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How to identify wood treatment levels
Don’t be surprised when you see different treatment levels on pressure treated wood that are supposed to resist rot equally well. The numbers depend on the chemical preservative used in the treatment. Every treated board carries a label. Check it to find the type
of preservative used. The preservative retention for both CCA- and
ACQ-treated lumber is .25 for above-ground applications and .40
for ground contact. CCA use has been reduced, so you’re more
likely to find ACQ when buying treated lumber.
However, the preservation retention is different for another
common treatment, called CA-B. The CA-B treatment level of .10
corresponds to an ACQ of .25 and a CA-B .21 to an ACQ of .40.
When a project calls for a certain preservation rate, it’s usually
for ACQ unless otherwise specified. Lumberyards don’t always
carry each type of treated wood at each level of retention. The cost
for a .25 ACQ-treated deck board is about the same as for a .40, so
some lumberyards and home centers don’t want the hassle of carrying
both. Instead, they often carry only ACQ .40.