Q: I want to make a curved band joist for a deck by cutting multiple kerfs in the back of a treated 2x10. Do all of those kerfs need to be field-treated with preservative?
A: The staff of JLC responds: According to American Wood Protection Association (AWPA) technical references (awpa.com), all cuts and holes in treated wood should receive a field-applied preservative, and here’s why. When wood is treated commercially, the liquid preservative is forced into it through the surface toward the center. So the amount of preservative is greatest on the surface and decreases toward the center. Also, the heartwood of otherwise “permeable species” doesn’t normally accept preservative treatment. Given those two scenarios, there might not be any preservative at all in the center of some boards. When treated wood is cut or drilled, areas with lower preservative retention (or none at all) are exposed and subject to attack by decay fungi or insects. Field treatment of cuts and holes re-establishes an envelope of preservative protection so that the wood will last longer.
Building codes say that field-cut ends, notches, and drilled holes in preservative-treated wood must be field-treated in accordance with AWPA Standard M4. Within that standard, preservatives for exterior use are copper naphthenate and oxine copper (also known as copper 8 quinolinolate). Copper naphthenate is suitable for field treatment of wood in ground-contact or above-ground exterior applications, whereas oxine copper is intended for exterior above-ground use only. Field treatment is also required for preservative-treated wood used in interior locations. For these applications, M4 specifies the use of an inorganic boron treatment that does not normally emit VOCs. So every kerf used to create a curved band joist—along with end cuts and drilled holes—needs to be treated with a topical preservative.
Whatever field-treatment preservative you choose, always follow the manufacturer’s application instructions and be sure to pay attention to any precautions—such as avoiding skin contact or applying the product in a well-ventilated area—spelled out on the product label. When applying the preservative to cuts and holes, use the method that provides the most complete coverage. For flat surfaces, application by brush is probably best. For drilled holes or the saw kerfs you describe, a spray application might be the best way to get complete coverage of all the exposed wood surfaces.