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Reference design values for Southern Pine lumber are tabulated in the Design Values for Wood Construction Supplement of the 2005 National Design Specification^® (NDS)^® published by the American Forest & Paper Association. Reference design values for untreated lumber also apply to lumber pressure treated by an approved process and preservative.

Reference design values are based on normal load duration and dry service conditions; they must be multiplied by applicable adjustment factors to determine adjusted design values. Adjustment factors for untreated lumber also apply to pressure-treated lumber with one exception – in Allowable Stress Design applications allowing an increase with the Load Duration Factor, C_D, that factor cannot exceed 1.6 for structural members pressure-treated with waterborne preservatives. As an example, common adjustment factors to consider when sizing pressure-treated joists for an exterior application include:

• Incising Factor, C_i – A reduction required for difficult-to-treat species that must be incised to meet AWPA treating standards. This reduction does not apply to Southern Pine because of its ease of treatability.
• Repetitive Member Factor, C_r – An increase allowed when at least three joists are in contact or spaced not more than 24” on center and are joined by a load distributing element such as decking.
• Wet Service Factor, C_M – A reduction required for lumber used under conditions where the moisture content of the wood in service will exceed 19% for an extended period of time, such as for uncovered outdoor decks regularly exposed directly to rain or other sources of moisture. Members that are protected from the weather by roofs or other means but are occasionally subjected to windblown moisture, such as for covered porches, are generally considered dry applications.

For more information about Southern Pine reference design values, refer to SPC’s Southern Pine Use Guide.

The spans shown below in Tables 6 through 8 are the maximum allowable length of a joist in wet service from face-to-face of its supports. Span lengths are given in feet and inches. Loading conditions are expressed in psf (pounds per square foot). Tables 6, 7, and 8 provide common deck live loads of 40, 60, and 100 psf, respectively. Deflection is limited to span in inches divided by 360 and is based on live load only.

These three span tables are excerpted from SPC’s more comprehensive publication, Maximum Spans for Southern Pine Joists & Rafters. The spans were computed using standard engineering design formulas for simple span beams with uniform loads. They include the wet-service and repetitive-member adjustment factors described above.

Decks are built in all shapes and sizes. The basic structural system is generally the same: decking is supported by joists, joists are supported by beams, which in turn are supported by posts. Posts are then connected or anchored to a foundation or footing, transferring loads from the deck structure to the ground. Recommended maximum span for Southern Pine decking is 24" o.c. (2x4, 2x6, R.E.D.). Typical joist spans are listed in Tables 14, 15 and 16 below, incorporating the wet-service factor C_M for adequate structural design in high-moisture conditions. Download PDF version of the Prescriptive Residential Deck Construction Guide from the American Wood Council.

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