For more than half a century, pressure-treated wood was a simple material to understand and purchase. Most of the PT lumber used in residential work was treated with chromated copper arsenate, or CCA. Most of the rest was treated with ammoniacal copper zinc arsenate, or ACZA. Different suppliers used different brand names, but all the wood — CCA and ACZA — looked, handled, and performed about the same.

And the performance was good: While treated lumber might check, splinter, or warp from the effects of weathering, it generally wouldn’t rot or get eaten by termites. This has been confirmed by decades of field experience as well as testing by manufacturers and the U.S. Department of Agriculture Forest Products Laboratory.

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