Shell Rot in Wood Poles

All preservative-treated wood poles commonly used in North America are subject to “shell rot” or surface decay below the ground line.  In southern pine poles, which compose roughly 85% of the poles in North America, this type of decay is most common.  Western species such as Douglas-fir and cedar are less susceptible to surface decay.  However; however, only the heartwood of these species are naturally decay resistance.  So, sapwood of neither Douglas fir nor western red cedar is naturally resistant to decay.  Therefore, as these poles age they can be subject to surface decay, though at later stages in life when compared to southern pine.

Wood destroying decay fungi are the most common wood-destroying organisms and they can be found in virtually any environment.  Decay fungi require four elements in order to live and attack wood: air, water, a favorable temperature, and food (in this case, the wood pole).  If you can remove any of these factors, then the fungi cannot live.  These four elements are most prevalent from the groundline to 18 inches below the groundline.   Beyond this depth, the soil becomes more anaerobic and oxygen is limited.  For any wood product, plywood, lumber, etc, the surface of the product has a tremendous effect on the strength.  For poles, the outer two to three inches governs approximately 90% of the strength.  Therefore, it is critical to inspect and protect this part of the pole to preserve the pole’s strength.  I have served as an expert witness on many cases in which a pole has fallen and caused property damage or death and most could be prevented by (1) having a pole inspection program and (2) using remedial preservative treatments at the groundline when an initial problem is detected.

Extending the life of a pole can be accomplished by in-place treatment with remedial preservatives.  In Bulletin 1730B-121, the Rural Utilities Service (RUS) recommends an 8 to 12 year cycle based on the decay conditions of the particular environment where the poles are installed.  Excluding remedial treatments from a pole maintenance program leaves owners with an inspection only program.  This “run to failure” strategy can have significant long-term negative impacts on our natural resources, skilled manpower, financial resources, and it increases the pole owner’s risk with regard to safety and reliability (Osmose 2017).

Externally applied preservatives vary greatly in their active ingredients, environmental profile, efficacy, penetration into a pole, and their ability to remain in the treatment zone so as to control decay for an extended period of time. Selecting an appropriate remedial treatment strategy can save pole owner’s millions of dollars by reducing the number of pole change-outs and reducing the risks associated with pole failures.  Most remedial treatments are applied by brush as a paste or as a bandage, similar to a large band-aid wrapped around the groundline.  The table below is from Freeman (2007).