Preservatives Used in Treated Wood: CCA, CA, MCA and ACQ

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In order to prepare wood effectively for use in marine environments, it is treated with a mix of preservatives and water to keep pests away and slow down its decay rate. These treatments are water-based as opposed to oil-based, which fully penetrate the lumber while still allowing an application of paint. In this post we will break down the most common preservatives used in the process of treating wood, and the suitable applications for each type.

  • CCA: Chromated Copper Arsenate – a combination of chromium, copper and arsenic which combined provide maximum protection. Due to its chemical composition, this preservative is most commonly used in marine construction and is considered to be the most durable.
  • CA and MCA: (Micronized) Copper Azole – these preservatives penetrate deep into the lumber and prevent rot from pests and excessive water exposure. This treatment is suitable for below and above ground, and freshwater uses. 
  • ACQ: Alkaline Copper Quaternary – contains a quaternary compound which includes ammonium and copper. This compound is particularly strong against insects, fungi and other marine creatures that can damage lumber.
What are Retention Rates?
When buying treated wood, you will find there are different retention rates as well as preservatives used to treat the wood. The retention rate in treated wood refers to the amount of preservative used to treat the wood. The retention rate is measured in preservative per cubic foot, or pcf. Typical retention rates are: 0.6 pcf, 0.8 pcf and 2.5 pcf. Generally speaking, the higher the retention rate, the lower the bio-deterioration rate.
  • 0.25 pcf (CCA) can be used in above ground projects
  • 0.4 pcf (CCA) can be used for ground and freshwater contact
  • 0.6 pcf (ACQ or CCA) is commonly used for structural poles, permanent wood foundations and projects that are exposed to ground contact, extreme weather conditions or freshwater use
  • 0.8 pcf (CCA) for projects in salty fresh water
  • 2.5 pcf (CCA) for projects that involve saltwater immersion