Natural durability describes how well a timber's heartwood resists decay. BS EN 350 sorts timbers into five classes, from class 1 (very durable) to class 5 (not durable).
The classes come from ground-contact tests on small heartwood stakes: a 50 mm x 50 mm stake of a 'durable' timber such as European oak lasts roughly 15 to 25 years in the ground. Larger sections, and timber kept out of ground contact, last much longer. Sapwood is not durable in any species.
