It's full-blown summer now, and people are spending more time outdoors on their patios and decks. So let me offer a summertime question for discussion. Which is better to use for an outdoor deck: wood, or a manufactured product (like Trex, Timber Tech, etc.)?
This is no more a black and white issue than most other green building questions. This can depend on the particular situation and the specific needs of a particular project. I'm not going to give you a definitive answer, because I don't think that there is one, any more than I do for most green building topics (other than greener is better).
First, there is the issue of material content. On the one hand, the manufactured products often use some combination of wood fiber (which is often sawdust and other scrap that would otherwise go to waste) and plastic (sometimes incorporating post consumer recycled material). On the other hand, wood is a natural material. It is sustainable, in that wood can be grown and harvested. There are some deck materials that have natural rot-resistant properties, but these tend to be more expensive. There is also the question of whether or not they are sustainably harvested, as well as the issue of shipping these materials.
Maintenance is another consideration. Wood decks need to be stained and cared for on a regular basis. Even with regular maintenance, a wood deck will wear out over time, and pieces will need to be replaced eventually. Maintenance for the manufactured products is supposed to be lower. However, I have heard of more than one architect I know who put in Trex decks for their own homes and now have some problems with its performance. Other manufacturers are competing in this field now, as well, and there may be better products available.
End of life is another question to be addressed. Making a material like Trex is a downcycling use, and there isn't much that can be done with an old manufactured deck other than to landfill it. Wood decking, if it is untreated, can be allowed to decay naturally. But, if the decking has been treated, it is not going to break down readily. Decking treated with preservatives (whether it was the older CCA — chromated copper arsenate — which is now outlawed because it was leaching arsenic, or the newer ACQ — alkaline copper quaternary compounds — or one of the other current alternatives) is meant to not break down. In the long term, it will eventually decay, but it will be a much slower process. According to a Wikipedia article, "A report published in Wood and Fibre Science (Vol 36 pp 119-128, 2004) concluded that soil contamination due to the presence of CCA-treated wood after 45 years is minimal." In the interim, however, you probably don't want it in your compost.
Finally, the workability and durability of the material factor into the question. Contractors and installers like the manufactured products because they are easier to use. The boards are straight and regular. Warping, twisting, and cupping defects are not an issue. However, many of the manufactured deck boards have a noticably fake appearance, or do not look like real wood at all. I think a case can be made for either product, as long as one takes a careful look at the material sourcing, with FSC certification for real wood or high recycled content for a manufactured product.