September 22, 2023

A Deep Dive into Grades of Pressure Treated Lumber: Your Ultimate Guide

Pressure-treated lumber extends wood longevity, prolonging the life of the material. The process uses chemical preservatives, which makes lumber resistant to insects and moisture. It also protects the wood in severe weather climates. The lumber lasts longer in any environment exposed to the elements or consistent moisture. 

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It is insect and fungal-resistant, helping it outlast natural wood. It is denser than untreated wood products. It also has increased strength. Pressure-treated lumber is fungal-resistant and fire-resistant. Pressure-treated lumber is available in multiple sizes for different applications. Although it comes at a price, it is worth it due to its durability and ease of use. 

Pressure-treated wood comes with lumber grades from Number 1, Number 2 to Number 3. The higher the grade, the fewer the defects. These defects come in splits or knots. This article will explore more on the lumber grades and their significance.

What is Pressure Treated Lumber?

Pressure-treated lumber is a process that uses high pressure to inject a preservative into the wood. The treatment process extends the longevity of the wood, adding more life to it. The purpose of wood pressure treatment is to force preservative chemicals into the cellular structure of the wood. 

The chemical is a barrier between the wood and deterioration agents. That explains how the service life of the wood increases. As early as 1940, wood was pressure treated with chromated arsenicals to protect the wood from rotting due to insect and microbial agent attacks. From the 1970s to the early 2000s, the wood used in outdoor residential settings was chromate arsenic-treated wood.

The Pressure Treatment Process

Step-by-step explanation

  1. Check wood for moisture content to ensure the moisture is not too high to accept the preservative treatment.
  2. Use a forklift to place the wood on a tram that will move the wood into a large steel cylinder, a vacuum pressure vessel. 
  3. The doors are closed and the cylinder is sealed.
  4. An industrial vacuum pump removes air from the cylinder and pulls air out of the wood.
  5. Fill the cylinder with the preservative solution.
  6. Apply pressure to the solution to force the preservatives deep into the wood cells.
  7. Adjust cycle times and pressure settings based on the retention levels needed and the species of the wood treated.
  8. The cylinder is drained once the cycle is complete, and then the industrial vacuum pump removes the excess solution from the wood.
  9. Run a final vacuum within the cylinder to extract excess preservatives.
  10. Open the door, remove the wood, then place freshly treated wood on a drip pad for 24 to 48 hours.

Chemicals used in treatment

  • Chromium
  • Copper 
  • Arsenic

Common Uses of Pressure Treated Lumber

Outdoor applications

Treated wood is for decks, fences, pergolas, and retaining walls. For outdoor structures, it is durable and resistant to moisture. It is an excellent choice for garden beds, walkways, and edging due to its resistance to decay and insect infestation.

Residential and commercial construction

Pressure-treated wood is also common in residential construction. It is for building exterior stairs, posts, sheathing, siding, porches, decking, and railings. Most fences, retaining walls, and garden boxes are treated wood. The wooden utility poles supplying power and communications are also pressure treated.

The Importance of Grades in Pressure-Treated Lumber

Ensuring quality and durability

Pressure Treated Lumber gives you a retaining wall that will stand the test of time. You do not have to worry about some durable hardwoods for a furniture piece if you have no idea what timber will work best for your project. When choosing the right wood for your project, pressure-treated lumber gives you timber that is fit for use. It offers timber durability when the hazard grades come in.

Industry standards and regulations

To produce pressure-treated wood, saturate the milled lumber with chemical preservatives. These chemicals minimise the wood’s natural vulnerability to insects and rot. Place untreated wood into a treating cylinder.

Grade 1: Select Structural

Characteristics and applications

  • Firm
  • Tight
  • String
  • Durable

Strength and load-bearing capacity

That is the highest grade of lumber for its strength and durability. You hardly find any defects in this grade of lumber. Knots are permitted in sizes as long as they are sound, firm, tight, and well-spaced. The strongest grade is Select, which is usually free of any knots.

Grade 2: Dense

Features and suitable applications

  • Multiple knots.
  • Irregular grain pattern.
  • It is used for projects like decks, retaining walls, raised flower beds, and barns.

Balancing strength and cost

Number two grade pressure treated lumber has irregular grain patterns, no limit on the number of knots, and is wet when you buy it. It tends to twist, cup, and bow due to the irregularities in the grain. It is used for sheathing and applications without a fine finish.

Grade 3: Stud

Characteristics and usage

  • The grade is #3-grade lumber and has fewer restrictions regarding knots and defects than other grades. 
  • It is not as strong as the other grades.
  • It works where it will not be visible and for light framing purposes such as bracing, crates, and pallets.

Structural support in framing

The wood appearance should be visible, such as deck boards, railings, and other visible structural members.

Grade 4: Standard and Better

Overview and common applications

Class 4 is for end uses where wood is in contact with or near the ground and frequently wet. 

Considerations for non-structural projects

Class 4 timbers have the lowest natural durability grade and last outdoors in an above-ground setting of 0 to 7 years and in-ground of between 0 and 5 years.

Grade 5: Utility

Features and applications

  • It has the largest amount of waste areas and coarser defects.
  • It is considered an economy grade of lumber. No. 5 softwood has the most waste areas and defects. 

Economical choices for non-critical use

It works in similar applications to No. 4, where it will not be seen but can make forms, molds, or models.

Grade 6: Economy

Characteristics and suitable projects

  • It contains numerous splits.
  • It has knotholes and similar defects, which are large areas of waste wood.

Lower cost options for temporary structures

The most temporary structures are formwork and scaffolding. Formwork provides a temporary structure used in concrete, and scaffolding is an elevated platform that supports construction workers, materials, and tools.

Comparing Pressure Treated Lumber Grades

Different projects require different types of pressure-treated lumber. Grades refer to the quality, strength, and appearance of the wood. Grade 1 has the strongest wood and is knot-free. Number one grade pressure-treated lumber mag has an irregular grain pattern and knots or knot holes along the edge. Number two grade pressure-treated lumber will have irregular grain patterns and no limit on the number of knots. 

Factors Affecting the Selection of Grades

Environmental considerations

  • Climate
  • Sustainability 
  • Recycling
  • Availability 

Project-specific requirements

  • Knots
  • Splits
  • Checks
  • Durability.
  • Strength.
  • Permeability.
  • Hardness.
  • Toughness.
  • Elasticity.
  • Workability.
  • Weight.

Understanding the Tag Information

Decoding the label

The wood is an abbreviated form, and the grade designation will be in the center. It indicates that a piece of wood meets established standards for strength and stiffness, offering information about the type of wood and how much moisture it contains.

Reading and interpreting grading codes

Identify the wood species in an abbreviated form and the grade designation will be placed in the stamp center. There are multiple industry-specific grades for different wood products. The most common are FAS, Select, #1 Common, and #2 Common, from best to worst.

Safety Precautions When Working with Pressure-Treated Lumber

  • Wear gloves and long sleeves.
  • Put on a dust mask when cutting it. 
  • Wash your hands after working with the product.
  • Only work with treated wood outdoors.
  • Dispose of dust and scraps properly.

Maintenance and Longevity of Pressure Treated Lumber

Care and upkeep guidelines

  • Seal and stain.
  • Cleaning
  • Sanding

Factors affecting the lifespan

  • Weather 
  • Seasonal temperature changes
  • Pressure treatment
  • Construction methods
  • Fasteners, brackets, and hangers used
  • Air circulation
  • The frequency of maintenance 
  • The application of sealants 

Alternatives to Pressure Treated Lumber

  • Cedar
  • White oak
  • Cypress 
  • Redwood

Common Myths and Misconceptions

  • Pressure-treated wood is a health risk.
  • Pressure-treated wood poisons surrounding plant life and soil.
  • Pressure-treated wood is not as strong as some of the other options.

FAQs about Grades of Pressure Treated Lumber

What grade of pressure-treated lumber is suitable for outdoor decks?

For outdoor decks, use Grade 2 or higher pressure-treated lumber.

Pressure-treated lumber comes in various grades: 1, 2, 3, and higher. Grade 2 or higher is best for decks due to improved durability and resistance to decay and insects. These boards have fewer knots and a smoother appearance, enhancing deck aesthetics.

Consider local building codes, and personal preferences. Consult with local authorities or a contractor for specific grade requirements. Also, consider board dimensions, treatment type, and maintenance when selecting deck lumber.

Can I use lower-grade pressure-treated lumber for structural projects?

Using lower-grade pressure-treated lumber for structural projects is not recommended due to potential durability and strength issues.

Lower-grade pressure-treated lumber often has lower preservative retention levels, making it less effective in withstanding moisture and environmental conditions. In structural applications prioritizing safety and longevity, use materials meeting or exceeding required structural standards.

For structural projects, choose pressure-treated lumber with appropriate grade and retention levels per building codes and engineering standards. This ensures wood can endure intended loads and environmental factors, ensuring safety and durability. Consult a structural engineer or building code authority for guidance on suitable pressure-treated lumber, ensuring structural integrity and safety.

Are there any health risks associated with pressure-treated lumber?

Yes, potential health risks are linked to pressure-treated lumber due to the chemicals used in treatment. These chemicals, such as chromated copper arsenate (CCA) or alkaline copper quaternary (ACQ), can lead to:

  • Skin Irritation: Handling without gloves may cause redness, itching, and sometimes, skin ulceration.
  • Inhalation Risk: Cutting or sanding can release airborne sawdust with treatment chemicals that might irritate the respiratory tract and lead to severe chronic or extensive exposure issues.

To minimize these risks:

  • Wear Protective Gear: Use gloves, long sleeves, and safety glasses to prevent skin contact and eye irritation.
  • Work in Well-Ventilated Areas: When cutting or sanding, ensure good airflow or use dust collection systems.
  • Clean Hands and Clothing: After contact with pressure-treated wood, wash hands and clothing to remove residue.
  • Avoid Burning Treated Wood: Burning it releases harmful chemicals. Proper disposal and adherence to local regulations are essential.

Stay informed about safety guidelines related to pressure-treated wood to safeguard your health and follow any regulation updates.

What is the typical lifespan of pressure-treated lumber?

The lifespan of pressure-treated lumber varies based on:

  1. Wood Treatment: The preservative used impacts lifespan. Some treatments suit above-ground, others ground contact or immersion. Stronger treatments like copper-based extend life.
  2. Environmental Factors: Climate and conditions matter. High humidity, rainfall, or saltwater exposure can hasten decay. Dry, arid regions may lengthen lifespan.
  3. Maintenance: Regular upkeep, like sealing and staining, extends wood life. Neglect reduces longevity.
  4. Installation Quality: Proper techniques, including drainage, ventilation, and avoiding ground contact, boost longevity.

Pressure-treated lumber lifespans range from 10 to 40+ years. Consider these factors when using it outdoors and follow manufacturer recommendations.

Can I paint or stain pressure-treated lumber?

Yes, you can paint or stain pressure-treated lumber.

Here are key tips for painting or staining pressure-treated lumber:

  • Allow Drying: New pressure-treated lumber is often damp from treatment. Let it thoroughly dry, depending on conditions, which can take several weeks to months. Aim for a moisture content of 15% or less.
  • Choose the Right Products: Use paint or stain designed for treated wood; these adhere better and provide excellent weather protection.
  • Prepare Surface: Clean the wood thoroughly, removing dirt, debris, and mildew with a brush or pressure washer. Lightly sand for a smooth finish.
  • Apply Multiple Coats: For best results and durability, apply multiple coats, following the manufacturer’s drying time recommendations.
  • Maintenance: Preserve the wood’s look and protection with regular reapplication, typically every few years, depending on climate and wear.

These steps ensure your painted or stained pressure-treated lumber remains durable and attractive.

Are there any restrictions on using pressure-treated lumber near water?

Using pressure-treated lumber near water has specific restrictions and considerations due to the potential harm caused by the chemicals used in the wood treatment. Here are the key points:

  • Direct Contact with Drinking Water: Avoid using pressure-treated lumber when it touches drinking water directly, such as in water tanks, well casings, or submerged structures. Instead, opt for approved materials like food-grade plastics or stainless steel for such applications to ensure water safety.
  • Indirect Contact with Water: When using pressure-treated lumber for outdoor structures near water, like decks, docks, or retaining walls, prevent direct wood-water contact. This is achieved by placing a waterproof barrier between the wood and the water source, such as a heavy-duty plastic liner or membrane. This safeguards aquatic ecosystems from chemical leaching.
  • Environmental Considerations: Consider the environmental impact of pressure-treated lumber near water. Chemicals like chromated copper arsenate (CCA) have been phased out in many places due to environmental and health concerns. Safer alternatives, such as alkaline copper quaternary (ACQ) or copper azole, are now preferred for being more environmentally friendly.
  • Regular Maintenance: Irrespective of the type of pressure-treated lumber used, perform regular maintenance tasks like sealing and staining to extend the wood’s lifespan and reduce the risk of chemical leaching over time.

Summary and Conclusion

In summation, pressure-treated lumber is more durable and less susceptible to decay, insect infestation, mold, and water damage. Select the right grade for your project for a lasting structure. Add paint or sealing to extend the lifespan of your lumber. 

David D. Hughes

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