Termites and Damp: The Detailing Limits That Decide How Long a Timber Building Stands

A timber building rarely fails because its members were too small on the day it was built. It fails years later at its feet, where subterranean termites and fungal decay, working together at the groundline, remove wood a little at a time until the remaining section can no longer carry its load. This paper turns that process into arithmetic. From stated rate assumptions — penetration rates by species durability class, multiplied by factors for ground contact, clearance height and climate wetness — it computes the expected time for a 150 mm post to reach structural loss, defined as 25 mm of penetration from each face, for a range of detailing cases. The study is simulated, not measured, and not load-tested. The results are stark. A non-durable, Class 4 post set in the ground in a moderate climate reaches structural loss in 6.2 years, and a moderately durable Class 3 post in 12.5 years. The same Class 3 post lifted onto a stone plinth with 450 mm of ventilated clearance and a termite shield reaches 104.2 years in the same model — a figure to be read as comfortably beyond a 50-year design life rather than as a literal prediction. In a wet climate the corresponding figures fall to 4.3 years for a Class 4 post in the ground and 10.8 years for a Class 4 post on a low 150 mm plinth. The paper also examines, and disproves by arithmetic, the widely held belief that a layer of charcoal or ash under a post stops termites for the life of the building: granted the most generous possible assumptions, such a layer is crossed by a mud tube in 3.0 days at the stated building rate, or 30.0 days at a tenth of that rate, and even if it blocked passage perfectly for a generous intact life of 2.0 years it would cover only 4.0% of a 50-year design life, extending a Class 3 post's ground life from 12.5 years to 14.5 years. The detailing standard originated here sets a minimum clearance, a maximum permitted exposure and a minimum durability class for each of three climate wetness bands, and it forbids ground contact for structural posts in all but the driest band with the most durable species.