Swelling & Bowing Composite Deck Boards

Here’s a review of composite decking from Howard B. As you’ll read, his deck isn’t even a year old, and it’s already exhibiting some serious problems:

We have a TimberTech “Earthwoods Evolution” deck at our new home. Within two weeks of installation, we began to see downward “bowing” near butt joints. This is about 7/32″ deep, with the deepest point about midway between the joists (16″ centers). The problem is progressive in the sense that it has spread to more boards, and it is also starting to appear farther from board ends. It is now about twelve weeks since installation…I’m a recently retired engineer, and my own theory is that this capstock material is absorbing moisture through the ends, which is causing swelling. With the boards firmly anchored on 16″ centers, it would not take much increase in board length to cause a significant bow Seems to me that this would be a common problem, but I haven’t yet seen it discussed or shown on-line.

Is My Composite Deck Dangerous?

Mr. B from Oregon says:

The house I am currently renting has a Trex deck on the back which is flaking, swelling, and warping. To date, my landlord has shown no inclination to do anything about the situation…are there any health hazards connected with flaking Trex materials? Should I not let my dog play and rest on the deck?

This can be a little tricky to answer, because there are several different composite decking “recipes”, and the manufacturing companies obviously aren’t eager to share their secrets. Whether or not your deck is poisonous depends on the type of plastic used. PVC or vinyl are known to be toxic.

In my research, I didn’t come across any reports of health issues related to the flaking problem. No doubt there’s some dust that could be inhaled, and that could pose a hazard—but the same could be said of all dust from any product, even natural wood. Of course, wood doesn’t tend to disintegrate like composites do…

Along with the flaking, another common complaint with composite decking is mold. Certain types of mold are hazardous to your health, so that’s something to keep in mind.

My bigger concern would be the deck’s structural stability. If the flaking and warping reached a certain point, I’d worry about the deck collapsing. It’s happened before. If you or your dog are on it when the boards start failing, that could certainly cause some health problems in the form of broken bones.

The True Cost of Composite Decking

Composite decking is expensive—certainly more expensive than low-end woods like pressure treated, and at least on par with high-end materials like ipe. The initial cost of composite material is compounded by the current state of oil prices. Don’t forget that composite is largely plastic, an oil-based product.

But this initial cost is just the beginning. If you’ve read some of the other posts on this blog, you’ll know of all the problems that plague composite decks. These problems led to decks being replaced.

Between warranties and lawsuits, the manufacturing companies will sometimes pay to replace the defective material, but that still leaves the customer high and dry when it comes to labor costs. Additionally, free composite decking only helps if you’re willing to install another composite deck and risk going through the whole ordeal a second time.

In light of all this, you have to seriously consider if composite decking will be worth it in the long run. Wood has centuries of proven performance, while composite decking has a decade’s worth of horror stories. Which would you rather choose?

The Downfalls of Capstock Decking

One of the chief problems with composite decking is the way it absorbs moisture.  This is the cause for symptoms like mold, swelling, and delamination. Capstock decking aims to solve this problem by covering the porous composite with a layer of durable PVC. The PVC acts as a shield, and in theory the solution is a good one.

There’s just one problem. The ends aren’t covered.

End Swelling

This is really unavoidable. Even if the capstock boards were end-sealed during production, the builder would just cut them open during installation. The exposed ends soak up water and expand, resulting in a flared look. The swelling can crack the PVC shell, further ruining the look of the deck. The uneven surface created by the swollen ends can pose a tripping hazard.

Unfortunately, once the board ends have flared, they never go back, even if the deck is completely dried out. This leaves the deck owner in the tough position of choosing between living with an ugly (and potentially unsafe) deck or paying replacement costs.

Structural Compromise

PVC is a fairly strong plastic, but it’s not impervious. When board ends swell, the surrounding PVC shell can crack, further exposing the composite core. More water gets in through these new cracks, and the swelling works its way through the entire board, cracking the shell as it goes. Once the shell is broken, the decking loses much of its strength and becomes more prone to breakage.

Environmental Health

One more thing. PVC isn’t a good thing. According to a study by the Healthy Building Network, “PVC is the worst plastic from an environmental health perspective, posing major hazards in its manufacture, product life, and disposal.”

Common Composite Decking Problems

Composite decking has a long history of problems. These issues continue to arise in spite of the manufacturing companies’ attempts to improve their product. If you install a composite deck, here are some of the things you can expect:


A delaminated composite deck.

This is the result of water getting through the plastic and into the wood fibers. The boards expand and shrink, causing the plastic binder to break down. What you’ll notice on a delaminated composite deck is flaking and, in extreme cases, splitting.


This is caused by exposure to the sun’s UV rays. It’s primarily just an aesthetic problem, but who wants an ugly deck?


Mold spots on composite decking.

This can be a problem with traditional wood decking as well, but composites take it to the next level. The wood fibers soak up water like a sponge, and within months or weeks of installation, dark spots can appear on the surface. These spots can be difficult to remove, often requiring the use of harsh chemical cleaners, and more often than not the mold simply returns a few weeks later.


Swelling around screw holes on a composite deck.

This moisture-related problem frequently occurs around screws and the ends of capstock boards—basically, anyplace where the inside of the board has been exposed. You can avoid screw hole swelling by using a hidden fastener system, but there’s not much to be done about end swelling.

Heat Retention

Composite decking is known for getting very hot in the sun, which poses the risk of burning your feet. Manufacturer’s advice? Choose lighter colors (but what if I like the look of dark decking?) and less dense boards (this strikes me as compromising the deck’s strength). Oh, and never walk barefoot on your deck. That’s hardly a solution; what if you have a poolside deck?

These are all problems presented by composite decking. While it’s true that some of them are shared by wood, composites seem to suffer from them worse. Also, many of the comparisons only seem to take into account lower-end woods like pressure treated, when there are far better wood options available, like ipe or cumaru, that don’t suffer from the problems inherent in composite or pressure treated decking.

Composite Decking Is Unsafe

I’ve talked about how composite decking is made. To summarize, it’s a mixture of wood fibers and plastic.

Now, while the plastic is supposed to keep water away from the wood, imperfect production methods can still let moisture in. This results in a familiar scenario if you’re used to working with wood decking: expansion. The wood fibers absorb water like a sponge, causing the entire board to swell. When the sun comes out and the board dries, it shrinks back.

This expansion/contraction cycle isn’t very forgiving on the rigid plastic component of composite decking, and the result can be anything from ugly—flaking surfaces like the one pictured below—to dangerous. The board can crack down the middle, compromising its durability under regular foot traffic. This deteriorating process is sometimes referred to as delamination.

Delamination on a composite deck.

This problem is so widespread, in 2009 the U.S. Consumer Product Safety Commission issued a recall on composite deck boards. Even more recently, reports keep coming in of injuries due to composite decking failure.

Composite Decking Is NOT Maintenance-Free

When composite decking first came on the scene, this was the feature most cited by its supporters. Because of the added plastic content, synthetic decking wouldn’t inherit all the maintenance headaches caused by traditional wood material. The arguments made sense; plastic wouldn’t absorb water, wouldn’t rot, and would repel insects. Homeowners who were tired of cleaning and sealing their decks were quick to jump on board.

And then the truth reared its ugly head.

The fact of the matter: there is no such thing as a truly maintenance-free material, and composite decking, in spite of its lofty claims, turns out to require more maintenance than some wood decks.

So what happened? While it’s true that plastic is waterproof, plastic makes up only a portion of a composite deck board. The rest is wood pulp, which acts like a sponge. The end result? Mold, and lots of it.

Mold spots on a composite deck.

On the plus side, the mold can be cleaned off (but wasn’t the point to not have to clean the deck?). Composite companies usually sell a special cleaner designed to handle this exact issue. These cleaners are corrosive; they definitely kill the mold, but they also eat away at the surface of the deck, allowing more water to get inside, which in turn creates a breeding ground for more mold…

Mold is just one problem composite decks face. Another is fading. Just like real wood, synthetic decking will lose its color when exposed to sunlight. The solution is the same as it is for wood: staining. Except, wasn’t composite decking supposed to liberate you from the annual hassle of deck staining?

Composite decking companies have changed their stance since the early days, opting to call their products “low-maintenance” instead of “maintenance-free”. But low-maintenance compared to what? Wood? Composite deck owners have reported similar (and sometimes increased) maintenance when compared to pressure treated pine.

Composite Decking’s Short Lifespan

Composite companies boast that their products have extensive lifespans, easily beating out traditional wood decks. There’s just one problem: the comparison is rigged.

Composite decking is sealed and protected from the elements during production. The finished, tested product then has a huge advantage over raw, untreated wood—and that’s exactly what the composite companies are testing against.

Turns out, a deck made of pressure treated pine, if properly maintained, will last just as long as a composite deck. The only real difference is the amount of work you need to put into it.

So, in terms of lifespan, composite decking and pressure treated pine are on roughly even footing. But what about other types of wood? Take ipe for example. Ipe’s been proven to last over 70 years, and it’s comparable to composites in price.

Alright, so a composite deck can be expected to last about 25 years, according to carefully controlled tests. Unfortunately, the real world is anything but carefully controlled. Do a little research, and you’ll find countless reports of composite decks only lasting a couple years.

Composite Decking Is NOT Eco-Friendly

Of the many claims made by composite decking manufacturers, this is among the most cited: their product is a more environmentally responsible option than wood.

This is due to the material’s production. One of the main ingredients is plastic and responsible companies source their plastic from used garbage or shopping bags. The other main ingredient, wood, is collected from sawdust, mill waste, and old pallets. In short, composite decking uses recycled materials.

That sounds great! Except it only addresses part of the eco-friendly problem.

There are essentially two main recycling systems: natural and man-made. Natural recycling is the process of decomposition, when organic matter is broken down and used to create new life. Man-made processes, on the other hand, are what we use to recycle plastic, metal, and other inorganic things that aren’t biodegradable.

A very important thing to note is that these two systems don’t mix well. Plastic won’t get recycled when put through a natural system, and wood won’ be recycled when put through a man-made system. So what happens when you mix wood and plastic together?

Composite decking cannot be recycled. Any sawdust created when cutting composite deck boards: straight to the dump. Scrap wood? More dump fodder. What about when the deck gets replaced? You get the picture. All that material becomes nothing but waste, destined to sit in a landfill forever.

But wait a second, when would you have to replace a composite deck? Isn’t this stuff supposed to last forever?

What Is Composite Decking?

Composite (or synthetic) decking is, essentially, “fake” decking. Rather than using traditional wood, Composites are a mixture of materials.

Samples of composite decking in different colors.

Each composite company’s exact recipe is a closely guarded secret, but synthetic decking generally consists of a filler, binder, and—sometimes—a coating.


The filler’s role is to give the composite decking substance. Usually it’s composed of powdered wood fibers (“wood flour”). Alternative fillers include rice hull powder and wheat straw.


The binder is responsible for holding the filler together and protecting it from the elements. It is made of one or more kinds of plastics, including:

  • Polyethylene – the weakest, but also the most common
  • Polypropylene – stronger, more rigid
  • PVC – strongest, but rarely used due to high cost

The binder is frequently sourced from recycled plastic, but recent quality problems with recycled material has prompted companies to move towards using virgin plastic instead.


Some brands of composite decking have a plastic coating. This is commonly known as “capstock”. The coating is often made of PVC, and is intended to keep moisture out of the wood fibers.