Southern Pine 2x4 Lumber Downgraded
It's official: effective June 1, Southern Pine #2 2x4 lumber
has been downgraded. Testing of randomly sampled 2x4s from
sawmills across the Southern Pine-producing region has shown
that the strength of the lumber has declined significantly
since the official design values were published several decades
ago. As a result of the new testing, official "design values"
for Southern Pine #2 2x4 wood have dropped by about 30%. Other
dimensions and grades are currently undergoing sampling and
testing; it appears likely that when this testing is complete,
the design values for larger dimensions and higher grades of
Southern Pine will likewise be decreased — by how
much, is yet unknown.
Coastal Connection has been following this story
closely, keeping in touch with the agencies involved, as well
as with industry organizations and academic researchers who are
studying the issue. It will take years for the full
implications of the Southern Pine strength downgrade to play
out across the construction industry. For now, here is a quick
Q&A on the important questions raised by this event.
• What are design values?
— Design values are published by official lumber
grading agencies based on randomized statistical sampling and
testing of lumber produced for market. Design values describe
the key characteristics of wood as an engineering material:
strength in bending, density, and tensile strength. The
published design values for all grades and species of lumber
are the numbers that engineers use to design wood structures.
Design values also determine the allowable spans for floor
joists and rafters, developed by the American Wood Council
(AWC), which are published in span tables and referenced by the
prescriptive code to guide the construction of buildings where
case-by-case design engineering is not required.
Because wood is a highly variable material, design values
are based on average strength measured in testing of many
individual samples. The design values are extremely
conservative: they are based on the tested value of the weakest
5th percentile of the wood. In other words, 95% of the wood in
the marketplace in any grade can be expected to perform better
than the published design value for that grade.
• Why is Southern Pine wood getting weaker?
— Not all Southern Pine lumber has gotten weaker. But
the average strength of Southern Pine lumber has gone down. In
particular, the weakest 5th percentile of Southern Pine lumber
in the marketplace has become weaker. This appears to be the
result of changes in the way Southern Pine is grown, harvested,
and sawn. Southern Pine is mostly produced in plantations,
rather than sawn from trees in naturally occurring forests.
Plantation growing practices have resulted in a high proportion
of young trees which were grown rapidly for frequent harvest,
with more weak "juvenile wood" and fewer annual growth rings
per inch. In addition, sawmill technology has changed in recent
decades to allow production of lumber from logs as small as 6
inches in diameter. As a result, a greater proportion of 2x4
lumber on the market now shows lower values for bending
strength and tensile strength than in decades past. However,
some Southern Pine, such as wood harvested from naturally
reproducing stands in the National Forest, is still as strong
Note: Forestry experts saw this coming a long time ago. For
a 1991 report from USDA Forest Products Laboratory's David
Kretschmann about weakness of Southern Pine sawn from
16-inch-diameter, 28-year-old trees, see:
Tensile Strength and Modulus of Elasticity of Fast-Grown
Plantation Loblolly Pine Lumber," by David E. Kretschmann
and B. Alan Bendtsen.
• Are other species of wood also getting
— Apparently not, but the jury is still out. After
testing of Southern Pine #2 2x4 lumber showed a surprising
decline in strength, the American Lumber Standards Council
(ALSC) Board of Review instructed grading agencies for all
other species of lumber in the U.S. market to conduct
comprehensive testing of all sizes, grades, and species of
lumber. Results of that testing for S-P-F #2 2x4
(Spruce-Pine-Fir), a lumber produced in Northern and Western
regions, have been reported — and the testing
indicates no decline in strength for S-P-F wood. In the coming
year or two, updated values for all species, sizes, and grades
will be reported. If the grading agencies see a change in
strength for any type of lumber, design values for that lumber
will eventually be changed.
• Will the change in Southern Pine design values
affect the building codes?
— Eventually, but not right away. The
Building Components Association (SBCA), an organization of
wood truss manufacturers, wrote a formal letter to the
International Code Council asking if the design value change
would result in an immediate change in building code
requirements in the field. On May 10, Tom Frost, Senior Vice
President for Technical Services at the ICC, responded: "The
ICC has made no determination regarding the lumber design value
reductions, the Board having denied the American Wood Council's
(AWC) request for an emergency amendment at this time." But
Frost repeated the comment of the ALSC Board of Review from
January 5, 2012: "All design professionals are advised in the
strongest terms by the Board to evaluate this information in
formulating their designs in the interim period."
Even if the ICC were to adopt an emergency amendment to the
International Residential Code (IRC) and the International
Building Code (IBC), that change would not take effect in any
states or cities unless the local authorities endorsed the
change. Wrote Frost: "Please note that section 2.4.3 of CP #28
states, in part, that €˜Emergency
amendments to any Code shall not be considered as a
retro-active requirement to the Code. Incorporation of the
emergency amendment into the adopted Code shall be subjected to
the process established by the adopting authority.'" (For more,
ICC Indicates Hands-Off Approach Regarding SP Design Value
Changes," SBC Magazine.)
• Does this mean the buildings I've already framed
with Southern Pine are unsafe?
— No. Nobody is predicting any kind of structural
failures as a result of this issue. Conventional wall, floor,
and roof framing is very conservative, with multiple members
sharing the loads and with many factors of safety built into
the span and load tables. There have been no failures reported
of wood in service related to this issue, as far as we are
• What about wood trusses made with Southern
— Trusses are an example of value-engineered design,
where the strength of a single truss chord or web member could
affect the performance of the entire truss or the entire roof.
However, even truss systems benefit from the redundancy of
using multiple trusses to share a roof or floor load. In any
case, manufacturers have had at least six months to adjust
their designs to reflect the new values for Southern Pine.
Truss makers now should be using the new design values in all
their engineering calculations.
• Should I adjust my framing practices to reflect
— That's a judgment call. If your building includes
any design engineering, your engineers should always use the
latest published design values. If
you€˜re already in construction on a
project framed with Southern Pine, with an engineered design
that used the old design values, you may want to have your
engineer re-evaluate those designs. If the building is designed
using the prescriptive code and the current span tables based
on the old Southern Pine design values, your building still
complies with code until such time as your local jurisdiction
changes its code.
Wall framing is extremely conservative, and will likely not
be affected greatly by this change. Roof and floor framing may
be affected more significantly. The biggest concern would be
with rare cases where a single member is relied on to handle
some sort of critical load — for example, in the case
of a knee brace on a deck frame. In those situations, using
larger dimension lumber, doubling up members, and carefully
selecting lumber with close grain spacing, straight grain, and
no knots, would all be good practice.
If you want to make allowances now for the changing design
values, there are updated, more conservative span tables
available, but they have not yet been incorporated into the
building code. The American Wood Council has published addenda
to its 2012 Wood Frame Construction Manual, as well as its
recommended amendments to the building code span tables, at
Codes and Standards Addenda and Amendments Related to Design
And remember: if you're framing a structure or building a
component where it's important to have full assurance of the
lumber's actual strength, you can always use machine stress
rated (MSR) lumber. MSR lumber is mechanically tested piece by
piece at the mill, rather than visually graded. MSR lumber's
performance is much more predictable and more uniform, making
it ideal for engineered applications or critical uses.