By Corrie Taliaferro, WIR Contributor

Ground Contact vs. Above Ground

Let’s say you’ve decided to construct some planter boxes for your backyard. Or a brand-new swing set for the kids. Southern Yellow Pine is an excellent choice for these outdoor projects, but to maximize durability, be sure to choose lumber that has been treated for Ground Contact.

Ground Contact lumber has been specially treated to withstand direct contact with soil, freshwater, and vegetation. It’s the right choice for decks, playsets, and walkways. Also, if your outdoor project uses beams that may be difficult to repair or replace, Ground Contact lumber is the way to go for those as well. Any joists or ledger boards that are critical for safety and/or performance should also be treated for Ground Contact.

ground contact deck
This ground contact pressure treated lumber has the best chance of resisting decay.

New Ground Contact Standards

In 2016, the American Wood Protection Association (AWPA) changed its guidelines regarding the use of Ground Contact treated wood. Before, only wood that was touching the ground, such as posts and beams, had to be treated. Under the revised standard, Ground Contact lumber may be needed for above ground use as well.

The AWPA now requires Ground Contact lumber when:

  • Wood is installed less than 6” above ground
  • Wood is used in a structure that doesn’t allow air circulation or water drainage
  • Soil, leaves, or other debris may build up and remain in contact with the wood
  • Wood is frequently wetted (docks, pools, hot tubs, etc.)
  • Wood is used in tropical climates

Also, the AWPA states, “Joists and beams shall be treated to requirements for UC4A [Ground Contact] when they are difficult to maintain, repair, or replace and are critical to the performance and safety of the entire system/construction.” This standard means that deck joists and support beams will need to be constructed with Ground Contact lumber since they are critical to performance and safety.

When Should I Use Ground Contact?

Feeling a little confused? To make things a little easier to visualize, take a look at the graphic below. It addresses nearly all the typical outdoor projects you might choose to take on, and shows where Ground Contact lumber is necessary.

ground contact guide

How Can I Tell I’m Buying Ground Contact?


To choose Ground Contact lumber at the building supply store, simply look at the labels on the end of each piece. They’ll say GC for ground contact, and they’ll have the code UC4A displayed as well. Here are a few examples of what you’ll see when you’re looking for Ground Contact lumber.:

What is Ground Contact from Wood. It's Real.



Is Ground Contact Treated Lumber More Expensive Than Regular Pressure Treated Lumber?

At retail, Ground Contact lumber costs just pennies more per linear foot than regular pressure treated SYP. And the peace of mind you’ll have, knowing that your project is built to last? Well, that’s priceless.


(Images via Koppers, via Wood. It’s Real.)