Did you know there are now different grades of pressure-treated lumber? Picking the right pressure-treated wood can make the difference between a lasting project and one with a short life.
Let’s say you walk into your local home improvement store to buy some pressure-treated 2x4s for an outdoor project, but when you inspect that green lumber you find a tag that says “Ground Contact”. It doesn’t say “pressure-treated”. Are you in the right spot?
Humans have been using wood for building for a long time and we have probably been trying to preserve it just as long. Olive oil, linseed oil, tree resin, tar, and modern chemicals are several ways we’ve extended the life of wood, especially in harsh outdoor climates.
Methods used to apply these preservatives include rubbing, brushing, soaking, or infusing by pressure treating. In this post, we are going to focus on pressure-treated wood.
What is pressure-treated wood? Is there a difference between Ground Contact lumber and pressure-treated lumber? Where and when should it be used?
All great questions! Let’s dig in!
What is pressure-treated wood
Pressure-treating wood refers to the process of infusing softwood species, primarily Pines, with chemical preservatives using a large pressure chamber. The fresh lumber goes in the chamber, it’s sealed, then depressurized creating a vacuum that removes air from the wood. The chamber is then filled with the preservative and re-pressurized, forcing the preservative deep into the wood. For a more in-depth look at the pressure-treatment process watch the video below.
These preservatives extend the life of the wood significantly by helping prevent wood rot caused by fungus and attacks from wood-eating insects.
Pressure-treated wood does not prevent weathering, however. It still absorbs water like regular wood, and over time, the sun and weather will take its toll on the exposed wood. It will check, cup, and warp like non-treated woods in the same environment. That’s why pressure-treated wood should be treated just like non-treated wood for outdoor use, by giving it some type of protective coating. If care is taken, it could potentially last 3 or 4 times longer than non-treated wood.
If you are planning an outdoor project that uses wood, you are going to want pressure-treated wood (PT) over standard, non-treated wood. Do not use pressure-treated wood for indoor projects. Stick with untreated woods with stains, sealers, and paints for indoor projects.
ground contact vs pressure-treated
Pressure-treated is used in residential applications such as decks, boardwalks, sheds, fencing, planter beds, gazebos, arbors, and thanks to newer and safer preservatives, even picnic tables and playsets. It is also used for bucking out rough openings for windows and doors.
Though most manufacturers using these modern preservatives say it is safe to use for planter beds growing edibles, some gardeners suggest lining the lumber with plastic to prevent any leaching into the soil. For a gardener’s perspective on this see Fine Gardening’s post on the safety of pressure-treated lumber in garden beds.
Not all pressure-treated lumber is the same. In fact, there are 12 different levels of pressure-treated wood that are available. These levels are divided into Use Categories (UCs) and were standardized by the American Wood Protection Association. It is in these UCs where we find the difference between Ground Contact wood and pressure-treated wood.
Below shows the use categories where pressure-treated lumber is used above ground only and Ground Contact.
- UC1, UC2, UC3A, UC3B are used above ground only.
- UC4A, UC4B, UC4C are used for ground contact or directly in the ground.
- UC5A, UC5B, UC5C are used for ground contact or directly in the ground and where salt or brackish water is continuously present.
- UCFA and UCFB are used where pressure-treated wood and fire retardant wood is required.
The higher the ratio of preservative in the wood the better the product can stand up to conditions that promote decay and damage from insects. The higher retention of preservative is what gives pressure-treated lumber the “Ground Contact” label. Retention is the amount of chemical preservative that is left in the wood after the pressure treatment process. The minimum retention levels vary for each use category depending on what type of preservative is used.
where and when to use
It is important to know which type of PT to use for your project. It would be unfortunate to accidentally use the wrong type of pressure-treated lumber only to have it rot in a couple of years.
The information above may feel overwhelming, but the reality is that it’s not too hard to determine what you need. Each piece of pressure-treated lumber should have an end tag that gives you everything you need to make a good decision. Plus, your local distributor should be able to guide you in the right direction. Let’s take a look at this end tag.
The very first line on the top says “Ground Contact General Use”. It also includes the use category, UC4A, and the preservative and pounds per cubic foot, (MCA-C) 0.15 PCF, should you need this information for your particular project.
For most residential outdoor projects where the lumber will come in contact with the ground or be directly in the ground, this lumber is the right fit for the job. You can also use Ground Contact above the ground, especially where components of your project may be hard to maintain over the next several decades.
For the most part, trim, fascia boards, deck boards, railing, and porch floors are all examples of places above the ground pressure-treated wood can be used. Planter boxes, structural posts, fence posts, outdoor stair stringers are all examples where Ground Contact should be used.
Some companies, like The Home Depot, have made changes to sell only Ground Contact pressure-treated wood to help prevent above ground lumber from being used incorrectly and failing prematurely.
The AWPA provides this great infographic to help you determine the right pressure-treated wood for your project.
Now you know how to decide which pressure-treated lumber is best for you. With proper care and maintenance, pressure-treated wood is a great product to build lasting projects.
Founder & Editor-in-Chief
I love old houses, working with my hands, and teaching others the excitment of doing it yourself! Everything is teachable if you only give it the chance.