The Deck Handbook: Part 8 Vinyl and Plastic Decking

Battling wood-plastic composite decking for market share are all-plastic decking products. These have quietly come on strong in the past dozen years with little of the publicity or bad press of the wood-composite products. As of this writing, all-plastic decking is generally superior to wood-plastic composites but this superiority comes at a price. Plastic decks tend to be more expensive than wood-composite decks.

Plastic decks have several advantages over wood-plastic composites. First, and foremost, since they contain no wood products in any form, the usual wood-destroying problems with swelling and cracking when the wood products get wet is eliminated. Further, they resist fading at least as well as the best capstock composite decking because the capstock is almost always some form of plastic, and plastic, with the right UV protection, has good fade resistance.

But, there are also disadvantages. The most important of these is seasonal expansion and contraction which is most pronounced in an all-plastic product. Plastic decking can be made to look somewhat like wood but it cannot be made to behave like wood. Expansion and contraction is the number one structural problem in plastic decks and requires special care and extensive experience to install the material correctly.


By far, the most common plastic used in plastic decks is polyvinyl chloride (PVC or vinyl). But PVC is expensive, and increasingly recognized as a bio-hazard throughout its life cycle — dangerous to manufacture, out-gassing hazardous chemicals throughout its lifespan — including dioxin, a known carcinogen — and is nearly impossible to degrade when its useful life is over. In consequence, decking manufacturers are experimenting with less expensive, more benign plastics. Perhaps the most promising is high-density polyethylene (HDPE), the plastic used to make milk jugs and plastic bags. It is cheaper and much more environmentally benign but also softer and less rigid.

Polyvinyl Chloride

Polyvinyl chloride or PVC is a synthetic plastic made from either petroleum or natural gas and is the third most common plastic after polyethylene and polypropylene. In its unmodified form, it is a white, brittle solid and has no commercial use. To be useful, chemicals that make it more flexible and softer called "plasticizers" must be added. The most common of these are esters of phthalic acid, called phthalates, of which there are over 25 in common industrial use. Many of these are linked to health concerns in humans, including asthma, allergies, breast cancer, hormonal disruption, elevated blood pressure and diabetes in children, and birth defects. Phthalates are not a problem when they are inside PVC. But, they don't stay inside the PVC and over time out-gas into the atmosphere. As they leach out, PVC becomes de-plasticized and eventually returns to its original brittle state. The extensive use of PVC means that quite an amount of these plasticizers are now loose in the environment, and showing up in human tissue, which has some health authorities concerned.

PVC is also frequently modified by additives to make it more impact, scratch and fire resistant, and less affected by changes in temperature. Chemicals to kill or retard bacteria and mold, and pigments are also common additives, as are UV absorbers. In exterior applications, it is particularly important that PVC is protected from ultraviolet light (UV), which tends to degrade the material very quickly without UV protection.

PVC is not well regarded by the eco-conservation crowd. It is not at all "green". Not only is it made from a non-renewable resource but its manufacture throws off some very nasty chemicals including monovinyl chloride, known to cause cancer, and uses massive amounts of energy.

PVC is used in the construction industry for siding, window frames, and exterior trim because it is relatively cheap, durable, and easily worked. It has become an increasingly popular choice for deck components because of its low maintenance requirements and ability to withstand the ravages of sun and water. Vinyl planks used as decking typically are embossed with treads or graining that enhance the look of the deck, maximize traction and disguise scratches and other normal wear and tear. Without them, vinyl is slippery and quickly shows scratches and dings.

Mineral-Added HDPE

One of the problems with PVC as the preferred material for plastic decking is that it is relatively expensive when compared to other plastics. High-density polyethylene (HDPE), the plastic used to make milk jugs and plastic bags, is cheaper but also softer and less rigid.

It is used frequently as the binder in wood-plastic composite decking, which often makes these products cheaper to produce and therefore less costly than all-PVC plastic decking.

Some plastic deck makers are experimenting with cores made of HDPE, both to reduce cost and to come up with a stronger product. The usual filler additive includes some sort of calcium carbonate. The technology was pioneered for the plastic bag industry where ground calcium carbonate had long been used as a low-cost filler in grocery bags to reduce cost, cut production time and add impact strength.

Manufacturers of modified HDPE deck boards include Engineered Plastic Systems (Bear Board and Lumberrock). These deckings are made of at least some recycled HDPE reinforced with one or more unidentified "minerals", probably calcium carbonate in some form, and, as one industry observer remarked "crushed rock". The manufacturer claims that the mineral additives increase strength and durability while decreasing expansion and contraction. However, none of these products have been on the market long enough to have a track record. So, we are in "wait and see" mode.

Other Compositions

Other plastic compositions have been tried. NyloBoard made NyloDeck out of recycled nylon carpet fibers and a plastic resin binder. An interesting idea. The board looks much like a typical non-capstock wood-plastic composite board but since the filler is plastic, not wood, it cannot absorb water and does not have the degradation and mold problems of the wood-composites. The product did not find a market, however, and ceased production in 2016.


There are two types of plastic decking: "cellular" and solid. "Solid" is a misnomer of sorts. The plastic is usually formed into a hollow shell to save weight and reduce costs. For this reason, it is also called "hollow plank". Until the turn of this century, all deck planks were solid plastic hollow planks. Several companies still make hollow plank decking but more and more manufacturers are turning to cellular plastics which seem to produce a better decking.

Cellular Plastic

Microcellular plastic or, more commonly, "cellular plastic" decking is the newcomer. It is barely ten years old. It is thought to be an improvement on solid vinyl decking. It looks and sounds more like wood, so homeowners like it, and it has the workability of wood that deck installers appreciate. Since its advent, the hollow plank plastic board is nearing extinction as manufacturers switch to the new technology.

Cellular plastic is formulated so that it contains air pockets or cells. The cells in the foam core are very small, on the order of 10 microns on average — so small that to the naked eye, the material looks solid but it is, in fact, primarily air. The foam core greatly increases the rigidity of the board but keeps its weight down, and uses only a fraction of the material of solid vinyl, thereby keeping the cost down. In the most common manufacturing process called "mono-extrusion", a lightweight plastic foam core and harder, more solid skin are formed at the same time. The skin is, however, thin and somewhat brittle. It is prone to cracking and splitting under sharp impact and when screws are driven into the material.

It has other problems as well. Users report that it can be stained by bug spray, suntan lotion, some food products and contact with rubber. Some products experienced excessive shrinkage, surface chalking, and warping. All of these issues sent composite chemists back to the laboratory to come up with better formulations.

Capstock Plastic

One solution is to co-extrude a solid plastic wrapper around a cellular plastic core. The wrapper or shell is called a "cap" in the industry and the resulting board is a "capstock" board. The solid plastic skin that may be a different color from the core. This is how manufacturers add darker colors, tonal variations, and deep wood-grain to plastic decking. Tonal variations are used to make the plastic look less like plastic and more like stained wood. Some of it is getting pretty realistic, if you don't look too closely.

Capstocking cellular plastic with more plastic may seem rather like overkill but there is a reason for it. Plastic is particularly susceptible to UV degradation. Without UV inhibitors most plastic will literally fall apart within a few years. The most common, and most cost-effective, UV inhibitor is titanium dioxide, a white pigment. The need to keep titanium dioxide at the surface of the plastic was one of the reasons that solid plastic decking could be made only in white or lighter colors.

Capstock has changed the picture. Wrapping unprotected plastic foam in a capstock containing titanium dioxide means that the UV inhibitor is guaranteed to be at the surface of the relatively thin wrapper, so deeper colors can be added to the material without fear of UV degradation.

Capstocking also reduces the cost of often expensive additives needed to make plastic more scratch and fade resistant. They need be added only to the thin wrapper, and not to the foam core. With some additives in the several-hundred-dollars-per-pound range, this is no small concern to manufacturers.

Capstock has also made it possible to add realistic stained wood coloring to vinyl, including variations in tone and shade, to the extent that vinyl decking is now often difficult to distinguish from wood except at very close range. (By the way, vinyl guys, embossing vinyl with woodgrain merely makes it look like embossed vinyl — real painted wood seldom shows woodgrain. It shows brush strokes if anything).

The clear advantage of foam core vinyl products over composite wood decking, even capstock composite decking is that the foam core cannot possibly absorb water, so there is no risk of the degradation that occurs when water gets under the capstock of wood-plastic composite boards.

The capstock coating is not just 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. 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.

The standard tests that decking materials must go through to be certified for use under building codes are designed for wood decking for which wear factors are known, and do not include 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. In any event, no one knows how long the capstock will last, and if it does wear through, there is no cure but to replace it, and the cost of replacement is yours. Wear-trough is not a defect covered by any warranty. It is ordinary wear and tear.


Plastic decking boards can be attached to the structural deck frame like any other type of decking material. Most of the time, direct screws through the board are used, although care must be taken not to compress or split the board. Manufacturers now provide stainless screws that are also color-matched to the new board colors. Vinyl deck boards can also be attached with hidden fasteners. Hidden fasteners eliminate puncturing the upper surface of the decking boards and give the deck a cleaner look and reduce the likelihood of water penetration. These fasteners are typically attached from the side of the board or from the bottom up and are generally more expensive and require more labor.

Solid plastic decking is often made with a nailing flange, similar to that incorporated into vinyl siding. The leading edge of the plank snaps into the prior plank. A fastener, usually a nail but it could be a screw, is used to then attach the flange to the wood joist of the deck. This provides reliable concealed attachment without the labor involved in using hidden fastener systems. Some vinyl planking has an even easier installation system. It is installed using an aluminum or vinyl track that allows the planks to be easily snapped into place — a definite plus for the do-it-yourselfer. The trade-off is that this decking is usually more expensive.

Vinyl is not usually strong enough for deck handrail posts. What looks like a vinyl post is often a wood post covered by a vinyl wrapper. However, recently manufacturers have started offering railing kits that feature posts strong enough to comply with the national building codes. These do not need wood reinforcement. For short lengths, vinyl will work as a railing and vinyl balusters are sufficiently rigid to stand on their own.

Considerable attention must be paid to alleviating the effect of thermal expansion and contraction during installation. Plastic decking manufacturers minimize the problem. Azek, for example, offers the following caution in its installation instructions:

"The expansion and contraction encountered may result in slight gaps, which might be noticed at the ends of boards or in splice joints."

In fact, gapping can have much more than a slight impact on the appearance of a deck. According to one end-gap calculation, a 16' long PVC deck board will expand up to 1/2" from Summer to Winter in the Mid-West. This means unsightly gaps at board ends during cold weather. HDPE moves so much that most manufacturers recommend hidden fasteners which allow the board to slip a little. Otherwise, it can bind and bow. Using Lengths over 13 feet are not recommended.

In short, plastic is not wood and while it can be made to look somewhat like wood, it cannot be made to behave like wood. To get the advantages of plastic decking you will have to accept the disadvantages which include unslghtly gapping with seasonal changes in temperature.

Benefits and Disadvantages of Using Plastic Deck Components


Plastic should be your material of choice if its higher initial cost is tolerable, and you want the lowest level of lifetime maintenance possible with today's commercial materials. However, using unconventional materials, it is possible to build a more durable, low maintenance deck at a cost much lower than that of plastic decks. See Deck HandBook. Part 11: The (Almost) Maintenance Free Deck.

Rev: 05/02/18

Rev. 05/20/18