Q. I use a lot of southern pine lumber, and I've heard that the design values for the material are about to change. How will that affect its use for things like framing or building decks?

A.Cathy Kaake, senior director of engineered and framing markets at the Southern Forest Products Association (SFPA), responds: Based on recent tests of visually graded 2x4 lumber, the Southern Pine Inspection Bureau (SPIB) made several changes to its 2002 rule for grading southern pine lumber. The new rule reduces some design values for bending (Fb), tension parallel to grain (Ft), compression parallel to grain (Fc), and modulus of elasticity (E and Emin). Included in this change are the four main species of loblolly, longleaf, shortleaf, and slash pine, as well as mixed southern pine. The SPIB didn't study the reasons for the change in strength revealed in the tests, but it may be due to changes in forestry management practices.

So far, the change affects only 2x2s, 2x3s, 2x4s, 3x3s, 3x4s, and 4x4s in No. 2 and lower grades. The revised design values will become effective June 1, 2012.

There will be no change for studs based on the IRC or the Conventional Light-Frame Construction section of the IBC, but species-specific tables for resisting wind loads will be updated. Span tables will also be updated, but because 2x2 through 2x4 material is seldom used for joists or rafters, that's unlikely to affect many users.

Truss manufacturers are expected to incorporate the new design values into their products between now and the June 1 effective date. It's likely that more trusses will now be made from machine-stress-rated (MSR) lumber and machine-evaluated lumber (MEL), neither of which are subject to the revised design values. The transition period is also designed to allow code officials to make necessary code amendments and prepare for enforcement. Because building codes may be enforced on a state, regional, or local level, exactly when enforcement begins in a given area will vary.

In general, though, projects permitted before June 1 should be able to proceed under the current values. According to standard code-enforcement practices, the design value in effect at the time a project is permitted (rather than when a project is completed) should govern.

New design values for other grades and sizes of visually graded southern pine are expected to follow testing of larger-dimension stock - including No. 2 and select structural 2x8s and 2x10s - that is now in progress and should be completed before the end of 2012. The SFPA will publish revised joist and rafter span tables after the new design values are available. More changes in design values probably lie ahead as other North American lumber species also undergo similar testing. You can find additional information at southernpine.com.