Pressure treated lumber is one of those inventions that changed the way we build. Pressure treated lumber replaced the need for endless maintenance of outdoor painted wood structures like decks, patios, and gazebos. Unless these projects were sealed, most had to be constructed from naturally rot resistant wood like redwood, which was expensive and in short supply.
Pressure treated lumber, known simply as “PT” in the construction industry, is usually made from some form of conifer like pine and spruce lumber. Pressure treated lumber, as the name implies, is treated not only with chemicals but pressure as well. These chemicals preserve the wood against water damage and taste bad to damaging insects like termites and carpenter bees.
Pressure treated lumber is available in a few different versions depending on what it will be used for. Generally, pressure treated lumber will be selected for its durability and resistance to moisture. However, some will also contain sealers and have been dried in a kiln, which extends the life of the board.
Here we will briefly discuss the different quality grades of pressure treated lumber, suggest when each might be used, and why you might choose one over the other.
What Are the Types of Pressure Treated Wood?
In general, pressure treated lumber used in construction can be categorized as premium, select, #1, #2, and #3 graded lumber. Not all pressure treated lumber manufacturers use the same terminology to designate one grade from another. Some manufacturers may use the term “select” to describe their best quality lumber, while most will use the term “premium”.
Pressure treated lumber must be graded by a government official who inspects the wood for defects like splits, grain patterns, knots, insect damage, and so on. In order of descending quality, pressure treated lumber is graded SS (premium/select), #1, #2, and #3. Beyond grade 3 lumber is considered useless in a construction scenario and is often used to make oriented strand board, known as OSB.
Pressure treated lumber is graded by the type of wood species and clarity, just like regular lumber, which is an indicator of the strength of the board. Boards that have a tight grain pattern and insignificant knots will be “more clear” than a board with an irregular grain pattern, knots, checks, or bark. Generally, the more clear a board is, the better the quality.
Premium Pressure Treated Lumber
Depending on the manufacturer, typically the best quality pressure treated lumber will have the word ”premium” stamped on the board or bar code tag. These pressure treated boards will be virtually knot free, straight, free of blemishes like bark, and in most cases will be dried in a kiln before delivery to the store.
Standard pressure treated lumber is notoriously wet when you buy it. Pressure treated lumber is literally soaked in chemicals that go all the way through the board. Eventually this moisture escapes through convection, but it often takes a hot summer after installation before the pressure treated lumber is considered dry enough to seal, paint, or stain.
Select Pressure Treated Lumber
Select grade pressure treated lumber can also represent the top quality a manufacturer has to offer, but it is usually considered a downgrade from premium. For manufacturers that use both terms, the difference between premium and select will usually indicate the presence of knots, but not on the edge.
A knot on the edge of a select board, in effect, makes the board weaker so a select board will not contain knots (or knot holes) within a ½” of the edge. Select boards will have typically straighter grain, as this will reduce the likelihood of the board cupping or curling in the future. Select pressure treated lumber is often used for the 5/4” floor boards on a deck for its smoothness.
Number 1 Grade Pressure Treated Lumber
Number one grade pressure treated lumber is another popular grade used for deck boards. Number one grade pressure treated lumber may have an irregular grain pattern and will often contain small knots or knot holes along the edge. Number one grade pressure treated lumber may have also been treated with water sealer or kiln dried, but not always.
Number 2 Grade Pressure Treated Lumber
Number two pressure treated lumber will rarely be treated with water sealer or be kiln dried at the factory. Number two grade pressure treated lumber will have irregular grain patterns, no limit on the number of knots, and is usually wet when you buy it. Number two grade lumber is more likely to twist, cup, and bow due to the irregularities in the grain.
Number two grade lumber is often the lowest grade lumber used for things like joists and posts, although many buyers will spend the extra money for number one grade when possible. Number two grade is often used for fence posts, retaining walls, and barns.
Number 3 Grade Pressure Treated Lumber
Number three grade pressure treated lumber will generally only be used for its ability to withstand ground contact. For example, most building codes require some form of insect and rot barrier between masonry and the wooden structure. Number three pressure treated lumber is often used for this purpose because it’s usually hidden from view and functions only as a spacer.
Other Categories of Pressure Treated Lumber
Scientifically speaking, pressure treated wood is classified into three general categories:
Borate is essentially a salt-based solution forced into the wood using hydraulic pressure. Borate works well for a while, but in most cases the solution will eventually leech out and release the borate into the environment. When the solution finally escapes the wood, there is little left to protect the wood from mold, mildew, or insects.
ACQ is a solution containing ingredients like copper and ammonium alkyl as a preservative. ACQ does a great job of protecting the wood from mold, mildew, and insects, but it should not come into contact with plants or animals, as it could be toxic.
Non-combustible pressure treated wood is created by forcing fire-retardant chemicals and other materials into the wood. The process essentially uses immense pressure to force the chemicals into the lumber, as opposed to simply applying some sort of fire-resistant coating.
These pressure treated materials are designed to not only prevent the spread of fire, but actually work to extinguish it. Non-combustible pressure treated lumber is used anywhere the building codes require it, such as in some public and commercial spaces like hospitals.
What Are Pressure Treated Lumber Grade Stamps For?
All pressure treated lumber will be stamped or tagged to indicate what the material can be used for. The notations of the tag or stamp prevent the builder from specifying incorrect lumber where the rating is important. Here is a brief explanation of the tags and the three pieces of information the builder needs:
- Rated Use
All chemically pressure treated lumber will have a tag or stamp indicating its recommended use. Pressure treated lumber designed for use a minimum of 6” from the ground is called Above Ground lumber. Lumber that can safely contact the ground is called Ground Rated, or Ground Contact lumber.
- Amount of Chemicals the Lumber Contains
The number of pounds of chemicals the board contains per cubic foot will indicate what it can be used for. Above Ground use means the lumber has .25/pound of chemicals in every cubic foot of wood. Ground Rated lumber will have .40/pounds of chemicals per cubic foot.
- Identifying the Preservative
The tag will also identify which process was used to preserve the wood. For example, a pressure treated board will usually have the letters ACQ, which indicates the board was preserved using an ammonium alkyl and copper type preservative.
What is the Best Pressure Treated Wood for Decks?
Most builders will choose either #1 or #2 grade pressure treated lumber when constructing a deck. As mentioned earlier, #1 lumber will usually be found on more elaborate decks where small details are important. Number two grade pressure treated lumber is likely the most popular overall, as it meets more building codes for strength, yet is less expensive that #1 grade.
If the project budget allows, premium or select pressure treated lumber may be used for handrails, guardrails, post caps, and other parts of the deck frequently touched by the user. Premium lumber and select lumber usually have rounded over edges, which helps prevent splinters and splitting.
Can I Mix the Different Grades of Pressure Treated Lumber?
You can absolutely mix the different grades of pressure treated lumber within the same project. For example, a common practice when building a deck involves saving the best lumber for the parts of the deck you touch, and using a lower grade for those parts you don’t.
Handrails, guardrails, benches, and deck boards will usually be built from the best lumber the budget will allow. Lower grade lumber can then be used for the posts, joists, spindles, or other areas of a deck where constant contact is unlikely. This practice does not weaken the structure, but might save the builder some money.
Pro Tip: If you are working with stainless steel screws, you can essentially make them go away because the screw head is small. When the screw goes in it displaces material, which you can tap back into the hole, covering the screw head.
What Is The Best Pressure Treated Wood For Fence Posts?
Generally, the best pressure treated wood for fence posts is pine or fir. The vast majority of wooden fence posts are 4” x 4”, or 6” x 6”, so pine and fir offer the least amount of waste and tend to be straight. Pressure treated fence posts offer very good resistance to damage from ground contact, especially when set in concrete.
Another popular post size is 4” x 6”, which adds 50% additional strength to the post in both directions. Four by six posts are also popular for use in raised garden beds, retaining walls, and landscaping barriers.
What is the Difference Between Above Ground and Ground Contact Pressure Treated Lumber and When Would I Use Them?
Pressure treated lumber is designated as either Above Ground or Ground Contact on the tag or stamp on the end of the board. In case you’re curious, manufacturers avoid putting the stamp on the wide side of the lumber because it would show after installation.
Building codes are concerned with the ability of pressure treated lumber to withstand remaining constantly wet while effectively repelling insects. A pressure treated board designed to be used above ground must be easily replaceable (like a swing set), or rated for ground contact (like a fence post).
As such, two different purposes were designated to differentiate between lumber that would be less than 6” above the ground, or higher. The idea is that pressure treated lumber that is accessible and ventilated can dry out, so it is less susceptible to damaging insects and rot. Pressure treated lumber within 6” of the ground however will struggle to stay dry, so it gets extra chemicals.
What Is the Best Pressure Treated Wood For Retaining Wall?
Most landscape designers prefer pressure treated landscaping timbers for small retaining walls. Many pressure treated landscaping timbers will be rounded on two opposing sides to resemble logs when they are stacked together. Most are attached with galvanized or aluminum timber spikes, which resemble a very big nail. These are suitable for retaining walls up to 48” tall.
The best option for retaining walls is often to use very long hot dipped galvanized lag bolts instead of spikes. For the same reason screws hold better than nails, bolts hold better than spikes. Most retaining walls can be built with ½” bolts, but if the timbers are large or the wall is tall, ¾” lag bolts are not uncommon.
What Are the Best Nails For Pressure Treated Wood?
If most professional deck builders have a choice, many won’t use nails at all. Most will use galvanized or painted deck screws instead. If the project will use nails, they must be hot dipped galvanized. Most deck nails are either #8 or #10 screw shank or twist shank, meaning they are very difficult to remove. The advantage of nails is speed, so a pneumatic nail gun is usually used.
Most pros use deck screws in modern deck construction because they can be installed quickly (although not as quickly as nails) and have great holding power. Screws can also be retightened, so as the pressure treated lumber dries out and shrinks the screws do not become a tripping hazard. Stainless steel screws are also popular and generally use a square drive head instead of a phillips head for strength.
What Is the Best Way to Attach a Pressure Treated Post to the Ground?
There are a number of ways to set a post, which means to permanently place it so that it remains plumb. Posts are strongest when they are perfectly plumb, which means to be perfectly straight up and down. When a post is plumb, it is also exactly 90 degrees perpendicular to a level surface.
Generally, the pros use special deck bases designed for the size of pressure treated posts they are using. In the old days, carpenters used lots and lots of nails, which simply didn’t have the holding power of today’s modern deck hardware. Joist hangers, corner supports, and deck bases are only a fraction of the hardware available for deck construction.
For example, when setting a pressure treated post on concrete, many pros will use a post base made from galvanized steel or aluminum. Some versions bolt directly to the concrete, while others provide a stand off feature that lifts the post away from standing water.
These types of hardware often add 10% to the cost of the materials, but the quality and time saved are often worth the investment. Plus, undamaged hardware can be reused over and over, so hardware is especially handy for newbies that make the occasional mistake.
#1 vs #2 Pressure Treated Lumber: When To Choose Which?
Number #1 Grade
Number one grade pressure treated lumber is probably the most common grade used for outdoor projects like decks, gazebos, trellises, and tree houses. These projects tend to be visible and number one grade strikes a good balance between cost and quality.
Number #2 Grade
Number two grade lumber is used primarily for projects like decks, retaining walls, raised flower beds, barns, and other structures requiring strength where the appearance is less important. Number two grade lumber will usually be the least expensive pressure treated lumber suitable for construction projects.
Premium or Select Grade
For contrast, premium or select pressure treated lumber will be used on projects that require lumber to be virtually free of visible defects. Premium or Select grade pressure treated lumber will usually also be treated with a water sealer (like Thompson’s Water Seal) and then gently dried in a kiln. This evaporates the moisture but leaves the chemicals in the lumber.
Species and Sizes Of Pressure Treated Wood
Pressure treated wood is usually made from some species of conifer, often southern yellow pine. Pressure treated lumber is available in the same sizes as regular lumber, with the exception of the deck boards.
Pressure treated deck boards are relatively unique in size because they serve a very specific purpose. Until manufacturers began milling boards specifically for decks, the deck boards were usually built from 2” x 6” or 2” x 8” lumber.
A new dimension, often called a five quarter (5/4) board, was designed in an effort to use less wood. A 5/4 board is slightly thinner than 2” lumber, but with proper placement of floor joists and stair stringers, this new lumber size saved many trees.
As the name implies, deck boards are 1 ¼” thick, which is more than strong enough for deck boards as long as the deck design is correct. Using 5/4 boards saves ½” of material with every board versus using 2” lumber, so the material savings is significant.
Considerations When Working with Pressure Treated Wood
Working with pressure treated wood requires a bit of extra protection to your skin and lungs. As mentioned earlier, most pressure treated wood contains harsh chemicals designed to impede organic life, like insects.
For the same reason you wear a mask and gloves when you bug spray on your garden, you should avoid contact with pressure treated wood and the sawdust. In addition, it’s a good idea to wear gloves when handling the lumber. As mentioned earlier, pressure treated lumber can be literally dripping wet when you buy it, so gloves are recommended to avoid skin contact with the chemicals.