By Martha Retallick, WIR Contributor

Fun fact: Nearly 90% of all pressure-treated lumber is Southern Yellow Pine. Why does SYP dominate this market? Because of the wood’s unique cellular structure – it’s tailor-made for pressure treatment.

So, you’re about to construct a deck, or any outdoor project, with Southern Yellow Pine. Before you head to the building supply store, here’s a lumber industry term that you’ll need to know:

Ground Contact

In simple terms, this is wood that has been specially treated to withstand direct contact with the soil. For your deck project, you’ll be using Ground Contact lumber for the deck’s supporting posts and stringers. The stringers are those notched boards that hold the stair treads.

jordan deck
You won’t need Ground Contact wood for the stair treads because they won’t be touching the ground.

How is SYP Treated for Ground Contact?

Now, let’s take a trip to a SYP treatment facility and learn how Ground Contact wood is treated. The process involves a sealed tank big enough to hold lo-o-o-ong pieces of lumber.

After the lumber goes into the tank, all of the air is vacuumed out. The vacuum makes the wood open its pores, which gets it ready for the next step, preservative treatment. The preservative flows into the tank and is pressurized so that it goes deep into the cellular structure of the wood. After several days of drying, the wood is good to go.


deck on ground
This ground contact pressure treated lumber has the best chance of resisting all of the things that want to eat it – we’re talking bacteria, fungi, and insects, including termites.

Why Ground Contact and Not Just Pressure Treated?

When a piece of SYP is treated for Ground Contact, that means it has a different proportion of preservative treatment than other pressure treated lumber. What this does is give that piece of lumber greater strength and durability.


Ground Contact vs. Above Ground

Ground Contact is one type of pressure treated lumber. When you’re at the store, you’ll find another type that’s called Above Ground. For your deck project, use Above Ground Exposed wood for flooring and railings that won’t touch the soil.

How do you choose Ground Contact from all that other lumber at the store? Simple: find the label that’s attached to the end of each piece.

Here’s what to look for on those labels:

ground contact label
Image courtesy Forest Products Laboratory


Ground Contact General Use

Use this wood in places where it will come in contact with the ground, fresh water, or in other places where it’s highly vulnerable to deterioration. Choose it for your deck’s supporting posts. Or for fence post and mailbox posts. The label will likely say GC for Ground Contact.

syp stoop

Ground Contact Heavy Duty. This is ultra-tough pressure treated lumber, and it’s especially suited for challenging environments. For example, if you’re building a deck for your beach house, Ground Contact Heavy Duty wood will be your best choice. It can stand up to the splashes of salt water that you may get during storms.

treated pine dock

Please remember: If the lumber is going to touch the ground, please make sure it’s labeled for ground contact use. Otherwise, you might be sad that your project didn’t hold up as long as you’d hoped.

Stay tuned for more articles on how to handle pressure treated lumber: building tips, fastener recommendations, and paint/staining tips.


(Images via Forest Products Laboratory, via Wood. It’s Real.)