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Q.Is treated southern pine as strong as untreated southern pine? Can I use joist tables for untreated southern pine lumber to size treated joists?

A.Jeff Easterling responds: The standard CCA treatment process does not weaken southern pine lumber, and span tables for untreated southern pine can be used to size treated joists provided the in-service moisture content of the lumber will not exceed 19% for extended periods. When the moisture content exceeds 19% for extended periods (on decks built near grade, for example), allowable spans are reduced by approximately 5%. Typically, outdoor residential deck construction does not require the use of wet service span reductions.

A complete span table for southern pine, including information on wet-service conditions, is available from the Southern Forest Products Association (P.O. Box 641700, Kenner, LA 70064; 504/443-4464) for $2 plus shipping.

Jeff Easterling is with the Southern Forest Products Association in Baton Rouge, La.