Pressure-Treated vs. Non-Pressure-Treated Weatherproof Wood

Pressure-Treated Wood Products

• Pressure-treated lumber is wood that has been treated with chemical preservatives and then put through a series of vacuum cycles to force the preservative into the wood grain. These preservatives make the wood durable and impervious to weather and insects. Pressure-treated wood is less expensive than naturally resistant wood. Different grades of pressure-treated lumber are used for different purposes. Lumber that has been dipped in the preservative for at least 5 minutes and kiln-dried is stronger and more durable than that which has received a brush application and been air-dried. Make sure pressure-treated lumber made from fir species is incised; incised products have tiny knife marks throughout the surface, making it easier for the chemical to penetrate the pores. This is necessary to make the softer woods reliably strong, water resistant and durable.

Naturally Resistant Woods

• Certain tree species are naturally resistant to the damaging effects of moisture and insects and so do not need pressure treatments. These include redwood, cedar, cypress and some tropical hardwoods. They have evolved in an environment of high moisture and abundant insects; over time, the trees became resistant to these factors. However, it is important to note that only the heartwood, not the sapwood, of these species are moisture- and insect-resistant. Do not buy common construction grade or streaky wood if you want a weather-resistant product.

Uses of Pressure-Treated Wood

• Pressure-treated wood can be used for a variety of purposes, and there are many products to choose from. Which pressure-treated product you buy will depend on what you are using it for. One of the most important factors is how much preservative has been forced into the wood. This is known as its retention level, and will determine its weather-resistance, durability and strength. For aboveground decking, look for a retention measurement of at least 0.15 lb per cubic foot with an additional water repellent. For general-use aboveground projects, look for 0.25 lb per cubic foot; for projects with ground contact, look for 0.40 lb per cubic foot; and for critical structure members, like wood foundations or supporting poles, specify a retention level of 0.60 lb per cubic foot.

Uses of Naturally Resistant Woods

• Redwood is considered one of the most attractive insect-resistant, rot-resistant and durable woods used for outdoor building. It is suitable for a variety of projects, but because of its high cost, it is usually used for decking and railings. However, the best redwood for outdoor building comes from old growth trees, so this is controversial; 95 percent of old growth redwoods have been logged. Tropical hardwoods share a similar story; while they are strong and highly weather- and insect-resistant, their usage presents environmental concerns. Cedar and cypress are viable alternatives; they are more plentiful than redwoods or tropical hardwoods, less expensive and also resistant to weather and insects. However, cedar cannot be used for structural supports, while cypress can.