When you want to use wood that is not naturally decay resistant in a wet application (outdoors, for example) or where it may be at risk for insect attack, you need to specify preservative-treated wood. This is lumber that has been chemically treated to make it unattractive to fungi and other pests. In the same way that you would specify galvanized steel where it would be at risk of rusting, you specify treated wood where it will be used in a setting conducive to decay. Also like galvanized steel, treated wood has a shell of protected material surrounding an unprotected core. So long as this shell stays intact, the core will be protected.
Wood does not deteriorate just because it gets wet. When wood breaks down, it is because an organism is eating it as food. Preservatives work by making the food source inedible to these organisms.
Properly preservative-treated wood can have 5 to 10 times the service life of untreated wood. This extension of life saves the equivalent of 12.5% of Canada's annual log harvest.
Preserved wood is used most often for railroad ties, utility poles, marine piles, decks, fences and other outdoor applications. Various treatment methods and types of chemicals are available, depending on the attributes required in the particular application and the level of protection needed.
There are two basic methods of treating: with and without pressure. Non-pressure methods are the application of preservative by brushing, spraying or dipping the piece to be treated. These are superficial treatments that do not result in deep penetration or large absorption of preservative. Their use is best restricted to field treatment during construction (for example, when a pressure-treated piece of lumber must be field cut), to cases where only part of a piece is to be treated, or to remedial treatment of wood in place. For example, mixtures of borate and glycols are used to treat sound wood left in place during repair of decay problems. The glycol helps the borate to penetrate dry wood, arresting the activity of any fungus which contacts it. The penetration of the preservative is still limited, and the most important function is to prevent undetected fungus left in place from spreading to sound wood.
Deeper, more thorough penetration is achieved by driving the preservative into the wood cells with pressure. Various combinations of pressure and vacuum are used to force adequate levels of chemical into the wood. Pressure-treating preservatives consist of chemicals carried in a solvent. The solvent, or carrier, is either water or oil. Waterborne preservatives have become increasingly popular over the last 20 years, due to the absence of odour, the cleaner wood surface and the ability to paint or stain the wood product. When a wood product will be used in an application known to present a risk, for example outdoors, pressure-treatment is recommended.
Types of Preservatives
The most commonly used wood preservative in North America for decades has been chromated copper arsenate (CCA), although this is changing since CCA began phasing out of most residential applications in January 2004. Replacing it are amine copper quat (ACQ) and copper azole (CA). These preservatives are all applied with pressure. They all have a green tone, which fades over time.
Borates are another class of waterborne preservative gaining in popularity. Borates are colourless, non-toxic to humans, and can be applied to wood with or without pressure. Their use is currently limited to applications which are protected from rain and other persistent sources of water. These include framing in termite areas and repair of decayed framing in leaky buildings where the main moisture source has been eliminated.
Creosote is the well-known black oily wood preservative, the oldest type of preservative still in modern use. It's now used in Canada almost exclusively for railroad ties, where its resistance to moisture movement is a key advantage. Pentachlorophenol in oil is mainly used for utility poles where the surface softening characteristics of the oil are useful in pole climbing.
Copper naphthenate and zinc naphthenate are two common preservatives used for treating field cuts.
With the exception of borates, wood preservatives become "fixed" in the wood after the treatment process. This means the chemical is virtually insoluble. In addition, the chemical is altered in the fixation process, such that treated wood is not considered hazardous to your health.
All wood preservatives used in the U.S. and Canada are registered and regularly re-examined for safety by the U.S. Environmental Protection Agency and Health Canada's Pest Management and Regulatory Agency, respectively. Health and environmental impacts of CCA are currently being re-investigated. Please see our FAQ.
Wood preservation is not an exact science, due to the biological - and therefore variable and unpredictable - nature of both wood and the organisms that destroy it. Wood scientists are trying to understand more about how wood decays to ensure that durability is achieved through smart design and construction choices where possible, so that as a society we can be selective in our use of preservatives.