Aluminum decking has several advantages over wood, composites, and pvc (all -plastic). It will definitely last longer and require less maintenance, perhaps no maintenance. Aluminum extrusions also have structural strength that composites do not have. This means you may be able to build your deck with joists that farther apart and still have a stronger deck.
Aluminum decking is an aluminum extrusion with a profile similar to the above illustration. It is very lightweight as well as strong. (This illustration is a close but not perfectly accurate representation. It is intended only to explain the concept.) Aluminum will, of course, never absorb moisture, and it will not warp.
You have the option of choosing an extrusion that will link the “planks” together in a way that makes the deck waterproof. This is not the only type of decking that can keep the space under your deck dry, but it is a natural solution.
And if you are extremely healthy and manage to outlive your deck, aluminum is never wasted in the recycling chain.
There are some disadvantages though. For one thing, it is more expensive, costing up to two times what a composite deck would cost. While the companies that make aluminum decking offer it in a range of pleasing colors, they make no attempt to make it look like wood. It will look like aluminum. It should be no surprise then that you are more likely to see aluminum in a commercial than residential setting.
A new method was developed in the 1940’s that used a chemical mix known as CCA for Chromated Copper Arsenate in a pressure treatment process that pushed the chemical into the grain. This made softwoods such as Southern Yellow Pine resistant to rot, and insects.
In December 2003, the pressure treatment industry in the U.S. stopped using CCA for wood that would be used in residential applications such as decks and picnic tables. Why? The chemicals might be good for wood, but they are very toxic to humans, and as a nation, we are becoming more careful to avoid poisoning ourselves. Ending the use of CCA in lumber was like taking lead out of gasoline and paint. In recent years, wood for residential projects has been treated with more benign chemicals such as Micronized Copper Azole or Alkaline Copper Quaternary (ACQ) or Sodium Borate. I don’t pretend to understand what these chemicals are, but they are apparently much less toxic than CCA.
Not all treated lumber is the same. There are levels of pressure treatment, and they are graded AG for above ground, GC for ground contact, and GCS for in-ground applications. in the meantime, wood is still being treated with creosote and CCA for commercial applications.
Concerns about the toxicity of treated lumber have fueled the growing popularity of alternatives such as composites, PVC, and aluminum. Everything we humans do or make has some kind of impact on the environment. We need to be aware of this and just try to choose the best options. Plastic alternatives to wood have an impact during their production, but then they are inert and stay around a long time. In many cases the material can be recycled.
Despite the popularity of composite decking, pressure treated wood is still very widely used. For one thing it is much less expensive. And, for the most part,composites are not used for the structure of a deck. You need the fiber strength of real wood. (A couple of companies have developed a structural composite, but its use is not widespread.)
Home Depot has a page with good information about treated lumber on their web site. It points out that the ACQ/CA (Copper Azole) treated wood tends to be darker in color and retain more moisture when you buy it, while the MCA treated wood tends to be drier. You need to make sure the wood is completely dry before you apply paint or stain. The way to know wood if dry enough to paint is to drop some water on it. If the water absorbs, the wood is dry enough. If the water beads up and doesn’t absorb, then it needs more time to dry out before you try to paint it.
Home Depot also says that “Ground Contact” as a rating of treatment is generally recommended. Wood rated Ground Contact has twice the preservative level as the Above-Ground rated wood.
Home Depot says “when used properly,” modern pressure treated lumber is “both safe and environmentally friendly.” However they also say that pressure treated lumber should never be burned because of the preservatives, and you should wear gloves, eye protection, and a dust mask when working with it. Screws or nails need to be hot dipped galvanized or stainless.
Over the past 75 years or so, we have gone from giving little or no thought to the risks chemicals pose to health and the environment to …. perhaps… being overly cautious. It is confusing. We have to go with the current advice of the experts and hope we are making the best choices for our families.
Now we have a new problem: how to dispose of old pressure treated wood in ways that are safe for humans and the environment. It turns out to be very tricky issue. You can’t burn it because that releases arsenic into the atmosphere. You can’t dump it in a landfill because that releases arsenic into the soil and ultimately the water table. The only really proper method is landfills with liners. States and local communities have not yet addressed this issue with a consistent set of laws, but regardless of what is allowed or not allowed where you live, it is something to worry about.
There is the option of returning to old fashioned construction and build outdoor projects with untreated wood… then just paint it every couple of years, but not, of course, with lead paint. This is actually my preference if I am using wood in an outdoor application. One reason is that I have some old pressure treated boards and “landscape” timbers at the back of my yard because I don’t know how to get rid of them. I know I don’t plan to purchase any more treated lumber.