... in some circumstances.
(But you may want to take a look at this before you get too carried away.)
|Is CCA OK?|
As fascinating as the pressure treating process is
not, it's generally not the defining factor for the piece of wood in question. As I have discussed Ad nauseam, (pun INTENDED) what makes the product decay resistant is not usually the process but the chemical used. (The recent exception to this is thermally modified wood where the process IS the preservative...but more on that later ) Wood can be "treated" with any number of chemicals. CCA is but one of a long list. Chromated Copper Arsenate, is the active pesticide used to poison wood and protect it from biological decay. The government of Canada blah blah blah on CCA treated wood is extensive. However, the "need to know" bit for contractors is that CCA has been voluntarily withdrawn by the industry as a whole for primarily residential applications. Oh it's still commercially produced and available for applications where it won't come into human contact. This includes PWF or Pressure Treated Wood Foundation applications, common in areas where structures are built to float over top of permafrost. So, that leaves a bit of a grey area surrounding plywood. Generally speaking, when pressure treated plywood is used it will not come into direct human contact in it's permanent application. Despite how awesome the idea is, you wouldn't build a residential deck out of treated plywood. Treated plywood applications in residential installations are very limited and almost always embedded behind cladding, roofing or some other membrane. It's used to create decay resistant roof assemblies like parapet walls and such. I have, in the not too distant past, seen it specified to be used as replacement wall
sheathing in multi family buildings that are being repaired due to water
and moisture ingress aka The dreaded "leaky condo's". More frequently, however, it is used to create a rain screen cavity behind exterior cladding.
So given these three very limited residential applications of CCA treated plywood it wood the risk of exposure by humans to arsenate is very minimal. Except in for one thing...
While looking for some information on the uses of pressure treated lumber I came across this article that talks about the dangers of CCA treated lumber. Or, more specifically, the ultra toxic smoke that is created when the stuff burns. The short version? There are ongoing concerns about the possible effects, namely increased risk of cancer and neurological problems, of treated wood fire smoke inhalation. Bottom line is that the stuff is nasty and you don't want to be anywhere near a fire involving CCA pressure treated wood. Common wisdom is that people generally run away from any structure that is burning so the staying away from the smoke is implied. For everyone, that is, except fire fighters. That's the cross over. That is who is most at risk for exposure to the toxic smoke created from burning CCA treated lumber and it's a big concern.
Specifiers have a responsibility, when recommending the use of CCA treated lumber and plywood, to think about all of the potential impacts that their choice has. The benefits must be weighed against the drawbacks and obviously every case is different. So with many drawbacks and available alternatives to CCA treated wood available why would anyone specify it? well, for starters it does have some advantages. For instance, it's less corrosive to metal than the ACQ alternative. That means that, even in moist service environments, the metal fasteners, (nails screws etc) used to hold the structure together will last longer before succumbing to the degenerative effects of rust.
So when looking at the risk to reward associated with specifying and using CCA treated lumber the ratio of treated to non treated wood in a structure is highly relevant. Sheathing and building an entire wall in CCA treated wood to prevent decay is definitely an effective strategy but would considerably raise the risk of unintentional exposure to the toxic elements say during an apartment fire. However limiting the use of CCA treated plywood to the small volume of strips that make up the rain screen cavity or critical elements in a parapet roof for instance limits the exposure to risk of toxic smoke while at the same time limiting the risk of premature fastener failure in these assemblies that are under constant shear.
So the reasons NOT to use cca treated plywood strips (high human toxicity) in a rain screen assembly are outweighed by the advantages of using them (lower risk of structural failure). Whereas the disadvantages of sheathing an entire wall with toxic materials outweigh the smaller advantages of using them when other alternatives are available.