Treated wood is ordinary wood that has been chemically treated to improve its resistance to rot, decay, and insect infestation. Treated wood is commonly used for outdoor construction projects such as decks, fences, siding, and other structures exposed to moisture and weather.
Different wood preservatives and treatment methods are used depending on the intended application. Modern wood preservatives can include copper-based formulations and other chemicals designed to protect wood from fungi and insects.
Pressure-Treated Wood Uses
Pressure-treated wood is commonly used for outdoor construction projects such as decks, siding, exterior stairs, ceilings, fences, and outdoor furniture. It can also be used for structural components and other applications where wood may be exposed to moisture or insects.
Pressure-treated wood is available for different exposure conditions, including above-ground and ground-contact applications. The appropriate treatment level should be selected according to how and where the wood will be used.
How Is Pressure-Treated Wood Made?
Pressure treatment forces wood preservatives deeper into the wood than a simple surface coating can. The exact process and chemicals vary depending on the wood species, preservative, and intended use.
Wood Selection
The first step is to select a suitable wood species because not all wood absorbs preservatives equally well. Some naturally durable wood species may require little or no chemical treatment for certain applications.
Sorting and Grading
The wood is sorted and graded according to its quality, thickness, size, and intended application.
The lumber may be cut to the desired dimensions before treatment. The treatment process is selected according to the type and dimensions of the lumber and the level of protection required.
Pressure-Treatment Process
The wood is placed inside a large treatment cylinder. A vacuum is first used to remove air from the wood cells.
The preservative solution is then introduced into the cylinder, and pressure is applied to force the preservative into the wood.
The pressure, temperature, treatment time, and amount of preservative used vary depending on the wood species, product, and required level of protection.
Removing the Wood
After the treatment process is complete, the wood is removed from the treatment cylinder.
Drying the Wood
The treated wood may contain a significant amount of moisture after treatment. It is therefore allowed to dry before certain finishing processes, such as painting or staining.
The drying time depends on the wood species, thickness, weather conditions, treatment process, and preservative used. Pressure-treated lumber may shrink, warp, or crack as it dries.
What Chemicals Are Used for Pressure-Treated Wood?
Different preservatives have been used for pressure-treated wood over the years.
Chromated copper arsenate (CCA) was historically one of the most widely used wood preservatives. In the United States, its use for many residential applications was voluntarily phased out beginning in 2004. CCA-treated wood is still used in certain industrial and commercial applications, so it is not accurate to say that CCA is completely banned.
Modern pressure-treated lumber can use preservatives such as alkaline copper quaternary (ACQ), copper azole (CA), and micronized copper-based formulations, depending on the manufacturer and intended application.
The specific preservative used should be checked on the product label or manufacturer’s information.
Types of Pressure-Treated Wood
There are mainly two common exposure categories for pressure-treated wood:
Above-Ground Pressure-Treated Wood
As the name suggests, above-ground treated wood is intended for applications where the lumber is not in direct contact with the ground and has adequate ventilation and drainage.
It can be used for applications such as:
- Deck components
- Railings
- Fences
- Siding
- Outdoor furniture
- Other above-ground structures
The appropriate treatment level depends on the expected moisture and insect exposure.
Ground-Contact Pressure-Treated Wood
Ground-contact pressure-treated wood is designed for applications where the lumber may be in contact with soil or exposed to conditions that can cause more severe deterioration.
It can be used both above ground and in ground-contact applications when the product is specifically rated for that use.
Soil contains moisture and organisms that can accelerate wood decay and insect damage. Therefore, lumber placed directly in or near the ground generally requires greater protection than lumber used in a well-ventilated, above-ground location.
Advantages and Disadvantages of Treated Wood
Advantages
- Insect Resistance: Pressure-treated wood is more resistant to certain insects and wood-destroying organisms than untreated wood.
- Durability: Wood preservatives help protect lumber against decay and deterioration, allowing it to last longer in conditions where untreated wood may deteriorate quickly.
- Moisture and Weather Resistance: Properly treated and maintained wood can perform well in outdoor environments where it is exposed to moisture and changing weather conditions.
- Low Maintenance: Pressure-treated wood generally requires less protection against decay and insects than untreated wood. However, it still requires inspection and maintenance, especially when used outdoors.
- Versatility: Pressure-treated wood can be used for a variety of outdoor applications, including decks, fences, retaining walls, stairs, and other structures.
- Availability: Pressure-treated lumber is widely available in different sizes and grades, making it relatively easy to find for common construction projects.
Disadvantages
- Chemical Treatment: Preservatives are used to protect pressure-treated wood from decay and insects. Because of these chemicals, treated lumber should not be used for applications where it is not approved, such as food-preparation surfaces or cutting boards.
- Warping and Shrinkage: Pressure-treated lumber can contain considerable moisture when purchased. As the wood dries, it may shrink, warp, or develop cracks.
- Drying Time: Some treated lumber needs time to dry before it can be painted or stained. The required drying time varies depending on the product and environmental conditions.
- Chemical Handling: Sawing, drilling, and sanding treated wood can produce dust containing wood preservatives. Appropriate protective equipment and safe working practices should be used.
- Environmental Concerns: Some wood preservatives can have environmental impacts, particularly if treated wood is improperly disposed of or burned.
Is Treated Wood Safe?
Modern pressure-treated wood is manufactured and regulated for specific applications, but “safe” depends on how the wood is used and handled.
Always follow the manufacturer’s instructions and product labeling. Avoid using treated lumber for cutting boards, countertops used for food preparation, or other applications where the product is not approved.
When cutting or sanding pressure-treated wood, wear appropriate eye protection and a dust mask or respirator suitable for the work. Wash your hands after handling the lumber, and keep sawdust away from children and pets.
Older CCA-treated lumber requires particular care because it contains arsenic. It should not be burned or used in applications where it may come into direct contact with food.
Is Treated Wood Safe to Burn?
No. Pressure-treated wood should not be burned in fireplaces, wood stoves, fire pits, or bonfires.
Burning treated lumber can release harmful chemicals into the smoke and leave concentrated contaminants in the ash.
The age of the treated wood does not make it safe to burn. Chemicals do not simply become harmless because the lumber has been treated for many years.
Dispose of unwanted treated wood according to local regulations rather than burning it.
Guide to Working With Pressure-Treated Lumber
While working with pressure-treated wood, follow these basic safety precautions:
- Wear gloves when handling treated lumber.
- Wear appropriate eye protection when cutting or drilling.
- Use suitable respiratory protection when producing significant amounts of dust.
- Wash your hands thoroughly after working with the wood.
- Keep treated wood and sawdust away from children and animals.
- Whenever possible, cut and sand treated wood outdoors or in a well-ventilated area.
- Do not burn treated lumber or use it as firewood.
- Follow the manufacturer’s instructions for handling, installation, and disposal.
Allow Treated Wood to Dry Before Finishing
Pressure-treated lumber is often wet when it is first purchased. It may need to dry before applying paint or stain.
Allowing the wood to reach an appropriate moisture condition can help the finish adhere properly and reduce problems such as peeling, cracking, and premature failure.
Always follow the recommendations of the paint, stain, and pressure-treatment manufacturers.
Can You Paint or Stain Pressure-Treated Wood?
Yes, pressure-treated wood can generally be painted or stained once it is sufficiently dry and prepared for finishing.
Before applying a finish:
- Make sure the wood is clean.
- Allow it to dry adequately.
- Test the surface if necessary.
- Choose a product specifically recommended for pressure-treated wood.
- Follow the manufacturer’s application instructions.
A penetrating exterior stain is often used for outdoor treated lumber because it can accommodate some of the wood’s natural movement.
How Long Does Pressure-Treated Wood Last?
The lifespan of pressure-treated wood depends on the treatment level, wood species, exposure, installation, drainage, climate, and maintenance.
Ground-contact lumber generally requires a higher level of protection than above-ground lumber.
Proper installation is also important. Even treated wood can deteriorate if water is continuously trapped around it or if it is installed incorrectly.
Final Thoughts
Pressure-treated wood is a practical choice for many outdoor construction projects because preservatives improve its resistance to decay and insects.
It is commonly used for decks, fences, stairs, retaining walls, outdoor structures, and other applications where untreated wood may deteriorate more quickly.
However, treated wood is not maintenance-free, and it should be used according to its treatment rating and manufacturer’s instructions. Proper handling is also important, particularly when cutting, sanding, disposing of, or working with older treated lumber.
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