This post is not meant to be a complete tutorial on Pressure Treated Lumber.
A complete presentation of all of the information is left better to manufacturer’s websites or like this one from the Southern Pine Council. I just want to introduce you to the fact that not all pressure treated lumber is the same. It seems most people think that “pressure-treated” is “pressure-treated.” The reality is that not only are there several different “grades,” of pressure treated materials, there are different types of chemicals that can be used to treat the wood as well.
One thing that will be true of them all is that each piece of lumber will have a tag stapled to it that explains what it is.
The tag will tell you how much preservative is in the material, what type of preservative was used—-along with a bunch of other information—-including the year it was treated and where.
It is important for the installer/builder to be familiar with what all this labeling means so that the correct materials get used for the right job.
The Grade of pressure treatment refers to the “amount” of preservative that is required to be forced (under pressure) into the piece of wood—-per board foot. These percentages run from 2.5 ounces per board foot for pilings used in salt-water (marine) applications, to .25 ounces per board foot for some deck railing and surface installations. When you get up into the higher number of ounces per board foot there are only certain species of wood that will actually accept that much preservative—-Southern Yellow Pine for example. (Don’t tell that to the NW timber industry however.) For all its other great characteristics, Douglas Fir is lousy at being pressure treated and Hemlock is likely not much better. For most purposes, wood that is going to be in contact with the ground will have a saturation level above .40 ounces. This is very typical of most deck structural posts, beams and joists. For wood meant to be buried—-as in Permanent Wood Foundation Systems (PWF’s)—-we want to see .60 ounces per board ft.
The problem is that because most people—especially “Weekend Warriors”—-don’t know the differences between all these grades of treated wood, we end up with the wrong grades being used in the wrong places. Another factor is that materials designed to be buried are only available at real lumber yards. You are probably not going to find them at Weekend Warrior hunting grounds like Home Depot, Lowes, or OSH.
Here is a picture of your garden variety .40 Hemlock pressure treated 6×6. Because the treatment of this wood “species” (at this grade) can’t penetrate all the way to the center of the wood—-the center just rots away.
So when you are using pressure treated lumber—make sure you have the correct grade and that the weekend warrior has good information.
Charles Buell, Seattle Home Inspector
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