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Pressure treated lumber is specified for many exterior applications because of its resistance to insect damage and fungal rot. But how does it compare to untreated wood, mechanically?

For example, consider a rim joist supporting the ground floor of a residential structure with a pier and post foundation. If the joist has been damaged by rot in a location that is impractical to fully protect from exposure, I might be tempted to replace the joist with a pressure treated member of the same nominal dimension (in addition to proper flashing) for added protection in that location.

Since this is an existing structure, by far the easiest approach is to use a member with the same dimension to replace the rotted joist. However, this relies on the new member meeting the same load-bearing requirements as the old member.

Building codes should provide enough wiggle room that in this particular example, there's not much of a safety concern for the homeowner. After all, the rotted joist had not failed, and it would definitely have less strength than the member was originally rated for. In practice, treated and untreated members may be manufactured from different wood species with different mechanical properties to begin with; for the purposes of this question, assume the species is constant.

Does pressure treatment result in a member with more or less strength in tension, compression or torsion? Does it affect the durability of the wood* in ways not related to rot or insect damage?

* Not the fasteners; that's a different issue that's pretty well-covered online. See this page from Simpson, for example.

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  • $\begingroup$ Anecdotally I have never noticed a difference in mechanical strength whether the wood was pressure treated or not. I'd say that species and selection of the particular piece can have a much greater effect on mechanical properties. For example structural select versus some random piece will make a big difference, and so will oak versus eastern white pine in both strength and density. $\endgroup$
    – Olin Lathrop
    Commented Mar 7, 2015 at 20:33
  • $\begingroup$ @OlinLathrop Absolutely; for example, the pressure-treated stock I've seen locally is Hem-Fir while the common wood is either Doug fir-Larch or SPF (stronger and weaker, respectively, than Hem-fir). It's the process itself I was interested in here, though. $\endgroup$
    – Air
    Commented Mar 7, 2015 at 22:33

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Pressure treatment does have a small, but documented effect on the strength of the member, particularly if it is 'incised' (has slots or holes cut into it as part of the pressure treating process.) If you're working to American codes, according to the American Wood Council, pressure treated wood is limited to a maximum duration factor of 1.6. This wouldn't matter for your example of fixing a house, because the duration factor would already be much smaller. Effectively, this just restricts the use of pressure treated lumber for resisting impact loads.

More importantly, if it is incised which is often the case, the NDS has a derating factor of 20% for the strength of the member except for compression perpendicular to the grain, which is not derated at all. Note that there is also a 'wet service factor' for any member that will be exposed to moisture, regardless of any treatment.

Pressure treated lumber intended for structural applications should have a research report that covers usage, labeling, and special instructions. These often have useful information. For example, here are the research reports for two random brands. (Warning: PDF links.)

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  • $\begingroup$ Exactly the sort of answer I was looking for, thanks. That derating factor for the incised variety is larger than I expected. $\endgroup$
    – Air
    Commented Mar 7, 2015 at 22:19
  • $\begingroup$ To be clear, it is the incising that causes the strength reduction per the code. I have heard that some species of wood are not incised while preserving even though it isn't as common. $\endgroup$
    – hazzey ♦
    Commented Mar 8, 2015 at 0:38
  • $\begingroup$ As I understand, the maximum duration factor applies regardless of incising, but yes, the strength reductions are only for incised lumber. I think the practice of incising depends on the kind of treatment and size of member as well as the species. $\endgroup$
    – Ethan48
    Commented Mar 8, 2015 at 1:08
  • $\begingroup$ @hazzey SYP doesn't require incising for good penetration (see this document, p. 2). Not very commonly used in this part of the country, though. $\endgroup$
    – Air
    Commented Mar 8, 2015 at 4:34
  • $\begingroup$ "...except for the compression perpendicular to the grain." Doesnt this mean its negligible in a rim-joist application? $\endgroup$
    – user5509
    Commented Mar 16, 2016 at 16:02

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