Outside building projects and even some damp areas inside require wood that is resistant to moisture damage. Using expensive wood that is naturally resistant may not fit the budget. Luckily, a more budget-friendly option of pressure treated lumber grades is available.
Pressure treated lumber has been infused with preservatives to prevent rot and insect damage. The greater the content of preservatives denotes the type and use of the treated wood. Like all lumber, the fewer knots, blemishes, or defects, the higher the grade.
In this article, we’ll explain pressure treated (pt) lumber, the different types and uses, and how it is graded. By the end of the article, you’ll have a better understanding of what different pressure treatments mean, and how to select the grade and type for the use you intend.
- What Is Pressure Treated Lumber?
- How Is Pressure Treated Wood Produced?
- Types of Wood Preservatives
- Uses for Pressure-Treated Wood
- Types of Pressure-Treated Wood
- Pressure Treated Wood Grades
- What is the Best Pressure Treated Wood for Decks?
- #1 vs #2 Pressure Treated Lumber
- Species and Sizes Of Pressure Treated Wood
- Pressure Treated Lumber Grade Stamps
- Tips for Working with Pressure Treated Wood
What Is Pressure Treated Lumber?
Protecting wood against rot and insects has been done for centuries. Some types of timber have natural abilities while others need a boost. In ancient times the Greeks used olive oil, the Romans tar, and others used pitch or lye. In the 19th-century railway companies used creosote to coat railway ties and timbers for bridges. Modern chemical baths and pressurization began in the early half of the 20th century.
Today, pressure treated lumber for residential use is commonly pine, spruce, or fir that has been immersed in a water-based chemical bath of preservatives. Pressurization then forces the liquid deep into the wood. The wood then is dried naturally or in a kiln to evaporate the water out, leaving the preservatives behind.
Different chemicals have been used in the preservative bath, with changes being made to limit health risks to humans and animals. Oil-based and creosote treatments are still used but since the preservatives leach out over time and with heat, they are considered too messy or hazardous for residential use. Those and other treatments are used for industrial purposes instead.
Benefits of Pressure Treated Wood
Pressure treatment is an affordable alternative to expensive naturally rot-resistant lumber. The preservatives provide protection against rot, insects, mold, and fungi, so the wood lasts longer. Treated wood is made from strong evergreen species making it suitable for most building projects.
The wood weathers better than untreated wood and won’t gray or discolor as quickly. Pressure treated wood can also be stained or sealed to enhance its color and for added protection.
How Is Pressure Treated Wood Produced?
Pressure treatment of lumber has come a long way in the past 100 years. The current process is mostly computerized and mechanized using modern technology. The process is similar for treating milled lumber, plywood, posts, or utility poles against rot and insects.
The treatment doesn’t penetrate deeply into the wood due to the impermeable nature of the wood cells. However, enough gets into the wood to provide a shell of protection that can’t be matched with a brush. Some wood goes through a machine that incises small cuts into the surfaces for deeper chemical protection.
The dry wood is loaded into a treatment cylinder which is then sealed and depressurized to vacuum as much air as possible from the wood cells. The chamber is then flooded with a diluted chemical solution, and hydraulic pressure applied for 20 to 60 minutes to force the solution deeper into the wood. The vacuumed cells more readily absorb the solution for better penetration.
At the end of the pressurized period, the solution is pumped out and stored for use on the next batch. A quick depressurization cycle then removes any excess from the wood before it is removed from the treatment cylinder.
The treated wood is then taken to the yard to dry naturally or to the drying kilns before shipping to the retailer. As the wood dries, the preservative is left behind to protect against insects, mold, mildew, fungal growth, and rot.
Types of Wood Preservatives
Different chemical solutions are used to create treated lumber. Here is a list and description of some of them.
Chromated Copper Arsenate (CCA)
CCA is a mixture of chromium, copper, and arsenic that turns wood green. It was widely used as a preservative from the mid-1930s to 2004. A study by the EPA in 2000 deemed it too hazardous for residential use, and in 2003 the lumber industry agreed to stop using it for residential purposes. CCA is still used for industrial purposes today.
Borates are sodium salts used in water-based pressure treatment solutions. They don’t discolor the wood and protect against insects, molds, mildew, and fungi. Unfortunately, the water-soluble nature makes them susceptible to leaching out in rainy or wet areas, or near standing water. However, they make an ideal solution for use in arid or dry locations.
Micronized Copper Azole (MCA)
MCA is an environmentally friendly water-based preservative considered safe for humans and animals and is an alternative to CCA. However, it shouldn’t be used where it comes in contact with food or animal feed.
The solution protects against insects, mold, mildew, fungus, and rot, and turns the wood a light brown hue. MCA can be used inside or out, above ground, in-ground, or in fresh-water contact residential areas.
Alkaline Copper Quaternary (ACQ)
ACQ is another environmentally safe preservative composed of soluble copper and either quaternary ammonium alkyl or substitute compounds. The solution protects the wood from rot, insects, fungus, mold, and mildew, and also turns the wood a brown tone. Similar to MCA, the treated wood shouldn’t be used in contact with food or animal feed.
Pro Note: All copper-based wood preservative solutions will corrode unprotected iron and steel brackets and fasteners. Use tar paper or bituminous tape to protect metal brackets and ho-dipped galvanized or stainless-steel fasteners, or those specially coated to prevent corrosion by wood preservative chemicals.
Additionally, all cuts and drilled holes should be treated with preservatives to prevent damage to exposed untreated wood.
Uses for Pressure-Treated Wood
Pressure treated wood has many outdoor uses and even is used in some indoor locations, especially where moisture may be a concern. Different types of treated lumber are used in many aspects of residential, commercial, and industrial construction where moisture or ground exposure may occur.
From the cedar shakes, facia, and trim of a house to the sill plate and wooden foundations or supports, pressure treated wood is common in residential construction. It is used to build exterior stairs, for posts, sheathing, siding, porches, decking, and railings too.
Many homes have fences, retaining walls, and garden boxes constructed of treated wood, and even the wooden utility poles supplying power and communications are pressure treated.
Basements and bathrooms are other areas where pressure treated wood can be used to frame interior walls to protect against moisture, mildew, and mold damage. Some lumber is also treated for fire protection and used throughout different areas of residential, commercial, and industrial construction. There are many other uses for pt lumber, including docks, boathouses, sheds, boardwalks, play structures, gazebos, and the list goes on.
Types of Pressure-Treated Wood
Walking into a lumberyard or store can be a daunting experience. Many DIYers and even some professionals aren’t familiar with the different types of pressure treated wood and their uses, or that the information is on the end-tag stapled to the board. Selecting the correct type of treated wood for a project often depends on how and where it will be used.
Pressure treated lumber is divided into different types based on the ‘retention level’ or amount of preservative retained in the wood after treatment. The retention level is measured in pounds of chemical per cubic foot of wood (pcf). The longer the wood is in the pressure chamber, the greater the content of chemical preservatives forced into and retained in the wood.
Pressure treatment doesn’t make the wood stronger or weaker, just more resistant to insect and moisture damage. The amount of chemical retained determines how resistant the lumber will be. The American Wood Protection Association (AWPA) sets the standards for the retention level required for the different chemical preservative compositions for specific types of use.
The result of having a unified rating means the end-tag on lumber identifies how well the wood will stand up in different conditions, regardless of the chemical cocktail it’s been exposed to. There are six different types of pressure treatment ratings, with different categories in some of them. The type identifies the amount of chemical retention which in turn determines how or where to best use the lumber.
Use UC3A pressure treated lumber for above ground exterior projects. The wood should be at least 6-inches above the ground, have good air circulation and drainage, and the components can easily be replaced.
Ground contact means anything within 6-inches of the ground or in contact with the ground, foliage, grasses, or over water or wet areas. It may have poor drainage and air circulation, or like a sill plate, is difficult to repair or maintain. Pressure treated wood for ground contact commonly has twice the chemical retention level as above ground rated lumber. UC3B or UC4A are best for ground contact use.
In-Ground Contact for Critical Use
Pressure treated wood used for in-ground or critical use has a greater retention level than timber used for above or ground contact. Use in-ground type lumber where there is a greater risk of deterioration or severe environmental exposure, or for critical structural components like foundation posts, exposure to fresh or groundwater, or utility poles.
The recommended type of treated wood for in-ground use is UC4B and UC4C; UC4B has a higher retention level than lower graded types, and UC4C has even more.
Wood treated with water-based preservatives for saltwater immersion has significantly higher retention levels than other types and categories. UC5A has almost double the weight of chemical protection as UC4C, and UC5B and UC5C both have more than three times the amount found in UC4C.
The table below identifies the types and their use based on the level of retention of preservatives.
|Types of Pressure Treated Lumber and Where to Use Them|
|UC1||Interior Dry||Stud framing & baseboards|
|UC2||Interior Wet||Sill & bottom plates, damp locations, basement framing, bathrooms, flooring, baseboards|
|UC3||A||Exterior Above Ground, Coated with Rapid Runoff||Protected decking, fascia & trim|
|B||Exterior Above Ground, Uncoated or Poor Runoff||Cedar shakes, exterior stairs, joists, beams, decking, railings and fence boards/pickets, any wood within 6” of soil & vegetation|
|Fence, deck & structural posts, ledgers, retaining walls, garden boxes, any wood within 6” of soil, vegetation, or freshwater, or poor air circulation|
|As with UC4A plus posts set into the ground, retaining walls, wood foundations & supports, freshwater contact, saltwater spray, garden & utility posts|
Extreme duty use
|Pilings or wood in direct installation in soil, gravel, concrete, freshwater, or extreme weather exposure|
|UC5||A||Marine Use, northern waters,
Salt or brackish water
|Contact & immersion in cold ocean waters north of Long Island & San Francisco, docks, piers, wharves, shore walls, boardwalks, & buildings|
|B||Marine Use, central waters,
Salt or brackish water
|Contact & immersion in warmer ocean waters south of San Francisco and Long Island, and similar to UC5A|
|C||Marine Use, southern waters,
Salt or brackish water
|Contact & immersion in warm Gulf Coast waters & south of Georgia, and similar to UC5A|
|UCF||A||Interior Above Ground
|Interior construction, no ground or moisture contact, fire partition walls, kitchens, hallway & stairwell framing|
|B||Exterior Above Ground
|Exterior construction, exposure to weather but not ground contact, porches, exterior stairs, window treatments|
Pressure Treated Wood Grades
Since 1960, lumber in North America has been graded at lumber mills by trained inspectors for structural strength and quality control. The lumber is inspected for knots, splits, and other defects that can weaken the wood’s structural strength.
All structural lumber used in the U.S. and Canada requires a stamp identifying it as grade SS, #1, #2, or #3. Building codes, inspectors, builders, engineers, and other professionals use the grades to make sure structures are safe for use.
Lumber must be stamped for strength by a trained inspector and identified with a legitimate stamp. The grading is standardized and consistent for structural strength across the U.S. and Canada. Some mills may then identify it A, B, C, or D for appearance based on how clear or free it is of knots and blemishes.
There is no standard for appearance grading and select or premium could be #1 or #2 structural grade, it just increases the cost to the consumer for better wood.
The size, location, and the number of knots and other defects, and the slope of the wood grain affect the structural strength of lumber. These factors, plus the moisture content at the time of milling, determine the grade assigned. The stamp identifies the wood species, grade, moisture content, inspection organization, and the mill it came from.
Dimensional wood from all grades is pressure treated for different uses. The grades are stamped prior to the wood being treated, so are the same for both treated and untreated. The preservatives don’t affect the load-bearing abilities, just provide protection against insect, mold, mildew, and rot damage. However, treated lumber may be kiln-dried or wet and will shrink, cup, and warp as it dries.
Here are the grades and types of lumber most commonly available, plus the percentage of clear wood you could expect for the appearance market or ‘grade’:
Lumber with few or no knots or blemishes may be called premium by some mills or retailers to capitalize on DIYers’ wish for better-looking lumber. There is no structural premium grade, it is strictly a term for bright looking wood free of blemishes or knots.
The lumber has the structural strength it is graded with but has fewer blemishes or knots, so a better appearance. Premium is usually #1 or #2 grade lumber.
The clear appearance makes it a choice for trim, furniture, and cabinetry. Premium is often identified as A, B, C, and D. ‘A’ has no visible defects, knots, or splits, ‘B’ has a few small defects visible, ‘C’ may have small knots on one side and none on the other, and ‘D’ may have pin size (up to 1/2”) knots or small defects on both sides.
Being 80% or more clear of defects, it is often used for cabinetry, furniture, and trim.
Select Structural (SS) is the highest grade based on strength and durability. It has a grain slope of 1 in 12, may have seasoning checks, and tight, well-spaced knots up to 2-1/4” on a 2×8. Select grade lumber may have a knothole every 4’ and splits can not be longer than the board is wide.
The lumber is stronger than #1 or #2 and is ideal for all structural construction applications. It can be used to span greater distances, an SPF SS 2×8 will reach 15’-3”.
- Its appearance ranking is 80% or better clear of defects.
Grade #1 Structural
Lumber structurally graded #1 or #1 & BTR (#1 and better) has a wood grain slope of 1 in 10. It is stronger than #2 or #3 grade lumber. Knots must be well spaced and tight, so they won’t fall out, and no larger than 2-3/4” in a 2×8. Lumber may have one hole every 3’ and any splits must not be bigger than the plank is wide.
The lumber is commonly used for all facets of structural construction, plus furniture, shelving, decking, railings, siding, post, and other purposes where its visible appeal can be appreciated. Structurally, an SPF #1 2×8 can be used to span distances of 14’-11”.
- Its appearance ranking requires it to be 75% free of knots or defects.
Grade #2 Structural
Number 2 grade lumber often has more knots and blemishes than #1 grade but may be clear enough on one face to be deemed select or premium. The wood grain has a 1 in 8 slope, and there may be bark edge or wane visible.
The lumber may have splits up to 1.5 times the board’s width, knots no larger than 3-1/2”, and one hole every 2’. Lumber stamped #2 grade is commonly used for framing, lintels, rafters, trusses, joists, beams, and fencing. Structurally, an SPF #2 2×8 can be used to span distances of 14’-11”.
- In appearance grade, the planks are 66% clear wood.
#3 Grade Structural
Lumber with more checks, splits, wane, well-spaced larger knots and holes, and a grain slope of 1 in 4 are stamped #3 or construction grade. The wood is commonly used for light construction or framing where it isn’t visible, or for bracing, packaging, and shipping purposes. Structurally, an SPF #3 2×8 can be used to span distances of 12’-4”.
- In appearance, it is at least 57% clear of defects.
Construction grade commonly refers to #1 grade lumber. It is used for all areas of residential construction from load bearing to trim.
Standard grade lumber has more defects and knots and is #2 structural grade wood. Commonly used for joists, trusses, and other load-bearing applications.
- The wood is 43% clear for appearance ‘grading’.
Utility grade is #3 grade lumber and has even fewer restrictions regarding knots and defects compared to other grades. It is not as strong as the other grades and is commonly used for where it won’t be visible, and light framing purposes such as bracing, crates, and pallets.
- For appearance, it must be 29% clear of defects.
Premium and select classifications and the percentage of clear wood are appearance grades used by some mills and retailers. The appearance doesn’t affect the structural grade, it just is superficially more appealing. The look of the wood is only important if it is highly visible, such as deck boards, railings, and other visible structural members.
What is the Best Pressure Treated Wood for Decks?
The best pressure treated wood for decks is Select, #1 or #2 grade wood. Within all grades, there are planks that will be freer of blemishes and knots and may be called premium. Decking materials such as 2×6 and 5/4×6 frequently are of better-looking #1 and #2 lumber.
#1 vs #2 Pressure Treated Lumber
A walk down the lumber aisle or through a lumberyard will provide some understanding of the differences between #1 and #2 grade pressure treated lumber, or increase your confusion. The most noticeable difference may be the price and scarcity of #1 grade lumber.
Most lumber in my area is listed as Prime or Premium depending on the box store and is stamped #2 or #2 & BTR. One even identifies its pressure treated pine fence boards as ‘D & better’ – so its appearance has small pin knots and some other blemishes.
No matter what it is called or tagged, it should bear a stamp on the wood surface identifying it as SS, #1, # 2, or #3. It should be noted, #2 & BTR does not mean #1. If the wood is #1, it is stamped #1.
The main difference between #1 and #2 is the slope of the wood grain, size, spacing, and the number of knots and holes, and the length of any splits. Construction grade is #1 and Standard is #2. Both can be used for load-bearing purposes or other outdoor applications. The species and grade, however, affect the spacing between supports with #1 often rated for greater spans.
The table below identifies the main differences.
|#1 vs #2 Pressure Treated Lumber|
|# 1 Grade PT Lumber||# 2 Grade PT Lumber|
|Strength||High strength and stiffness||Moderately high strength and stiffness|
|Wood Grain Slope||1 in 10||1 in 8|
|Knots||Tight and well-spaced, and no larger than 2-3/4” on a 2×8||Well-spaced and no larger than 3-1/2” on a 2×8|
|Holes||1 every 3’ or more||1 every 2’ or more|
|Pitch & Pitch Streaks||Unlimited||Unlimited|
|Splits – a through separation||Not bigger than the plank is wide||1.5 times the width of the board|
|Wane (bark edge)||1/4 the thickness and 1/4 the width full length, or not greater than 1/2 the thickness or 1/3 the width for 1/4 of the length||1/3 the thickness and 1/3 the width full length, or not greater than 2/3 the thickness or 1/2 the width for 1/4 of the length|
|Checks – separation across or through growth rings due to seasoning||Unlimited surface seasoning checks with through checks limited to splits||Unlimited surface seasoning checks with through checks limited to splits|
|Skips – surfaces that were missed or failed to clean||Maximum of 10% of the plank||Maximum of 5% of the plank or 2’ or less in length|
|Shake – lengthwise separation through or between growth rings||Limited as splits if through or 2’ long on the surface only||Limited as splits at ends if through or 2’ long if well separated, or maximum of 3’ or 1/4 the length if surface only|
|Warp – bow, crook, cup, twist, or any combination||1/2 of medium||Light|
|Stain – variations in natural color||Sapwood stained or firm heart or red heart stain||No limit to sapwood stained or firm heart stain or red heart|
Species and Sizes Of Pressure Treated Wood
Pressure treated lumber comes in a variety of lengths, widths, and thicknesses and from different species of softwood. The type and dimension influence the strength of the timber.
Different Kinds of Softwoods That Are Pressure Treated
The kinds of softwood available in different geographic regions often determine what species is available for pressure treatment. Southern yellow pine (SYP) is very strong and is common in the eastern U.S. A mixture of spruce, pine, and fir (SPF) dominate much of the northern U.S. and eastern and central Canada and isn’t as strong as SYP.
Douglas fir, fir, and hemlock dominate in the western U.S. and Canada, with Douglas fir ranking the strongest of the commonly milled and treated softwoods, while the fir and hemlock are slightly stronger than SPF.
Species of Wood
The main tree species used in the manufacture of pressure treated lumber are softwoods that regionally predominate in the mill area. Southern yellow pine in the eastern U.S., spruce pine and fir in the northern U.S. and eastern-central Canada, and Douglas fir, fir, and hemlock in the western U.S. and Canada.
Southern Yellow Pine
Longleaf, shortleaf, loblolly, and slash are some of the native southeastern pines that are classified as SYP. It is a very strong and stiff softwood, and dense too. It holds nails, screws, and other fasteners well, and is commonly used in residential, industrial, and commercial construction. SYP holds almost 85% of the U.S. pressure treated market. It has a unique broad grain cellular structure that allows for uniform and deep penetration of chemical preservatives.
Douglas fir has a tight close grain cell structure making it stronger and less prone to warping and twisting than pine. However, the tight grain also makes it less receptive to pressure treatment. Older growth wood is better but more expensive and difficult to find.
Pressure treated lumber is available in 1”, 5/4”, 2”, 4”, and 6” nominal thicknesses, and widths up to 12”. Treated posts or ties readily available at yards and stores are 3×5, 4×4, and 6×6; 8×8 is also available but may be a special order. Lumber, posts, and ties range from 8’ to 16’ in length, although longer may be ordered. Pressure treated plywood is also available in thicknesses from 1/4” through 3/4″.
Pressure Treated Lumber Grade Stamps
Lumber is stamped after it is milled and prior to being pressure treated. Pressure treated lumber commonly is end-tagged with an information label before leaving the plant.
The stamp identifies:
D Fir-L or Doug Fir for Douglas fir or larch (L)
SYP for Southern yellow pine
H-F, Hem-fir, or Hem for hemlock-fir or hemlock
S-P-F for spruce, pine, or fir
Grade of Lumber
The grade of lumber is noted – SS, #1, #2, #3, or STUD – and the price reflects the quality.
The amount of moisture in the wood at the time of milling is noted. Unseasoned or green lumber has a moisture content of 19% or greater and subject to shrinkage and warping as it dries. It is stamped S-GRN for surface-green or AD for air-dried. Lumber that is KD (kiln-dried) or S-DRY (surface-dry) has a moisture content that is lower, between 16% and 19%.
Lumber that is imported or for export requires the designation KD-HT, which means it has been kiln-dried and heat-treated to kill insects. KD and KD-HT are often more expensive and less susceptible to warping and shrinkage. MC-15 identifies lumber as having 15% or less moisture content.
Registered Logo or Symbol
All lumber must identify the association or agency accredited with overseeing the grading – MLB is the Maritime Lumber Bureau.
The certification identifies the grade rules used to grade the lumber – for example, ALSC is the American Lumber Standard Committee and NLGA is the National Lumber Grades Authority.
A number that identifies the mill where the lumber was produced. It often links with the logo or symbol of the association that oversees the grading at that mill.
There may be a secondary stamp that identifies the visual grade as Premium or Select A, B, C, or D.
Pressure treated lumber also has an end-tag stuck or stapled to one end. The tag identifies its proper use – Above Ground, Ground Contact, etc. There is a barcode with product information, registration logo or symbol noting the standards followed, and the use category (UC4A for Ground Contact).
The type of preservatives, retention level, and year it was treated are identified on the label. The accredited inspection agency, manufacturer, and plant are also commonly on the label but may be available in the bar code.
Tips for Working with Pressure Treated Wood
Working with pressure treated lumber is similar to working with untreated planks. Gloves protect against slivers and water-soluble chemicals; it is also advisable to thoroughly wash hands before eating or drinking. While most residential pressure treated lumber is deemed safe for use, dust and particles can still be harmful.
Wearing safety glasses or a shield protects the eyes from dust and debris when drilling, cutting, or sanding, and a dust mask will prevent inhalation of dust and chemicals. Treated wood will shrink as it dries and expand when wet and may acquire surface checks and cracks over time. Overly dry wood often needs a pilot hole before fastening to prevent splitting.
Pressure treated wood may require several weeks or even months to dry before it will absorb stain or paint. The chemical treatment makes the wood resists insects, mold, mildew, and rot but not moisture-proof. It needs to be treated every 2 to 3 years (depending on the environment) to protect it from moisture damage.
Pressure treated lumber has been treated with a chemical solution to protect it from insects, mold, mildew, and rot. There are different types of treatment and lumber grades for different uses.
I hope you have a better understanding of what pressure treatment means and which grade and type is best for your project.