect damage, but it is neither stained nor sealed when received from the lumber yard. Sealer is applied by the deck builder, and sealing is part of the builder's price for his or her labor of building the deck.
Composite decking is sealed at the factory and does not need to be re-sealed when the deck is built. The sealing is part of the manufacturing, and the cost of sealing is built into the price of the material.
Sherwin-Williams Exposure Testing
You can bet that any claim that a product is particularly resistant to rot or deterioration is going to get a lot of attention from the paint companies. After all, their bread and butter is protecting stuff from rot and deterioration.
Here are some results from a Sherwin-Williams study of what happens to composite decking after 3 years of exposure. The dark gray and black spots are mold growing on the boards. Every composite tested grew bumper crops of mold and mildew.
The pictures tell the whole story.
Once sealed, a wood deck needs to be resealed periodically to maintain its structural integrity. Without sealing, treated pine decks have a lifespan of 15-20 years.
There are as yet no true "lifetime" sealants. They're coming, but they are not here quite yet. The new preservative treatment for pine, ACQ (alkaline copper quaternary) that uses no arsenic, is not only safer and longer lasting but takes stain and sealant better. New multi-year sealants and coatings have made the annual chore of cleaning and resealing a thing of the past. The re-sealing interval for wood decks is now every 3 to 7 years. (See Part 6: Staining, Sealing and Maintaining Your Deck.)
Composite decks need no initial sealing because the plastic binder in the product is also the material's sealer. The question is this: is the binder such a durable sealer that it will last for the lifetime of the deck without being renewed?
The answer is, unfortunately, "no". Eventually, the deck will need to be re-sealed. Why? Two reasons:
The UV inhibitors in the binder are not permanent. No one has yet come up with an inhibitor that can be used in a composite mix that lasts more than a few years. The basic problem is that the UV-inhibiting additives are themselves broken down by exposure to UV radiation and decline in effectiveness over time. This is the primary reason composite decks turn gray.
Decks get walked on. Walking on the deck eventually wears through the plastic that "seals" the wood particles that make up the bulk of the material in a composite deck board. The plastic used in most composite decking is polyethylene (See "Binders" above), a soft and not very tough plastic. It will fairly quickly wear away in normal use. As will the tougher plastics like PVC and polypropylene. It just takes a little longer. So, even if the factory's binder creates a perfect seal, and that's not possible, that perfection is soon compromised in the normal course of using the deck.
A failed plastic seal that no longer protects the wood fiber filler is the fundamental flaw of wood-plastic composite decking.
It is simply not possible to keep the plastic from being worn away or compromised by UV. Once the protective plastic binder is gone. The wood particles in the decking are exposed to weather and all sorts of unpleasant things begin to happen.
Probably the first thing a homeowner will notice is white spots on the deck. These are exposed wood particles bleached by the sun and rain. This is a sure sign that the deck's seal has been damaged. Exposed particles, besides creating unsightly white spots, swell when they get wet, and can cause the decking to split and degrade over time.
Then comes the mold. The wood fibers in composite decking provide a very friendly home for mold and mildew which happily puts down roots into the unprotected wood flour. Once well anchored in the wood particles, mold and mildew are almost impossible to eradicate.
Wood flour in composite decking has no natural protection against mold and mildew. Wood in its natural state contains chemicals that fight off fungus. If wood did not have them, all trees would soon die of fungal infestations. Some species are so well protected by their internal chemicals that they are virtually mold and mildew proof — western red cedar, for example.
But, the wood flour used in composites has had most or all of these protective chemicals removed, not intentionally, but as by-products of the processes that clean the wood of unwanted elements. So wood flour is very vulnerable to mold and mildew when the wood particles are exposed. Trex, the largest manufacturer of composite decking, has been sued more than once due to excessive mold and mildew build-up on its composite decking, and had to pay out big dollars in settlement. The only sure cure for this condition is prevention — sealing to restore the barrier that seals the wood particles from the elements. That requires sealants.
Looking Clean, But Not Looking New
Every time you restain your wood deck, it looks almost brand new. So every five years or so, you have a new-looking deck to enjoy.
Every time you deep wash your composite deck, it looks clean, but not new. Any fading — and there will be some — is still with you. So, what you end up with is a clean, but old looking, faded deck.
You can cure fading and discoloration on some composite decks by staining them using a specially formulated composite deck stain. But, if you stain your deck, you will have to periodically re-stain it to keep it new looking, just as you would a wood deck. So, you end up with a choice, a faded, old-looking, but clean deck or the chore and expense of regular re-staining to keep it looking new.
The sealant available for composite decks is not very different from that used on wood decks, so once you start sealing a composite deck, the sealing interval is about the same: every 3 to 7 years. Non-capstock composite decking is no different than wood decking when it comes to the need for sealing.
Never Needs Staining?
Any number of manufacturers claim that their composite product never "needs" staining. The key word in this phrase is "needs".
The bright, crisp composite decking color you get fresh from the factory is not going to be the color you see a year or even 3 months, after installation. All composite decks fade. Non-capstock decks fade a lot. All eventually fade to a drab weathered gray that looks little different from the color of dingy, weathered wood.
Capstock deck boards, with their plastic shell, fade less than uncapped composite boards. Most will fade a little, but because the exposed material is plastic, the fading is minimal. Some capstock manufacturers, such as Fiberon and Timbertech, now guarantee some of their products against excessive fading. But, most continue to specifically exclude color changes, fading and graying from their warranties.
At some point all composite decks are going to look old, faded and dingy. Whether and when a composite deck "needs" staining is essentially an aesthetic decision. Your car never "needs" washing. It will happily run for many years as dirty as a coal pit. Your clothing never "needs" ironing. A severely wrinkled shirt still works as a shirt. We wash our cars and press our shirts because we don't like the look of dirty cars or wrinkled shirts.
If you don't mind a faded, graying, tired-looking deck, then it's true: your deck never needs staining.
On the other hand, if faded, gray and tired-looking is your preferred aesthetic, you can get the same effect by never staining or sealing a treated pine deck, and for far less than the cost of a composite deck.
The theory behind the "never needs staining" claims in the early days of composite decking was that by coating the wood filler material with plastic containing UV inhibitors, the wood would be protected from the two elements that cause fading — sun and water.
Experience has shown that the theory does not work as well as expected. Rather than each tiny wood particle in the WPC mix being coated with plastic, the particles tend to clump together during manufacturing and it is the relatively large clumps that get coated, not the individual particles. Once the plastic coating is penetrated, as, for example, by the wear and tear of walking on the deck, sun and water can reach the wood powder inside causing fading over time.
Add to that the fact that chemicals that protect against UV do not provide either perfect or permanent protection. The chemicals evaporate and deteriorate over time. No UV inhibitor lasts much past five years.
"Cooking" Composites: More Art than Science?
Excerpt from an Article By Craig Webb, Pro Sales Magazine
Creating composites and PVC often is compared to cooking, but lots of chefs would be sorely tested if they tried to whip up a deck board or piece of trim. That’s because, unlike in a kitchen, you can’t count on the ingredients to be consistent from one day to another.
Wood and plastic typically make up roughly 85% to 95% of the ingredients’ total weight. Most of the wood consists of cast-offs from flooring factories. And, except for virgin PVC flakes, all the plastic had a previous life as well. WPC and PVC manufacturers work constantly to get reliable streams of raw product, but if the wood flour has a different mix of species or the recycled plastic shipment was heavier on low-density polyethylene film, the manufacturer may need to adjust its formula.
Technology challenges are no less daunting. Consistently and accurately extruding a product–basically, cramming a hot, doughy mixture of plastic and wood through a specially shaped opening is hard enough. Co-extrusion–pouring a syrupy plastic shell over the inner core in such a way that it bonds exactly where you want at exactly the right thickness–is an added challenge that manufacturers didn’t feel comfortable producing commercially until a few years ago.
It’s not a highly repeatable process,” says Tom Gramlich, chief operating officer of TimberTech, which makes composite and PVC decks, porches, railing, and trim. “Every night, our operator is fighting a different set of circumstances than the night before.”
You can tell how tricky a business this is through the scrappage rate: Industry-wide, manufacturers say, roughly one-eighth of what’s produced gets rejected before it ever leaves the factory.
What is certain is that making WPCs and PVC products requires a careful mixing of ingredients not just for performance, but for cost. One way to do that is by literally trimming away some of what’s produced. It’s a principal reason why a lot of lower-priced composite decking is flat on just one side and scalloped on the other–you use less mix to create a board.
Advanced Environmental Recycling Technologies (A.E.R.T.) has found other ways to limit cost runups. It gets its plastic from recycled goods rather than virgin PVC, and over the years it has learned how to convert ever-lower grades of polyethylene into usable feedstock. (Trex also gets most of its plastic through recycling.) As for wood flour, generally the more finely ground it is, the more expensive it gets. A.E.R.T. uses relatively large flakes of wood. The result, however, is a rougher-looking board than its competitors.
But if you really want to scrimp, ultimately you need to examine your additives – the chemicals that make trim whiter, deck boards stiffer, cores less (or more) foamy, and surfaces less likely to fade in the sun or crumble. Like spices, a little additive can cost a lot. [A]n additive can set you back hundreds of dollars per pound.
Anatole Klyosov, a biochemist and former WPC executive, says several lawsuits this past decade involving WPCs appeared to arise from failure to use enough additives. In one case, he says, a manufacturer didn’t add enough color pigment, thus leading to fading. Another case stems from a lack of antioxidants.
“I’m astounded that we sold as many boards as we did five years ago,” one decking exec says. “Because five years ago, those boards didn’t do what they promised…”
Some manufacturers are very up-front about color fading. ChoiceDek and Trex are among those manufacturers who will show you post-fading colors so you can see what your deck will really look like after a few months. What none will show you is what they will look like after three years.
Testing organizations like Consumer Reports are conducting tests that look at, among other things, color changes over time. Coatings manufacturers like Sherwin-Williams have already finished testing some of the composites, and the results are not encouraging. After three years, most un-capped composites tested were gray or nearly gray (See the sidebar "Exposure Testing" on this page).
Once fading starts, there is no cure but to seal the deck with a sealant that protects against water and UV rays from the sun to prevent further fading. If you want to restore the original color, the only solution is to stain the composite. Almost all uncapped composites can be stained. But, don't expect the manufacturer to advertise the fact — suggesting that composites can be stained implies that they may need to be stained, which is contrary to the advertising claims of most composite deck manufacturers.
Generally, however, buried somewhere in their cleaning and maintenance instructions, manufacturers will recommend a stain for use on their decks. Use the recommended stain. But, if you stain the composite, you have to periodically re-stain the composite. Manufacturers are silent as to how often re-staining may be required, but our experience is that it should be done every 3-7 years; or at about the same schedule as a wood deck.
So, while it is true that most composites do not "need" staining, the choice may be to live with the often ugly faded color of the deck or bite the bullet and stain.
How "Green" Is this Stuff?
On the surface, composite materials look mighty un-"green" with lots of unpronounceable chemicals, some of which can be highly toxic. So, are you trading convenience for some sort of health or ecological disaster when you select composite decking materials? Probably not. But, neither are composites especially environmentally friendly.
On the minus side, all wood-plastic composites use some very eco-unfriendly chemicals. The most unfriendly is plastic, particularly "virgin" or new plastic made primarily from petroleum. The most common plastic binder is polyethylene. While no plastic is environmentally benign, analyses by the Healthy Building Network (HBN) concluded that polyethylenes possess fewer chemical hazards and environmental health impacts than other common industrial plastics such as vinyl (PVC), making them marginally more eco-friendly. (For a complete report on and ranking of the environmental impact of various plastic and composite materials by the non-profit Healthy Building Network go to Guide to Plastic Lumber). But some manufacturers use vinyl (PVC) which is decidedly not green. HBN's impact study concluded that "[t]he manufacture, use, and disposal of PVC poses substantial and unique environmental and human health hazards." But, it has the advantage over polyethylene that it is a stiffer material, less likely to sag or bounce, so it is attractive to composite makers.
To their credit, some manufacturers use a high percentage of recycled plastic — plastic water and soda bottles, milk jugs and those ubiquitous plastic shopping bags are the most common sources. By recycling plastic that would otherwise end up in landfills for many centuries before it deteriorated, these producers provide a valuable ecological service. Quality control problems with recycled plastics have, however, caused a few manufacturers to back away from recycled plastics and use more virgin plastic.
Also on the minus side, all composites use an enormous amount of power to manufacture and the generation of that electricity is a primary source of greenhouse gases. This "embedded energy" as it is called by environmental folks, detracts from the products' greenness.
On the plus side, most composite manufacturers use waste wood products as filler, primarily sawmill waste, but also post-consumer recycled wood products such as used wood pallets. Some companies, such as Advanced Environmental Recycling Technologies, Inc., use very little new wood, and the new wood they do use is such poor wood that it has almost no use other than as firewood or filler for composite decking.
By comparison to real wood, however, composite materials are a distant second on the greenness scale.
Wood, on the other hand, has a pretty good environmental scorecard. A life-cycle assessment of the relative environmental impacts of redwood compared to poly-vinyl (PVC) decking and a composite decking by Dr. Elaine Oneil, Executive Director of the Consortium for Research on Renewable Industrial Materials (CORRIM) found that redwood contributed substantially less to global warming, ozone depletion, smog, acidification, water pollution and respiratory illness than any of the man-made decking products. The study also found that redwood required much less energy to produce and transport than did any of the three artificial decking materials. The composite decking material required 21 to49 times more fossil fuel energy than did the natural wood, and 15 to 33 times more total energy.1
In a comparative life-cycle assessment of western red cedar and composite decking by the environmental advocacy group, Dovetail Partners, Inc.,2 the results were strikingly similar. Cedar contributed substantially less to global warming, acidification, respiratory illness, water pollution, and smog than did the composite decking, used less than half of the fossil fuel energy required by the composite material, and half of the total energy.
The study concluded that composite decking materials have substantially higher environmental impacts in comparison to a naturally durable solid wood product such as western red cedar, and while composite materials may be made completely or nearly so, from recycled content, the overall environmental impact is nonetheless large compared to traditional solid wood decking products.
Treated pine, which is immersed in a copper solution that resists mold and insects, is probably the least eco-friendly of the wood decking materials, but even it outperforms composites. The environmental engineering firm AquAeTer looked at the environmental impact of the two materials at the three life-cycle states: production, use, and disposal, and concluded that treated wood out-performed wood plastic composites at every stage.3 It was particularly frugal with fossil fuels. Composites used 14 times more fossil fuel than treated pine throughout the life-cycle of the products.
So, while 100% recycled composites can be somewhat "greenish", they typically have been found by life-cycle studies to be a lot less green than most common deck woods, even treated wood. And this difference is even more pronounced if the natural wood is harvested from managed forests. (To learn more about how forests are managed and which wood comes from certified forests, take a look at the Forest Stewardship Council).
Most composite manufacturers now shy away from calling their products "green", and instead simply state that they use post-consumer and industrial waste (if they do, some don't), leaving it to you the consumer to decide if the stuff is really environmentally friendly.
Also, keep in mind when choosing between natural wood and wood-plastic composites that wood smoke is toxic, but gases from burning plastics include formaldehyde, phthalates, hydrogen cyanide and dioxins which are very toxic to humans. Good thing decks are outside.
A Lifetime Deck?
No responsible manufacturer now claims its composite deck is a "lifetime deck", although some dealers and installers still use the phrase in their advertising. There are no "lifetime" decks: no lifetime wood or lifetime composite decks. Only aluminum decking has any claim to being a "lifetime" products, and even that's questionable.
When composite deck manufacturers compare their products to wood decks, they always compare to unsealed wood decks. Comparing composite decking to unsealed wood is very much "apples to oranges" because composite decks are sealed at the factory while raw lumber is not sealed until you or your builder seals it. Generally, a sealed and maintained wood deck lasts about as long as a maintained composite deck.
If treated pine is properly sealed and maintained, it will last nearly forever. So, will a composite deck. The difference is not how long a well-maintained deck will list, but how much work is required to maintain it. Wood decks may require somewhat more maintenance over their lifetimes than do composite decks. But, the difference is not very large, and the gap is constantly getting smaller as coatings manufacturers improved their products for sealing and staining decks.
Unfortunately, one place where composite decking will probably last a lifetime is in the landfill. There is so far no recycling for composite decks, and those that end up in the dump will probably be there for about 2,000 years. Treated wood, on the other hand, returns to nature in 10 years or less, and untreated wood in as little as 2 years.
The Disadvantages of Composite Decking
While composite decks have few real advantages over wood decks, they do have some real drawbacks: A high initial cost, heat retention and excessive expansion and contraction.
High Initial Construction Cost
Expect just the cost of composite decking material itself to be a two to five times more expensive than treated pine (ACQ) decking. But, that's not the whole cost of a composite deck. Here are some of the other factors that add significantly to the total cost of a composite deck.
More Elaborate Framing: Composite decking is denser and heavier than wood, but not as rigid. Most composite decking boards are a mixture of wood dust and polyethylene plastic. Polyethylene is a soft, springy, flexible plastic. It needs more support from the decking structure to keep it from bowing and sagging. Deck joists are typically spaced 16" to 24" on center for wood decking, but many composite products have to be spaced 12" on center to achieve the same feeling of rigidity. Most manufacturers, when pressed, admit this problem; and somewhere in the fine print most "recommend" a 12" on-center structure. This extra-rigid structure adds to the cost of the deck.
Proprietary Fastening Systems: Wood decks can be attached to supporting joists with screws through the face of the deck board, or by using hidden fasteners. It's your choice. But, there is often no choice with composite decking. Not all, but a good many composite decks are designed to be attached only with proprietary hidden fastener systems. These can add to the cost because not only are the proprietary systems pricey, but there is about twice as much labor involved in using hidden fastening systems.
High-Priced Accessories and Railings: The price of composite deck boards is very competitive. Producers compete with each other in quality and appearance, but also in price — keeping the price as low as possible for the basic deck board product. But, once you buy a manufacturer's decking, you are probably going to have to buy its accessories and deck railing systems if you want the rest of your deck to match your decking. Here's where they get you. Accessories such as trim boards, nosing, and railing kits are very expensive. These are the profit makers for the composite decking companies, not the decking itself, and manufacturers are not at all bashful in pricing these items.
One serious drawback to composite decking is ignored by everyone except engineers and scientists, and certainly never talked about by composite manufacturers. This is the problem of heat retention. If you live in a place like most of Nebraska with hot, hot summers, you need to be very aware of heat retention.
A deck exposed to direct summer sun gets hot. A wood deck gets hot enough so that walking barefoot is mildly uncomfortable. All composites and plastics get hotter than wood — some get hotter than concrete. It's merely a function of density and color. Dense materials retain more heat and get hotter than less dense materials. In our backyard tests, composites reached Fahrenheit 109° to 111° (Celsius 42.77° to 43.89°) on a sunny summer day where the air temperature was a relatively mild 85° (31.67°).
Wood contains a lot of air pockets and is usually at least 10% water, so it is not nearly as dense as composites.
Dark colors absorb more heat than light colors. In our heat retention tests, dark stained wood was typically Fahrenheit 5° to 10° (Celsius 2.8° to 5.6°) hotter than natural, unstained wood. With composites, the difference was slightly smaller.
This does not mean don't build a composite or vinyl deck on the sunny side of the house, but it does mean either building in a place that does not get much sunshine or providing some shade. If natural shade from trees is not available, consider a pergola, awning or roof. You are going to need it. The other alternative, building on the shady side of the house also has a downside: mold and mildew. These seem to form more readily where the deck does not get direct sunlight.
Expansion and Contraction
All materials expand when heated and contract when cooled. Wood expands and contracts, but very little along the length of the board. Most expansion and contraction in wood is across the width of the board. The movement along the length of the board is so small that it is usually largely ignored as inconsequential. For many thousands of years, carpenters have contended with expansion and contraction across the width of wood boards and learned how to compensate for this wood movement.
Composite decking is not wood. It contains wood, but it also contains plastic, and plastic is one of the all-time great expanders and contractors. A composite board not only expands across the width of the board but also along its length. A lot. The seasonal movement along the length of a 20' composite board may be as much as one inch.
This means that special processes must be used in manufacturing and installing composite decks to account for this un-wood-like expansion and contraction. For guidance, we can look to the experience of the vinyl siding industry that has been contending with the problems of plastic expansion for over a half-century. Many of the techniques of installing composite decks are taken from the vinyl siding professionals. But, there are still many problems that have not yet been conquered.
The consequence to you as the composite deck owner is that the nice, tight, wood-like joints in your deck when it was installed in July may be wide, unsightly gaps in February. And, since plastic does not expand and contract evenly, those gaps may still be there the following July. There is not a lot that can be done about it — it's the nature of the material. Composites may look like wood, but they are not wood, and they don't act like wood. So, don't expect a perfectly convincing wood-like appearance. It just won't happen.
Mold and Mildew
Composite decking that is not protected by a plastic wrapper or capstock seems to have a particular susceptibility to mold and mildew. Trex and Fiberon, major manufacturers of uncapped products, have been sued over this issue in class action lawsuits and had to pay out large amounts of money to affected homeowners.
How mold and mildew get a foothold is well understood. The plastic in the composite mix is compromised, and water gets past the protective plastic and into the wood flour.
This can happen in many ways, but the most common is just the daily use of the deck. After a while, foot traffic on the deck wears off some of the plastic coating that surrounds and protects the wood particles in the composite material, exposing them to the environment. This is the cause of "white-spotting" as the sun fades the particles. It also gives mold and mildew an opening to root in the deck.
Natural wood contains mold-inhibiting chemicals, and treated pine (treated with copper) is possibly even more resistant to mold. Copper, even in small amounts, is anti-microbial — it kills mold and mildew, although no one yet fully understands exactly how the process works. What we do know is that mold, mildew, and most bacteria cannot survive in the presence of copper. In tests on colonies of E. Coli bacteria conducted by the EPA, 99.9% of the bacterial colony was killed after two hours of exposure to copper. So, as long as the copper treatment remains in your treated pine deck, it is fairly safe against embedded mold and mildew.
The wood flour used in composite decking has been chemically stripped of most of the chemicals that are wood's natural protection against fungi, not intentionally, but as a by-product of other processes. So unless some sort of anti-fungal material is added, wood flour can be a very attractive home for mold and mildew. Manufacturers caution in their care instructions that keeping the deck free of pollen and debris that may be food sources for mold is important to assuring a mold-free deck. What they forget to tell you is that the wood flour itself is a food source for mold. So, if it is not protected, or the protection wears off, your composite deck becomes a giant smorgasbord for many species of mold and mildew no matter how clean you keep it.
Cleaning does is knock off the surface mold, but does not touch the root structure embedded deep inside the composite. Tthe mold will quickly spring back up. After lawsuits against major deck companies for embedded mold problems, most manufacturers have begun including some sort of anti-microbial agent in their composite decking slurries, but so far these do not seem to be having much effect. The number of mold complaints does not appear to have diminished
How to Buy a Composite Deck
If despite all the risks and uncertainties, you decide you cannot live without a composite deck, then buy wisely. The key to buying a composite deck is, like any major purchase, research, research, research. Here is the information you will want to have to make your decision… (Continues).
1. Oneil, E. et al, Comparative Life-Cycle Assessment of California Redwood Decking, 2013.
2. Browyer, J; Fernhol, K.; Howe, J.; and Bratkovich, S.; Wood-Plastic Composite Lumber vs. Wood Decking: A Comparison of Performance Characteristics and Environmental Attributes, 2010.