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Treated Lumber

Pressure treated lumber is lumber that has had a chemical preservative forced into it. It is rated according to its “retention level.” What does “retention level” mean? It is the amount of preservative that remains in the wood after the pressure-treatment is complete. The number refers to the pounds of preservative per cubic foot of lumber.

Treated lumber has different levels of treatment to it, and the levels listed depend on the type of chemical preservative used. Two different types of treatment – CCA- (Chromated Copper Arsenic) and ACQ- (Alkaline Copper Quaternary Compounds) have preservative retention levels listed as .25 for above-ground application and .40 for ground contact lumber. CCA is growing less common so ACQ will probably be found more readily.

Another common treatment known as CA-B (Copper Azole) has a treatment level of .10, which corresponds to the .25 for ACQ, and CA-B .21 corresponds to ACQ’s .40 rating.

Most lumberyards or building supply stores won’t carry each type of treated lumber at each retention level. So if a particular type is needed, special ordering may be necessary. The good news is that because the cost of ACQ-.40 and ACQ-.25 are about the same, supply stores carrying only type will generally carry the higher rated product.

Most roofing projects require above-ground ratings. The use of ground contact rated lumber often requires corrosion resistant fasteners such as copper and stainless steel. However, since copper is the primary compound most pressure treatment chemicals, most standard fasteners may be prone to corrosion. Consult the roof system manufacturer for clarification on which fasteners and types of treated lumber they recommend.

For more information on treated lumber, visit Simpson Strong-Tie’s website at



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