Pressure Treated Wood is Not All the Same

By Eric Battey, Project Manager | November 14, 2019


Wood has been an invaluable building material for thousands of years, but in certain situations, it is prone to infestation, decay, and rot; limiting its usefulness for many applications. To improve the wood’s properties, chemicals compounds can be infused into the wood grain under intense pressure under a process called “pressure treatment.” These chemical compounds provide protection from the elements, bacterial and fungal growth, and even pest infestation. Because of this, pressure-treated wood is an excellent material for exterior use, particularly decks.

The common mistake that can easily be overlooked is that not all pressure-treated wood is the same. Depending on the type of chemicals and preservatives applied, there are different grades and allowable uses for pressure-treated lumber.

The American Wood Protection Association (AWPA) ( has designated the following Use Categories that can be found on a typical lumber stamp:

UC1 Interior Dry
UC2 Interior Damp
UC3A Exterior Above Ground, Coated with Rapid Water Runoff
UC3B Exterior Above Ground, Uncoated or Poor Water Runoff
UC4A Ground Contact, General Use
UC4B Ground Contact, Heavy Duty
UC4C Ground Contact, Extreme Duty
UC5A Marine Use, Northern Waters (Salt or Brackish Water)
UC5B Marine Use, Central Waters (Salt or Brackish Water)
UC5C Marine Use, Southern Waters (Salt or Brackish Water)
UCFA Interior Above Ground Fire Protection
UCFB Exterior Above Ground Fire Protection

As you can see, there are several use-categories and when installing deck or porch posts, you can see how this may get over complicated and fast. Rather, we always suggest using a post base that will avoid direct contact with the soil and/or surrounding condensing materials, such as concrete. Not only does this create a standoff from damaging elements, but it’s also a standard connection and typical for deck and/or porch construction.

Of course, there are other alternative materials to pressure treated wood.  These include composite materials, masonry or metal, such as aluminum or steel.  While readily available, these options may prove to be a more costly option and you’d still need to create a connection.

If you are constructing new or adding onto your existing deck and need structural design services or help determining the best material for your project, give us a call today for our expert opinion!

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