This may seem like a simple question that can be answered with either a yes or no. Yet, there are a number of questions within this question that should also be addressed, and we’ll do our best to ask and then answer them for you.
What is Pressure-Treated Lumber?
Both pine and cedar are the most common woods that receive a pressure treatment. The purpose of the treatment is to prolong the lifespan and usefulness of the lumber for outside use. Pine is the more common of the two, as it is an environmentally friendly choice because it grows so quickly.
The wood is processed under high pressure to force a solution of water and chemicals deep into the wood fibers. The chemicals used in this treatment are:
- Alkaline copper quaternary (ACQ);
- Copper Azole (CA); and/or
- Micronized copper azole (MCA)
These chemicals act as preservatives and react to the wood fibers to slow down decay and frustrate insect infestation, including termites and fungus growth, and to protect from other conditions that lead to rot and structural degradation.
The wood absorbs a great deal of water in this process and will arrive at lumber yards and hardware stores still wet. It can take a month to completely dry, depending on where and how it is stored, and the helpful chemicals that protect the wood remain inside the fibers after the water has evaporated.
Two Types of Pressure-Treated Wood
There are two types of pressure-treated wood, separate and apart from the type of wood:
- Above-ground. Above-ground pressure-treated wood should be used only when it can be more than 6” above the ground and when there will be good air circulation and drainage.
- Ground-contact. Ground-contact pressure-treated wood can be used both above the ground and in contact with it. It will have twice the chemical concentration after the water has evaporated. It should be used when there will be less than 6” of clearance, or there is poor air circulation.
Within these two types, there are also different grades of pressure-treated wood:
- #1 Grade. This is the highest grade, and the pressure-treated lumber will be free from defects like knots, as well as a clear surface appearance.
- #2 Grade. The lesser grade, this pressure-treated lumber will have defects and the occasional knot.
Your project will determine which of the two grades you will choose, and there is a price difference between the two grades.
Uses of Pressure-Treated Woods
Outdoor decks are a common pressure-treated wood project, as you might imagine. Fences and pergolas, too. Wooden walkways through your garden, handicapped ramps, fresh-water docks, and other outdoor projects with exposure to the elements are also common.
Since fence posts will sit in the ground, ground-contact pressure-treated wood will be required. If the fence itself will be within 6” of the ground, you’ll want to use ground-contact pressure-treated wood as well. Since caps are well above-ground, you can use either type.
Raised garden beds that rest on the ground will require ground-contact type pressure-treated wood. If you are building a deck and a portion of it will sit in direct contact with the ground, all of the wood for the project should be of the ground-contact type.
Pressure-treated lumber should never be used for any indoor projects. The chemical treatments should not be a part of any indoor projects. It is entirely for outdoor use where rot from water, insects, and fungus is of concern.
We should mention, also that there is some pressure-treated lumber that is treated with additional chemicals for water-repellent qualities. The tags on such lumber will indicate that these water-repellent chemicals have been added, just as the tags will indicate between above-ground and ground-contact types.
Pressure treatment will not prevent weathering. It will extend the lifespan of the wood, though, as it will prevent rot and decay. Pressure treatment can extend useful life to between 20 -25 years before it must be replaced.
Can Pressure-Treated Wood Be Painted?
Remember that the pressure-treating process involves the pressurized penetration of water with preserving chemicals deep into the wood fiber. Paint does not take well to wet wood, no matter what kind of paint you are using – latex- or oil-based.
Depending on the “expert” you speak with, fully drying pressure-treated wood can take from one to three months to fully cure. Again, how and where it is stored after treatment will have a great impact on drying and curing time for pressure-treated wood.
You certainly want to wait until you are quite sure the pressure-treated wood is fully dry before you paint it. To test whether the lumber is dry enough to paint, you can sprinkle a little water on the wood; if the droplets are readily absorbed, the wood is dry enough to paint or stain. If the water beads on the surface, the wood is not dry enough to finish and will need more time.
One thing to keep in mind is that pressure-treated wood will warp and crack during the drying process. This is another reason why you want to wait for paint or stain until it is fully dry. You wouldn’t want to paint the wood only to have it crack afterward and need to be painted again.
After allowing ample time for drying, the question then becomes how to paint pressure-treated wood. A primer is essential as preparation for the paint you have chosen. A primer that will also cover any imperfections in the wood, something like KILZ, is a good choice. It will cover any knots and imperfections well and prepare the surface for the paint and color of your choice.
Lighter colors might be a bad choice for pressure-treated lumber, too, as it will require additional coats to give a good finish appearance to what is otherwise an inexpensive grade of wood, separate and apart from the pressure-treated grade. A darker color for PT wood is a smarter choice, along with at least one coat of primer, if not two, and two coats of paint.
This video discusses how long to wait before painting PT wood.
And then, here are a few tips and tricks for painting PT wood.
Untreated Wood For Outdoor Use
All of that having been said about pressure-treated lumber, let’s consider the main question of this article – do you need pressure-treated wood for an outdoor project if you are only going to paint it anyway? Would untreated wood do just as well for outdoor use with paint?
The answer really has more to do with lifespan. While untreated painted wood might last you 5 years, with an occasional new coat of paint, we’ve already told you that PT wood can last 4 or 5 times that.
If you go one step further and topcoat the paint with a sealant, you will extend the life of untreated wood by a number of years, yes. A marine sealant, for instance, something like Thompson’s Waterseal, or even a few coats of polyurethane, will add to the lifespan of untreated wood used for outdoor projects. Any film finish that will provide waterproof protection and thus delay rot or decay will extend that lifespan.
The question is whether it will extend it 4 or 5 times to make the difference. Most agree that for outdoor projects, it really is a better choice to use the lumber that is produced for outdoor use that will last for 25 years. That lumber is PT, and you can even use PT wood with waterproofing treatment that will extend its life even more.
So, do you need PT wood for an outdoor project if you are going to paint the project? The answer is no. The question really would be, why wouldn’t you use PT wood for outdoor projects since it’s treated for that purpose and will last longer?
For us, we can’t think of a reason why we wouldn’t choose PT wood for an outdoor project.