What are the dash marks in the outdoor lumber in phoenix for?

[Colleen F.]

Ok so in RI we use pressure treated wood a lot outdoors for decking and such,  but here in AZ desert area you seem to use dimensional lumber for everything. I know you don't need pressure treated in the dry areas.  However, the wood for decking and stairs etc has these little dash marks in it. They obviously put them there for something but I have been looking and can't find an explanation for what this is called and why they do it.  I think it may have something to do with drying but just throwing it out there for the construction experts. Anybody have an explanation?

[Jim K.]

@Colleen F.

When you buy pressure-treated dimensional lumber in Arizona, although it might come out of the same mill (it doesn't, usually), it is typically not the same wood species and it is typically not kiln-dried to the same moisture level as it is in Little Rhody.

The wood species that deal best with the higher levels of dryness are more resistant to the pressure-treatment process compared with others. The most common way to get the solution in on those species is to punch, or incise, those little slits in the lumber.

The stuff they use to pressure treat the wood was historically also different in the incised timber. Not too long ago, the two main solutions were CCA (chromated copper arsenate) and ACZA (ammoniacal copper zinc arsenate). The CCA produced the familiar green tint we know all about in the Northeast. It didn't penetrate some species well, so the ACZA solution was used instead on that wood. ACZA tends to turn the wood more yellowish with blue spots and only a hint of copper green. So a lot of the ACZA (western) wood was further dyed brown, liked Erik B.'s picture. This situation lasted for many years. The CCA typically worked better than the ACZA, the ACZA was widely considered safer than the CCA.

Fast forward to 2004, when the EPA banned CCA. There are other chemicals that are used, but it's mostly ACZA now. But since it doesn't produce the same pronounced green tint, there are a number of mills that are now adding a green dye to AZCA-treated PT wood sold in the northeast.

Just like Florida oranges ripen green and are soaked in Citrus Red No. 2 to be able to better compete with California oranges in the marketplace.