The Deck Handbook: Part 4
Wood-Plastic Composite Decks
When composite decking first hit the market around 30 years ago; it was trumpeted as the best thing to happen to deck building since decking screws replaced nails. Company advertising promised a new era of nearly indestructible, maintenance free decks that would last your lifetime.
Turns out, it just wasn't true.
Within a very few years significant flaws in the material began to surface. The most common complaints were that it scratched and gouged easily, sagged, and warped, delaminated, cracked and splintered; and developed mold and mildew problems that could not seem to be cured.
Manufacturers have scrambled to cure some of the more egregious problems, and as a result composite decking is getting more sophisticated — and more expensive. But, it may be that the basic concept of wood-plastic composite decking is fundamentally flawed — it simply does not work in a product intended for prolonged outdoor use, and it may be that cannot be made to work given the technologies currently available.
In theory, it ought to work.
Wood plastic composite (WPC) materials are made of a combination of wood powder or dust (which is called "wood flour" in the industry) and a plastic binder. The idea behind this combination is that the wood fiber helps shade the plastic from UV rays that cause the plastic to deteriorate, while the plastic coats the wood particles to protect them from water, mold and insect damage. It should be a perfect marriage.
The practice is getting closer to the theory, but the products are still far from ideal. Wood-plastic composites are very much a work in progress, and like any developing technology have quirks and kinks that have not been worked out.
The basic flaw is that the plastic protective coating is very imperfect, and very fragile. The soft plastics used in most products are not very durable. Ordinary use of the deck wears away the protective coating, exposing the wood particles to sun, water and microbes, especially the fungi that we call mold and mildew.
Trex Settles Product Lawsuits
September 14, 2004
Trex and Exxon-Mobil (the former owner of Trex) have agreed to settle a class action lawsuit. The suit claimed that Trex decking rotted, splintered and degraded, contrary to widespread advertising and marketing claims by the company, and that Trex failed to live up to its warranties against product defects. After initially calling the lawsuit "frivolous", Trex agreed to stop all advertising claims that its deck products do not require sealant and are maintenance-free. Trex also agreed to replace any defective product sold between 1992 and 2004.
Trex settled a similar suit by one of its large distributors in 2000 for fraudulent business practices in which it was claimed that Trex materials tended to "disintegrate, crumble, turn pink, turn blue, spot, bubble, blister, contain lumps, contain hidden defects such as metal objects…, or grossly warp if exposed to sunlight or weather" and that the company failed to honor its warranty. The actual settlement terms are secret.
GeoDeck Products Recalled
August 16, 2005
Proving that even the experts can be fooled, the GeoDeck composite decking products, a top rated composite decking by Consumer Reports in 2004, were recalled for safety issues in 2005.
The Consumer Products Safety Commission announced a voluntary recall of certain GeoDeck products manufactured by Kadant, LLC for dangerously rapid degradation, especially in hot climates. Although no one has yet been hurt, the recall was prompted by several hundred confirmed reports of composite deterioration to an unsafe condition. Kadant, LLC is a subsidiary of Kadant, Inc. Kadant sold the assets of its composites business to Liberty Diversified Industries. Kadant, LLC used the money received from the sale to pay warranty claims and has reportedly paid out $4.6 million on its defective GeoDeck product. When the money ran out in September, 2009, it simply ceased business and shut down its recall web site.
EON Declares Bankruptcy
January 7, 2009
CPI Plastics Group Limited, the maker of EON decking, announced that it has been unable to withstand the ravages of the current economic crisis and was seeking relief in bankruptcy. Its CEO and all of its directors have resigned. The company is being liquidated under Canadian bankruptcy law.
Trex Sued For a Third Time
January 13, 2009
Trex has been sued again for warranty violations. In the lawsuit, a Washington homeowner claims his two-year old deck began degrading, cracking and rotting and is now unusable and unsafe for his family. The homeowner claims Trex offered to replace the damaged boards but refused to pay for associated labor costs. In exchange for the new materials, Trex asks for a signed release freeing the company from further liability. The lawsuit alleged Trex's warranty documents violate the federal Magnuson-Moss Warranty Act, a consumer protection regime.
After first responding that it had fully honored its warranty, Trex settled the lawsuit in 2010, agreeing to replace any defective materials and paying part of the labor cost of replacing the defective products.
Trex Sued Yet Again
February 13, 2009
In a class action filed in California, Eric Ross, et al. v. Trex Company Inc., et al., Case No. 3:09-cv-00670-JSW, SD CAL, 2009 plaintiffs alleged that Trex manufactured and sold defective decking materials that were susceptible to surface flaking, mold, mildew, fungus, or other spotting and discoloration. Trex agreed to pay $8.25 million to settle the case, which, as the Court admitted was just a fraction of the actual cost of repairing the damage. Trex, as usual, admitted no liability.
Louisiana-Pacific Decking Recalled
May 13, 2009
The Consumer Products Safety Commission with the voluntary cooperation of Louisiana-Pacific Corp (LPC) has recalled composite decking materials made by the company and sold under the trade names LP WeatherBest®, ABTCo, and Veranda®. The recalled decking can prematurely deteriorate and unexpectedly break. Consumers can fall through broken decking and suffer serious injuries. LPC has received 37 reports of composite decks breaking, resulting in 14 injuries.
CorrectDeck Sheds Warranty Liabilities in Bankruptcy
July 2, 2009
Correct Building Products, LLC, manufacturer of CorrectDeck, a composite decking product line, has filed for reorganization in bankruptcy and a quick sale to GAF subsidiary Building Materials Corp. of America (BMCA). Once completed, the CorrectDeck product line will be added to GAF's newly formed Decking Systems business division. The sale, under Section 363 of the Bankruptcy Act allows sale of "distressed" assets free of liens and warranty claims. (Editor's Note. GAF renamed the product Duralife Siesta, then discontinued making the decking entirely in 2011.)
Fiberon Settles Class Action Lawsuit For Mold and Staining of its Composite Decking
March 5, 2013
Fiberon, then organized as Fiber Composites, L.L.C., was sued in a class action (Fleisher v. Fiber Composites, LLC) for staining and mold build up in its non-capstock composite decking. The suit claimed that the decking contains manufacturing defects that caused "extensive mold, mildew and other fungal growth" that manifested itself as a uniform spotty discoloring across the entire deck, and that Fiberon failed to honor its warranty. Although Fiberon denied the allegations, calling them "completely false" and stating that the mold and mildew is caused by improper homeowner maintenance, it nonetheless agreed to a settlement in which it pays compensation to owners of its products sold under various brand names. The number of customers involved has been estimated at 150-200,000. A final settlement order was entered on March 5, 2014.
GAF sued over defective DuraLife and Elk Cross Timbers composite decking
March 4, 2015
GAF, former manufacturer of the DuraLife and Elk Cross Timbers brands of composite decking which exited the composite decking business entirely in 2011, selling DuraLife back to its original owners, now organized as Integrity Composites, L.L.C., has been sued by 30 named defendants in U.S. District Court in New Jersey. The Omnibus Amended Master Class Action Complaint alleges that the composite decking is defective in that it cracks, warps, swells and expands beyond what is reasonably acceptable, harbors mold, mildew and fungus regardless of proper installation and maintenance resulting in unsightly discoloration and spotting; all contrary to GAF's representations that the decking was resistant to sagging, warping, rotting, scratching and wear and tear, even under heavy traffic; did not require staining or sealing, was easy to install, low maintenance, long-lasting and would outperform wood decking.
In 2016 the defendant agreed to compensate deck owners for damages, including a fixed sum for the labor to repair the decks.
Green Tree Composites, LLC Settles Class Action
Green Tree Composite, makers of Monarch Decking products, settled a class action lawsuit in which plaintiffs alleged that Green Tree sold defective decking and then refused to honor claims, stating that it was no longer in business. Greentree settled for $470,000. (Stolzenburg v. Green Tree Composites LLC., Case No. 4:14-cv-1533, ED MO, 2014)
The earlier, very wide gulf between consumer expectations and actual product performance has been narrowed somewhat, not so much by making the products better, but also by scaling back the advertising rhetoric and outlandish claims.
No composite manufacturer now advertises its products as "never needs maintenance" or "never needs sealing". Trex made these kinds of claims early on, but was sued when its decking faded, deteriorated and stained, and had to agree to never again say such things in its advertising (see Sidebar: "Trex Settles Product Lawsuits").
Composite decking manufacturers now acknowledge that their products will fade and change color, will stain, and may need a periodic re-sealing. But, after all the improvements in composite deck products, and there have been many, they are still having a hard time justifying their relatively high purchase price for the very marginal improvement over wood decking that some, but not all, offer.
You will pay at least 75% more for composite material over standard treated yellow pine. If you select a high-end capstock material, the premium might be as high as 400%. For less than this price, you can have a hardwood deck of Ipe (Brazilian Walnut) or another luxury, exotic wood with many times the durability of plastic or composite decks. The premium price of composite decking is worth it only if the materials actually do reduce the time and cost required to maintain the deck.
The question is, do they? Do composite materials perform well enough to justify their higher cost? In this section of The Deck Handbook, we're going to find out. We want to know what actually works and what is just hype. Have these materials finally become so much better than wood decks that their relatively high price is worth it?
Let's take a look.
What Is Composite Decking?
Wood-plastic composite (WPC) is not new. In fact, the technology is well over a century old.
The world's first true thermosetting plastic, Bakelite (or Baekelite) was a wood-plastic composite developed in 1907 by Leo Baekeland, a Belgian-born American chemist living in Yonkers. He was looking to improve the durability of wood by impregnating it with a synthetic resin. He developed a resin that he called polyoxybenzylmethylenglycolanhydride. When combined with a wood powder filler a hard, moldable material resulted. It had "1,000 uses" — or so Baekeland predicted.
Its first use was as an electrical insulator, an application at which it excelled because it was non-conductive, could be formed into any shape, and was heat resistant. During World War II it was used in everything from warplanes to mess hall trays. (The ugly brown trays, and matching ugly brown coffee mugs, are familiar to anyone who served in the military up to about 1975. Bakelite was extremely durable and could handle abuse by unwilling soldiers condemned to KP — which is why the damn things seemed to last forever. They are still available and outside the U.S., used widely.)
Early radios often had Bakelite cases, and those ubiquitous black table model telephones distributed in their millions by the Bell System up to the 1960s had Bakelite handsets, which were lighter and more comfortable to hold than earlier cast steel models.
Hoover upright vacuum cleaners made after 1935 were encased in a Bakelite cover that gave Hoovers their iconic look. Bakelite applications, in fact, far exceeded Baekeland's predicted "1,000 uses", including 78 rpm records, jewelry, toys, camera bodies, guitars, pistol grips, appliance cases, chessmen, poker chips, billiard balls, coffins, medical equipment, kitchen utensils, tool handles, dishware and saxophone mouthpieces, to name just a few. By Baekeland's death in 1944, bakelite was already being used in over 15,0000 products.
Today's composite decking boards are made about the same way as Bakelite. They are composed of a filler, usually powdered wood fibers ("wood flour") that make up the bulk of the board, a plastic binder, and a variety of additives that facilitate manufacture, stabilize the plastic and help protect the material from UV damage.
The binder glues the mix together, protects the filler from the elements, and gives the board its rigidity. The plastic used as the binder in today's composite decking is a little more advanced than Baekeland's polyoxybenzylmethylenglycolanhydride1, a resin derived from phenol exposed to formaldehyde. The plastic binder in composite decks may be any of a number or petroleum- or natural-gas-based plastics.
One 2003 study identified the types of plastic used in WPC as follows:2
High-Density Polyethylene (HDPE) is a soft plastic commonly found in shrink wrap, milk jugs and other disposable containers, garbage bags and plastic toys. It is used by 83% of all composite decking boards.
Polyvinyl Chloride (PVC) is used in building materials, artificial turf, furniture, shoes, credit cards, sports equipment, raincoats, luggage and shower curtains. It is a little tougher than HDPE, wears longer and is more difficult to scratch and mar. It is used in 9% of deck boards.
Polypropylene is used for packaging tape, rope, carpets, thermal underwear, reusable containers and auto parts. It is the binder in 7% of composite boards.
Other plastics are used is less than 1% of decking boards. These include binders made from bio-oils rather than petroleum or natural gas.
Unfortunately, all of these are relatively soft plastics, unlike polyoxybenzylmethylenglycolanhydride which is very hard.
Polyethylene is the most common plastic in the world, and one of the most versatile. It is used to make grocery bags, milk jugs, and those ubiquitous plastic water bottles. It is manufactured in many forms with varying properties, including PEX, a cross-linked variety used to replace copper in water pipes, and ultrahigh-molecular-weight polyethylene (UHMWPE) used to make ballistic (bullet-proof) vests.
But, the most ubiquitous forms are high-density polyethylene (HDPE), used to make shrink wrap, milk jugs and other throw-away containers, garbage bags and plastic toys, and low-density polyethylene (LDPE) used in grocery bags, film wrap, and throw-away plastic bottles. These are the compositions most used in composite decks. Unfortunately, while strong, the plastic is not rigid and not very wear resistant. How hard it is to scratch a plastic toy? Not very, and in a composite mix, the plastic does not get any more resistant to wear and tear. Polyethylene is most commonly used in its high-density form (HDPE), but can be mixed with less expensive low-density polyethylene to save cost. LDPE has even less structural rigidity than HDPE.
Polyethylene is the world's most recycled plastic, and many composite deck manufacturers use at least some recycled polyethylene. Unfortunately, after being combined with wood flour in a composite mix, is it no longer as recyclable. Some German WPC manufacturers offer to recycle their own decking materials, but no U.S. manufacturer does so.
Poly-vinyl Chloride (PVC)
Poly-vinyl Chloride (PVC) is used as a binder and as the cap material in capstock boards (see below). It is more rigid than either of the other two common plastics, and more resistant to wear. It is degraded by UV from sunlight so UV-absorbing additive must be included in the composite mix. The common fungi in the Aspergillus family also degrade PVC. Aspergillus is a common constituent of the mold and mildew that infect composite decks, which is one reason it is important to keep a composite deck free of mold and mildew.
PVC is a problematic material with known health and environmental risks. Its principal component is chlorine, a chemical used to kill microbes in swimming pool water and to make poison gas, but chiefly used to make PVC. PVC is the only plastic, and the only major building material, made from high volumes of chlorine gas. Vinyl production consumes more than 40% of the chlorine gas produced in this country. That is the largest use of the gas in the world. By comparison, only 5% of the nation's chlorine gas is used to disinfect water — including wastewater and sewage treatment.
PVC is about 50% chlorine by volume, and it is the chlorine that makes PVC hazardous from cradle to grave. It is dangerous to manufacture and continues to be a bio-hazard throughout its lifespan. It out-gasses chemicals nearly constantly, including dioxin, a potent carcinogen for which there is no known safe dose. (That chemical smell that fills the air when you open a new vinyl shower curtain is partly dioxin combined with up to 107 other organic compounds, most of them not at all good for you.)
In 2007, the Green Building Council (USGBC) released its report on PVC which concluded that "risk of dioxin emissions puts PVC consistently among the worst materials for human health impacts."
Polypropylene is a strong, fairly rigid plastic, less susceptible to expansion and contraction, and is the binder of choice in some higher-end composites, although it is, like polyethylene, not particularly wear-resistant. Its other common uses include packaging and labeling, rope, carpets and thermal underwear; reusable containers, and automobile parts. It is, however, very affected by UV radiation from sunlight, which degrades the plastic, causing cracks and crazes that become more severe as exposure continues. UV-absorbing additives such as carbon black must be used with polypropylene to protect the material from the effects of ultraviolet.
The filler gives the composite decking material its strength and rigidity. In most composite decking the filler material is a powdered wood cellulose called "wood flour".
Wood flour is not just finely ground wood particles. It is heavily modified from natural wood. The lignin and other components of natural wood are almost completely removed in processing, leaving just the cellulose in a very small particle form.
Most often a fairly random mix of wood species is used — after all, once the wood is reduced to a flour, it makes little difference whether it started life as a premium hardwood or scrub pine. Oak and other high-tannin woods are, however, generally avoided because tannin tends to stain. Some manufacturers use a specific species of wood.
The wood filler makes the material stiffer and reduces expansion, but generally does not make the board any stronger than a pure plastic board. If you find the notion that a material can be stiff but not strong a tad confusing, think of a glass shelf. The glass is stiff, but not strong enough to hold a heavy load. A WPC plank is stiff enough not to sag under its own weight when installed per the manufacturer's instructions, but will not hold nearly as much load as natural wood without deflecting or breaking.
A wood-plastic composite board will expand and contract less than an all-plastic board, but much more than a natural wood board. And, while wood expands hardly at all along the long axis of the board, WPC does. So excessive gaping between board ends can be a problem.
Wood is naturally hygroscopic, It likes water. Over several million years of evolution, wood has evolved to absorb and store water whenever it can. A tree's ability to draw in water helps ensure its survival. It's good for the tree, not so much for a deck.
In a deck board composed partly of wood flour water is a problem. Water absorption can cause swelling, deformation, mold built-up and other undesirable results. As a consequence, some manufacturers are actively looking for alternative filler materials that do not absorb water but still provide sufficient stiffness and expansion control.
Rice hull powder is one such material. Rice hulls are plentiful, cheap, relatively lightweight and don't absorb water. They also result is a material that is more impact-resistant — a plus for decking that usually takes a considerable beating in ordinary use.
Natures Composites located in Torrington, Wyoming uses locally abundant wheat straw as the filler in its TerraDeck decking. According to a company spokesman, wheat straw is stronger than wood powder, more water resistant, and does not contain starches or proteins that may be a food source for mold. The company does have some rather impressive data from the University of Wyoming that says wheat straw filler is marginally stronger and more resistant to impact damage than wood-flour composites, But, it is no more resistant to deflection caused by seasonal expansion and contraction, and like all composites, special installation measures are needed to control and disguise expansion and contraction.
Asian WPC manufacturers are experimenting with bamboo fiber as an alternative to wood flour filler. As bamboo is one of the most environmentally friendly materials, bamboo fiber in WPC is being heralded for its "green" advantage but does not appear to have any other advantages over wood flour. A study by Jing Feng3 et al. published in 2014 found that wood and bamboo fillers absorbed water at about the same rate, and are equally likely to provide a medium for mold growth. Other natural materials such as flax, jute, hemp and kenaf fibers have a strength to weight ratio higher than fiberglass, which is encouraging experiments to see if they can be used to add structural rigidity to composite boards. So far, however, there are no commercial WPC boards using these materials. They are still experimental.
Capstock Decking — Plastic Coated Composites
If most of the problems associated with composite decking result from water getting into the filler through small chinks in the binder, why not solve the problem by enclosing the whole thing in a plastic shell? Plastic is impervious to water, so problem solved.
That's the thinking behind the newest incarnation of composite decking — the capstock or capped decking board.
Manufacturers, seeking to counter the historical problems with unprotected wood-plastic composites have begun wrapping a solid plastic shell or "cap" around a WPC core. The resulting plank is known in the industry as a "capstock board", or just "capstock". The waterproof shell is intended to protect the wood content of the interior filler from moisture.
Almost all composite manufacturers now offer a premium capstock board. The process of binding the plastic capstock to the underlying wood-plastic composite material is very new. It is much too soon to tell how it will work over the 20- or 30-year expected lifetime of a deck. All we can say at this point is that it has promise. But, so did uncapped composite when they first came out, and that promise has proven largely false. The tradeoff with capstock is that the material is much more costly than uncapped composites. The co-extrusion process is a more complex and expensive technology, and a capstock board uses much more plastic. Plastic is expensive, so the more that is used the higher must be the price.
To work well, the plastic cap must enclose the entire board. It doesn't. The manufacturing process used, called co-extrusion, coats the top, sides and (maybe, but not always) the bottom of the board, but not the ends. The board is extruded as one continuous length. To get individual boards, the extrusion is sliced every 10 to 20 feet. The sliced end is not then capped. And, if it were, when the board is installed it will be re-cut to the needed length by the installer, so again, the unprotected core WPC would be exposed.
The Capstock-PVC Paradox
A capstock composite board is made of wood-plastic composite (WPC) wrapped in a plastic shell.
The thin plastic shell is intended to protect the vulnerable WPC from the elements. But, it is only partly effective. The WPC is still exposed at the cut ends of the board, so the WPC can still get wet, swell and permanently deform the board (See the main article).
We can get rid of the WPC, however, by making the plastic outer shell thicker with some internal bracing so the shell does not collapse when walked on. With these changes, the rigidity provided by the WPC is no longer required — the plastic shell provides its own structure.
Without the WPC we also…
Of course, if we modify our capstock board like this, we end up with plastic decking. Plastic decking is capstock without the internal filler. Which begs the question: why buy capstock decking when you can buy a PVC deck board for about the same price and have many fewer problems?
By the way, the capstock guys are trying to keep this on the QT. So, mum's the word.
After studying the behavior of capstock decking, Shane O'Neill, founder of Compositology LLC, a technical consulting firm focused on composite building materials reported in Deck Builder, the magazine for deck professionals, that the unprotected ends of capstock boards can swell and deform when the board is exposed to water, and the deformation is permanent. He wrote:
“With capstock decking, you have sealed a WPC - which wants to absorb water - in a protective wrapper. This offers many advantages, but the uncapped ends of the boards are free to pick up water just like before. Since only the ends of capstock decking take on much water, that's where the swelling happens. I've measured the water absorption rate through the ends of a capped WPC and found it to be more than six times higher than through the cap. Unfortunately, the real kicker is that once the decking flares, the flare never fully goes away. Even if you completely dry the deck, the swelling may go down some, but the board will never be the same.”
The reaction of capstock manufacturers to the problem of capstock end flare has been largely denial. The response we received from a representative of Tamko, the manufacturer of EverGrain composite decking is typical: "We have not had any reports of deformation at the ends of our boards."
Once the companies move beyond denial (which, based on the history of the industry, will not happen until someone is sued), hopefully, they will come up with a solution. Right now, installers are trying their own fixes, including sealing the board ends with sealants designed for end grain on wood boards — at best a stop-gap measure.
How quickly will the relatively soft and very thin plastic outer covering of a capstock board wear out from normal use of the deck?
Astoundingly, no one knows.
The capstock coating is not just a water seal and a pretty surface that has all the nice woodgrain embossing and "genuine" wood coloring, it is also the wear layer. It is where all the wear from constant walking on the deck takes place. The coating is paper thin and the plastic mix used as the coating is not very resistant to scratching, and other damage from regular use.
The Wearability of Vinyl Tile
Because there is no information about the wearability of vinyl capstock coatings, we looked at its closest analog, vinyl flooring, to get some idea of how long capstock can be expected to last.
Vinyl flooring is of two types. The high-end product is called Luxury Vinyl Tile (LVT) and is composed of several layers. Each layer has a particular purpose. The tile in the illustration at right has five layers, two of them wear layers. Some LVT tiles have more, some as few as three.
1. Wear Layer: The wear layer gives the tile an easy clean surface that does not need waxing and provides stain and abrasion resistance. It is usually polyurethane and may include silica beads and aluminum oxide for even greater wear resistance. Some vinyl tiles, including this one, now include added UV or durability layers on top of the wear layer for even more surface protection. The thickness of wear layers can vary from 4 mils (a mil is 1/1000 of an inch) on economy tile up to 40 mils on high quality commercial tile.
2. Printed Layer: A high definition printed layer that gives the tile it color and pattern. In this case, simulated wood grain similar to that which appears on most capstock. It may also be embossed to give the tile texture.
3. Core Layer: The spongey vinyl layer that gives the tile its resiliency or "bounce".
4. Base Layer: The vinyl body of the tile usually formulated to add stability to the tile and to adhere well to the mastic used to attach the tile to the floor.
So, in better vinyl tile the wear does not take place on the vinyl itself but on the polyurethane wear layer. The lifespan of this type of tile is from 15 to 30 years.
There is a second type of vinyl tile made without a separate wear layer — the color layer doubles as the wear layer. It's called Vinyl Composite Tile (VCT) and is considered low end vinyl flooring — not especially popular because it requires periodic waxing to maintain its appearance. The life expectancy of this tile, with proper maintenance, is 10-15 years.
VCT is the closest analog to capstock decking. Capstock, like VCT, does not have a tough polyurethane wear layer protecting the vinyl capstock layer. There is no essential difference between the two products. With VCT its fresh, new look is restored by periodic waxing. No capstock manufacturer suggest that its decking be waxed, so expect scratches and mars to dull the deck over time. There is nothing in vinyl capstock that suggests it will wear longer than VCT, so expect a lifespan of between 10 and 15 years, less is heavily trafficked areas.
For comparison, a well maintained treated pine deck has a life expectancy of 20 years or longer.
But, despite the known risk of wearing through the capstock shell, there is no information about how long it will last on a deck board. None, zero, zip zilch, bupkus..
The standard tests that decking materials must go through to be certified for use under building codes were designed for wood decking for which wear factors are known. Wood does not require extensive wear testing, so wear testing of capstock shells has not been done.
Or, perhaps some manufacturers have done it, but they are not sharing, which is in itself a troubling notion. If the results were good, you can bet they would be heavily advertised. Manufacturers talk extensively about how realistically their capstocks emulate natural wood, and how little their colors fade over time, but not a single manufacturer talks about how long its capstock lasts in normal use.
So, at this moment all we know about the wearability of capstock decking is that we don't know anything about the wearability of capstock decking..
But, we do know that when the surface cap material does wear through — and it will — there is no repair. The only solution will be to replace to boards, assuming they are still available at that time. In any event, if the capstock does wear through, you will end up footing the whole cost of replacing it. Why? Because, it's not a defect, it's ordinary wear and tear. Sorry!