Decking Material: Wood
Wood is a renewable resource that sequesters (removes) carbon dioxide from the atmosphere as the tree grows and locks it away until the wood decays or is burned. Wood can come from Forestry Stewardship Council certified forests or can be reclaimed wood from demolitions. Wood can last many years but it does weather and requires periodic application of stains and sealers. Wood is susceptible to dry rot, termites, splintering, and cracking. When a wood deck is demolished and removed the material can be reused, burned as firewood (unless it is pressure treated wood!), and composted in municipal composting facilities that chip it and turn it into soil.
Decking Material: Composites
Composite decking can range from fiberglass to plastic while most are a mixture of plastic and wood fibers. Composite decking manufacturers claim long-term cost as the main advantage to using their product. Not only do composite decks last longer, but they require far less care. While composite decking may cost more upfront, the lower cost of maintenance puts it ahead in the long run. Decking is available in many textures and colors and never needs to be stained or sealed. Composite decking is resistant to rotting and insects, and it even meets the strict fire codes in California. Composite decks are said to last over 30 years but in reality it is probably less since owners and tastes may change over time.
The Environmental Benefit of Composite Decking
In addition to lasting longer and not needing to be stained and sealed, composite decking manufacturers make several other environmental claims. Composite decking manufacturers such as Trex use recycled materials in their products. Trex claims that much of its plastic comes from 1.5 billion plastic shopping bags, or about 7 out of every 10 in the US. They also don't use any virgin wood, using mill waste and old pallets instead. The plastic and sawdust is combined to form decking that is attractive, long-lasting, and maintenance-free.
The Problem with Composite Decking
What makes it so environmentally friendly on the material-sourcing and manufacturing end also makes composite decking unsustainable on the installation and end-of-life side. When you mix an industrial material (plastic) that is recyclable with a biological materials (wood) that is compostable you get a material that is neither recyclable nor compostable. This is what William McDonough and Michael Braungart call a "monstrous hybrid" in their groundbreaking book "Cradle to Cradle: Remaking The Way we Make Things." As a result, any sawdust or construction scrap is destined for the landfill. Since perfect collection of sawdust is nearly impossible a miter saw will spread plastic/wood sawdust throughout the construction site. If your deck uses 100 twelve-foot boards and you are averaging three cuts per board with a 1/8-inch saw blade you will produce 8 pounds of this monstrous hybrid, a significant portion of which may blow away in the wind. I also estimate at least 100 pounds of scrap that will go to the landfill, even after you make as many composite birdhouses as you can.
The Bottom Line On Decking
What it all comes down to is personal preference and values. The environmental impact of composite decking can be minimized by planning all cuts to minimize scrap and by carefully capturing the sawdust. A composite deck will last longer and doesn't require the annual application of stain and/or seal. On the other hand, a wood deck is made from a natural and renewable material that won't end up in the landfill, but you do have to apply fossil fuel-based chemical stains and sealants.