DIFFERENTIATING FIRS
I enjoyed your destructive wood testing and your patience in waiting eight years to report comprehensive results (see PS December 2023, “Wood Rot Prevention Eight Years Later”). It is useful to note that the phrase “fir” is used rather indiscriminately and is worth a little refinement. Douglas fir is a naturally rot resistant species but is difficult to find. It is found untreated on older boats often in pretty good shape. I have dug up 25-year-old treated wood foundations made of Douglas-fir (labeled DF) and was very pleasantly surprised at the condition of the wood. Hem-fir (Hem, Hem-fir, H-F) is a generic label for a range of trees, nominally hemlock or fir. It is the most common kind of “fir” available (at least in the upper Midwest). Treated or not, it is terrible at resisting moisture and rot. Ponderosa pine (PP) is also commonly used in treated lumber and is a modest performer. Southern yellow pine (SYP) is also a great rot resistant species and is found in premium treated lumber. You may also find treated spruce, pine, or fir lumber (S-P-F) which is also a terrible performer. When buying “treated” lumber it is important to read the grade stamp for species as well as the intended use designation—above ground or in ground. The USDA Forest Products Lab in Madison, Wisconsin is a great source and their scientists are remarkable approachable about anything woody.
