Pressure treated wood can last for up to forty years or more. Precisely how long it will last depends on the chemicals used in the pressure treatment, the project type, the wear and tear the wood will endure, the species of wood, the amount of exposure to harsh wet environments and how well it is maintained. Each of these factors and their effect on pressure treated wood’s longevity is described below.
During the pressure treatment process, preservative chemicals are injected into the wood in a pressurized, vacuum-sealed tank. The type and amount of chemical preservative used and the depth of penetration are key to determining how long the wood will last.
Until 2004, pressure treated wood was treated with a chemical known as chromated copper arsenate (CCA). Questions about its effects on the environment and health caused the construction industry to cease using CCA-treated wood for most residential applications. However, it is still used in heavier applications such as shakes and shingles, wood foundations and utility poles. In CCA-treated wood, copper and arsenic are fungicides, arsenic functions additionally as an insecticide and chromium provides UV protection. Once treated with CCA, it is impossible to remove it from the wood, which makes the wood extremely long-lasting.
Wood used in lighter-duty, residential applications is treated with amine copper quat (ACQ), copper azole (CA), or ammoniacal copper zinc arsenate (ACZA), each with a limited range of applications. Many other chemical treatments are also available.
It is important to use the right hardware and fasteners with the type of pressure treated wood used. The chemicals will corrode metal that is not manufactured to withstand the treatment chemicals. This corrosion will impair the integrity of the project and impair the life of the wood.
The project type will dictate the necessary kind of pressure treatment in the wood, which in turn will affect the life of the wood. Different projects expose the wood to different stresses on the wood. Those stresses and nature of use require different amounts of preservative for adequate protection. The amount of preservative needed is called “retention level.” Retention level means the amount of preservative that is retained in the wood after treatment. Each preservative has its individual retention level for these applications.
Each piece of lumber is tagged with a label indicating its appropriate use. Appropriate use ranges from the lightest treatment of UC1, suitable only for interior dry conditions, all the way up to the heaviest treatment of UC5C, suitable for use in saltwater marine conditions.
Species of Wood
Some wood species absorb and hold the treatment chemicals better than others and thus are longer-lived when used in appropriate applications. The best wood species are:
- Pine or Cedar – Southern pine is one of the most commonly-used species for deck framing in the eastern United States. Its logs are strong and high in sapwood, which absorbs the preservatives very well. Red pine and Ponderosa pine are used in the northern United States and Canada
- Douglas Fir – Predominantly used in western United States and Canada
- Hem-Fir – Western species that is weaker and more prone to warping and splitting than Douglas fir, but very receptive to preservation
In high humidity or sub-tropical or tropical climates, the wood will last longer if it is wood treated for ground contact or heavy-duty ground contact. As the name indicates, wood graded for ground contact applications is normally used for projects and project parts that are in the ground or in contact with the ground, debris, leaves or vegetation. It is also suitable for fresh water applications or those exposed to daily moisture. Wood graded for ground contact use has two times the chemical retention levels and protection compared to wood graded for above ground applications.
The life of the wood can be prolonged with proper maintenance. The right care and maintenance can protect the wear caused by rain, sun, and extreme temperature changes. On decks and exterior structures, apply a water repellent about six weeks after construction is completed, and then annually. Inspect the fasteners as well to determine if nails, screws or bolts are working loose due to corrosion or wood failure. Wood that is rotting can impair the integrity of wood that it is in contact with, so be sure to replace any wood that is showing signs of decay. Weather resistant stain should be applied and reapplied as needed.
Consult With the Lumber Experts at Curtis Lumber and Plywood for More Information
Selecting the wrong kind of pressure treated wood for your application can cost time and money. Worse, it can risk the safety and integrity of the project. The lumber professionals at Curtis Lumber & Plywood can make sure you get the right materials to make your project a success. At Curtis Lumber, you’ll find expertise, quality and a large selection of different types of treated lumber at competitive prices.