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Frequently Asked Questions

Does pressure-treated lumber have the same design values as lumber that is not pressure-treated?

AF&PA'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, Ci
Reference design values shall be multiplied by the following incising factor, Ci, 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/ft^2. Incising factors shall be determined by test or by calculation using reduced section properties for incising patterns exceeding these limits.

Table on Incising Factors

The wet service factor, CM, 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, CM

(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.

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