Use of Treated Wood in Residential and Light Commercial Construction

This blog is intended to be a useful and simple summary for anyone that uses, or specifies, treated wood materials in residential and light commercial construction. Also provided are links to useful pdf documents that describe and illustrate the proper use of treated wood in this type of construction. Too many times I’ve seen treated wood products used incorrectly and the results can be devastating.

For some background information I think it’s important to talk about the industry standards associated with treated wood and wood-base materials which are published by the American Wood Protection Association (AWPA). The AWPA is a consensus based association whose members meet regularly and work to develop, update and maintain all the standards associated with treated wood and preservatives. Each year the AWPA publishes its Annual Book of Standards. One of the most important standards in that document is AWPA Standard U1, also known as the Use Category System. As stated on the AWPA webpage, “The Use Categories are a shorthand method of describing the various hazards to which wood products may be exposed.” Standard U1 provides detailed information on the use of several treated wood and wood-base materials in residential and commercial construction based on the exposure conditions that the materials will be subjected to while in service. Also provided in AWPA U1 are the approved preservatives and the required minimum preservative treatment retention levels for all the approved products listed in each Use Category. At times it may seem intimidating to get through AWPA U1, but ultimately it’s pretty straightforward.

For a description of the various use categories in AWPA U1 here is a link to the AWPA webpage where an 18 page excerpt is provided (full copies of AWPA U1 are also available for purchase on the AWPA webpage):

Defined in AWPA U1 are the five use categories: UC1, UC2, UC3, UC4 and UC5. Let’s start with UC1 and UC2. Both of these classifications are for interior uses where the products will not be in contact with the ground (UC1 is interior-dry, UC2 is interior-damp). These may be for uses where protection against insects, such as termites, is important. Use Category 3 is provided for exterior applications of wood, in above ground, exterior construction. UC3A is for protected materials and UC3B is for exposed materials. These include products such as coated exterior millwork and trim (i.e. protected, UC3A) as well as decking materials (i.e. exposed, UC3B). Use Category 4 (4A, 4B and 4C) is intended for products that will be in ground contact or fresh water exposures. UC4A is the most common Use Category for residential construction and is referred to as General Use which includes products such as fence posts, deck posts, ledger boards, joists, beams and decking. UC4B is defined as Heavy Duty use, and includes products such as permanent wood foundations, timber posts and building poles. UC4C is defined as Extreme Duty and products products such as foundation and fresh water pilings as well as utility poles. Finally, Use Category 5 (5A, 5B and 5C) deals with all the various products used in marine exposures around the coastal US.

One of the most important interpretations of AWPA U1 when specifying products in residential and light commercial construction is when products listed under UC3B should instead be interpreted as UC4A products instead. For example, deck joists, ledger boards, and cantilevered beams that support exterior deck structures are all, technically, above ground - exterior construction (i.e. UC3B). However, because they are structurally important, are critical to the overall performance of the structure, and they can be difficult to replace, they are considered UC4A exposures. Products listed under UC4A are required to have higher preservative retention levels than products listed under UC3B. Therefore, specifying UC4A treated products for these structurally important uses ensures more protection from decay and insect attack, thus providing a longer service life and added safety.

Below are three different pdf links with very useful information for any professional, contractor and homeowner when it comes to the use to treated wood in residential construction. I’d particularly love to see a poster version of the infographic in all the home centers across the US.

The infographic below can be found on both the AWPA and WWPI webpages. This one is from the AWPA webpage. Of particular importance is footnote 1.

The WWPI pdf below provides descriptive information for specifying treated wood in residential construction per AWPA Standard U1:

The pdf link below can be found on the WWPI webpage. The information provided is also included in the 2018 International Residential Code: