AWC’s National Design Specification® (NDS®) for Wood Construction (2001 and later), outlines the following for pressure-preservative treated lumber (similar provisions are provided for structural glued laminated timber):

4.3.13 Pressure-Preservative Treatment
Reference design values apply to sawn lumber pressure treated by an approved process and preservative. Load factors greater than 1.6 shall not apply to structural members pressure-treated with water-borne preservatives.

For structural sawn lumber incised to increase penetration of preservatives, the NDS outlines the following:

4.3.8 Incising Factor, C_i
Reference design values shall be multiplied by the following incising factor, C_i, when dimension lumber is incised to parallel to grain a maximum depth of 0.4″, a maximum length of 3/8″, and a density of incisions up to 1100/ft². Incising factors shall be determined by test or by calculation using reduced section properties for incising patterns exceeding these limits.

The wet service factor, C_M, applies to dimension lumber (including those preservatively treated). According to NDS section 4.3.3:

Reference design values for structural sawn lumber are based on the moisture service conditions specified in 4.1.4. (19% or less for lumber). When the moisture content of the structural members in use differs from these moisture service conditions, reference design values shall be multiplied by the wet service factors, C_M

(Similar provisions are provided for other structural wood products.)

To order a copy of the NDS, which is part of the Wood Design Package, call the AWC publications dept. at 1-800-890-7732 or visit the website.