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Different Types of Composite Decking Boards
Composite decking is made up of a mixture of wood particles and plastic which is then heated to make the plastic soft and pliable. It is then pushed through a die in a process called extrusion or pressed in a mold to make the board shape. Early versions of composite decking had some problems. It was, after all, a new concept and therefore a work in progress. Despite being mixed with molten plastic, typically high density polyethylene (HDPE) from plastic grocery bags and milk bottles, the wood content still absorbed moisture and caused warping and other problems. In recent years, the process has been dramatically improved to produce boards that have the qualities we look for including moisture resistance and stability. Each company has chemical engineers who have developed unique formulas that are proprietary and certainly well guarded secrets, so we can't know exactly what differentiates one company's product from another's. A number of companies have improved the formula to an extent that they can offer "uncapped" decking with great color, wood texture, moisture resistance, scratch and stain resistance, etc.
One solution to the moisture issue has been to "cap" or "shield" the composite with a thin layer of HDPE or other plastic. Then it would be the plastic skin that would have the coloring, wood grain texture, stain resistance and so on. This solution is not universal. Some composite decking is capped and some isn't. The illustration above shows capping on all four sides.
Some composite decking is capped on three sides. One theory is that leaving the bottom uncapped, the composite will have a chance to dry out if it does absorb moisture. Some companies offer a premium version that is capped on all four sides and a less expensive version that is capped on three sides. A disadvantage of three-sided capping is that only one side can be up. That is really not much of a disadvantage because you only need one side to be up. Four-side capped decking or uncapped decking in some cases is promoted as being reversible (either side up), the advantage of which is that you can vary the appearance for a more natural look… not every board exactly the same.
Most composite decking boards are available in both a "solid" profile (above left) or a "grooved" profile (above right). The advantage of the groove is that it accommodates a hidden fastening system (see illustration below).
A number of the composite decking companies have their own proprietary hidden fastener. Trex may have been the first with its Hideaway® clips. Other companies provide grooved boards knowing that contractors can purchase appropriate clips from a variety of sources. The illustration above is oversimplified to show the concept. The fasteners not only hold down the boards; they also automatically set a proper gap between the boards (usually 1/4"). A metal c-shaped clip is needed at the edge of the first board. The fasteners are often formed of a special reinforced plastic, but can also be made of steel. An open deck requires a gap between the decking boards for water drainage. The exception is when the open deck is one level above a patio, and the homeowner wants the patio below the deck to be dry. Then the deck needs to serve as a sort of roof with a water barrier system and have a slight slope so water can run off the side of the deck. Trex has developed a proprietary system for this.
For decades…you could say for generations of builders, the way to attach decking to the 2x6 joist below was to drive long screws through the decking into the joist, using two screws every 16 inches where the decking lined up over the joist. Not every builder or do-it-yourselfer wants to use hidden fasteners, so the solid option for composites is available for those who want to drive screws (no one uses nails anymore) through the decking. A problem that showed up early on is that regular deck screws cause a sort of raised ring of plastic around the head of the screw. This problem has been ingeniously addressed by a specially designed screw.
Another variation of a composite decking board is the "scalloped" profile, shown above in three variations. The scalloped board has less material providing the dual advantage of lower weight per board foot and lower cost. Veranda® brand decking, for example (center example above), which is made by Fiberon and sold exclusively by Home Depot, is an economy brand.
Many of the companies that make composite decking boards also offer a "porch" board. While decking board is typically 5-1/2" wide, porch board is generally more narrow at 3-1/2" wide. The other difference is that porches, by definition, have a roof over them and are therefore not getting rained on or covered with snow. That means that porch boards are laid down with no gap between them. The "tongue and groove" profile shown above is popular for porch boards.
Note: All of the illustrations on this page were created exclusively for this web site.
