While the materials for a composite deck are more expensive than most wood, installation costs are fairly comparable. The same foundation of pressure-treated wood is used for both wood and composite decks, but there are several differences between the way wood and composite decking is gapped and supported.
Note: There is now a plastic lumber, USPL's TriMax, which can be used for structural applications. However, unless accommodations are made to oversize the joists and/or space them more closely, the deck can be bouncy [source: Hardy].
Manufacturer's specifications will guide the size of the gaps required for the composite decking. Gapping (between boards and between the decking and an abutting wall) is necessary to accommodate the expansion and compression that is inherent in composite decking. To determine how many fasteners you'll need for a deck installation, multiply the number of joists by the number of decking boards.
Composite decking is weaker than wood. Installing a composite deck on a foundation originally used for a wood deck will require more joists. You'll also have to adjust the framing if the decking is installed in a pattern, like a herringbone or tile pattern [source: Hardy].
Drainage and airflow are crucial to keeping composite decking from degrading. To install composite decking over a solid surface, you need to build a sleeper system. The surface below the deck needs to be pitched for drainage or outfitted with a gutter system, such as TimberTech's DrySpace Drainage System, designed to drain the water from underneath the decking.
Also, the type of composite decking will dictate certain parts of the installation. For hollow decking, you need to use a picture frame design or a starter strip. The starter strip covers the end of the first piece, hiding the fact that it's not solid. Hollow decking also needs a screw at every junction where a joist meets the decking board [source: Descoteaux].
One final note: While most composite decking manufacturers offer warranties on their products, the warranty will be void if the installation does not precisely follow the manufacturer's directions, because a bad installation can result in a degraded, unsafe deck.
To learn more about composite decking, visit some of the following sites.