Wood joists or structural floors must be of what kind of wood if they are within 18 inches of exposed ground?

When wood joists or structural floors are within 18 inches of exposed ground, it is crucial to use preservative-treated wood. This requirement stems from the need to protect the wood from moisture and ground contact, which can lead to wood decay, insect infestation, and ultimately structural failure.

Preservative-treated wood undergoes a treatment process that helps prevent deterioration due to fungal growth and insects, making it suitable for environments where it may come into contact with soil and moisture. This is particularly important in areas prone to damp conditions. By utilizing this type of wood, the risk of damage over time is significantly reduced, ensuring the longevity and safety of the building’s structural components.

In contrast, naturally durable wood is not a requirement in this situation, and while it can offer some protection, it may not be sufficient in areas with high moisture exposure. Using any wood type or untreated wood would increase the risk of decay and structural failure, making them unsuitable choices for joists or structural components near the ground.