Factors Affecting Durability
The durability of any wood species varies greatly for different conditions and applications. All wood species are durable when kept dry because fungi, the primary decay organisms, do not attack wood that is well below 25-percent moisture. Also, sapwood almost always lack durability. Even with species that have a reputation for excellent durability, only the darker interior wood, known as heartwood, has any significant ability to resist decay.
The pine family includes species such as eastern white pine, western white pine and longleaf pine. Most pine species are classified as slightly resistant or nonresistant to decay, which means that under moist conditions, they could exhibit significant deterioration in less than five years. Eastern white pine, a common lumber species, is moderately resistant, meaning it can be expected to remain in good condition for more than five years but less than 20 years.
Cedar is extremely durable and often sold for exterior applications, but the reality is not so simple as that. Cedar is classified as resistant or very resistant to decay, meaning it should last more than 20 years even under moist conditions. But this classification only applies to cedar heartwood, which is increasingly unavailable because cedar lumber now primarily is cut from small second-growth trees with a high proportion of sapwood. It is even possible currently available cedar heartwood is not as decay-resistant as expected because the trees are harvested at a younger age. Nevertheless, cedar usually is more durable than pine.
When durability is absolutely essential, the most reliable choice might be pressure-treated lumber. The treatment process uses high pressure to force preservative chemicals deep into the cellular structure of wood, resulting in a product that is highly resistant to decay. Research indicates that treated wood in contact with soil remains in good condition for more than 40 years. With proper manufacturing practices, treated lumber can be more consistently durable than ordinary lumber containing unpredictable proportions of heartwood. The primary disadvantage of treated lumber is the inherent toxicity of the preservative chemicals.