Companies that manufacture composite decking boards make them by creating a mixture of small wood fibers and plastic polymer, such as polyethylene or polyvinyl chloride. They dye the mixture and then form it into 2-inch by 6-inch decking boards, often patterning the surface to resemble a wood grain. The resulting hybrid looks like wood but has the flexibility of plastic. It will support weight if reinforced from underneath by decking joists, but it is unsuitable for structural purposes. You can cut and paint it, but sanding it will remove the surface pattern, and most manufacturers recommend against this.
Composite decking is not subject to UV deterioration and won't crack or shrink. Installing it is comparable to installing wood boards, and, in some cases, the boards come pregrooved so you can install them with hidden fasteners and avoid surface nails or screws. Manufacturers typically supply a 25-year warranty against wear and fading, and although composite decking is not maintenance-free, it never has to be refinished and requires less maintenance than wood boards. Most composite decking is made from recycled materials, so composite decking in serviceable condition represents a reduction of plastic waste in landfills.
The wood fibers used in the manufacture of composite decking tend to absorb moisture, causing the boards to expand in moist conditions. Moreover, the moist fibers can promote the growth of mold, which can grow and cover the surface with unsightly and toxic black spots. Composite wood boards will sag under their own weight if not properly supported from underneath, which is a disadvantage if your deck has widely spaced joists. Finally, the environmental advantage of using composite decking is nullified once the boards are worn out. They cannot be further recycled and usually end up in landfills.
No matter how closely composite decking boards resemble wood, they retain an element of artificiality that, in some circumstances, may not be a disadvantage. Despite the claims of manufacturers, however, time will take its toll on their appearance, and your options for restoring it are more limited than for wood boards. Whereas you can power-wash and refinish wood decking, most manufacturers don't recommend power-washing composite boards because the pressure from the spray tip can gouge the surface. If the color fades or the surface becomes excessively dirty, you usually have to paint composite decking.