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