Types of Pressure Treated Wood

Pressure treated wood has had a preservative applied under pressure in a vacuum, allowing the preservative to seep into the wood. There are three main types of treatments, differing only in what type of preservative is applied.

Waterborne Treatements

Waterborne treatment is an umbrella term covering several preservatives that are water-soluble. Common preservatives in this category include alkaline copper quat, micronized copper quat, copper azole and sodium borates.
Wood treated with waterborne preservatives is normally used in residential, commercial or industrial structures.

Waterborne Preservatives and Wood Strength

Waterborne preservatives can affect wood strength. Generally speaking, it weakens the wood slightly. This effect can be exaggerated by several factors in the treatment process, but redrying temperature has the greatest effect.
For this reason, the American Wood Preservers Association has adopted limits on redrying temperatures. When purchasing waterborne treated wood, look for wood endorsed by this association.

CCA and CCA-C

Chromated copper arsenate, commonly referred to as CCA, is a type of waterborne wood treatment that was once widely used, but has been taken off the market for general use.
The wood treatment industry was pressured to take products using CCA off the market due to the use of arsenic in the formula, but the product is still used in some highway and agricultural applications.
Some literature will refer to CCA-C. This is a type of CCA.

Creosote

Creosote is used strictly in industrial settings. The most common uses of wood treated with creosote are railroad ties, guardrail posts, utility poles and marine structures.

Creosote Controvery

Recent years have lead to some debate over the safety of creosote. According to the Environmental Protection Agency (EPA), it was charged in 2008 that creosote is a possible human carcinogen.
After researching the claim in cooperation with Health Canada's Pest Management Regulatory Agency, the EPA ruled that wood products treated with creosote would be allowed in the United States, but they are regulated.

Oil-borne Preservatives

Oil-borne preservatives, also referred to as penta preservatives, are preservatives that are oil-soluble.
Oil-borne preservatives can be used in residential and other structures, and have benefits over waterborne preservatives. The main advantage is the lack of swelling in the wood compared to waterborne treated wood. If you choose oil-borne treated wood, you will pay more for this advantage. It can also be difficult to find in some areas.
Common oil-borne preservatives are Chlorpyrifos/IBPC, Copper Naphthenate and Pentachlorphenol.

References

• "Pressure Treated Wood FAQs"
• "Effects of Waterborne Treatment on Wood Strength"
• "Creosote and Its Use As A Wood Preservative"

Resources

• EPA Re-registration Eligibility Decision for Creosote