The first published guidelines for comparing stem failure in regards to decay were released in the early 1990’s. Mattheck, 1994, “The Body Language of Trees; A handbook of Failure Analysis”. Claus Mattheck proposed a .30 to .35 t/R ratio threshold of high risk of stem failure from a central column of decay for full crown trees. He noted that failures begin to accelerate in surveys, as the ratio begins to fall under .25 t/R. This became generally interpreted as a .30 t/R ratio or a 30% Remaining Wall!
The diagram above represents a 24” (inside the bark diameter), of a stem or branch, with roughly, a center column of decayed wood, (the diagonal lines). The t (above) is the outside shell of sound non-decayed wood. The diagram indicates that the internal wood decay has consumed roughly 2/3 of the cross-section. The t/R ratio is simply the ratio of the sound wood shell thickness (t), without the bark, to the radius of the cross section (R). The above example has a t/R ratio of 30%, or .30 or roughly 1/3 of the radius is sound! Guidelines for evaluating trees for decay!