ProWood curved deck for deck entertaining

New Standards for Treated Lumber

The American Wood Protection Association (AWPA) recently changed its standard on lumber treated for ground contact use.

The change: Above-ground applications must now adhere to (UC4A) ground contact requirements when the wood will be in any of the following scenarios:

  • Difficult to maintain, repair or replace (such as deck joists and ledger boards) and are critical to the performance and safety of the entire system/construction
  • Installed less than 6 inches from the ground
  • At risk for poor air circulation (little airflow)
  • Likely to have prolonged contact with vegetation or damp leaves
  • Exposed to frequent moisture
  • Used in tropical climate

Download our Technical Bulletin for details about this change (download Spanish version).

Choose the right treated lumber for your project.

Wood treated to (UC3B) above-ground standards is ideal for use in light-duty situations where wood will not be in contact with moisture or the ground. Railings, balusters and certain deck board installations may be appropriate for above-ground treated lumber.

Wood treated to (UC4A) ground contact standards is ideal for general-use situations where the wood may contact moisture, the ground, or be installed in a manner that subjects the wood to comparable ground contact conditions – such as poor ventilation, tropical climates or frequent exposure to moisture. 

Deck joists, beams and ledger boards must be UC4A ground contact material; this is a big change and an important one

Posts, stair stringers and certain deck board installations also require ground contact treated lumber.

Wood treated to (UC4B) heavy-duty standards should be used in very severe environments or for critical structural components such as foundation piling. Always consult local building codes for additional requirements for specific structures.

Wood treated to (UC4B) heavy-duty standards should be considered when the wood will be used to support a permanent structure.

  • UC3B treated lumber is for above ground, not for near ground, light-duty use
  • UC4A treated lumber is for ground contact, general use
  • UC4B treated lumber is for ground contact, heavy-duty use

Ground Contact Use Chart

Download our Treated Lumber Standards Chart.

ProWood® products are available in Ground Contact (UC4A) standards:

  • 5/4-inch decking
  • Dimensional lumber
  • Plywood
  • Appearance-grade boards

Find ProWood products sold near you.