Specifying treated wood

• Treatable species
• Grades of lumber and plywood
• Model spec

Treatable species  

The following species of wood can be effectively treated with CCA preservative in accordance with American Wood-Preserver's Association standards.

Although these species are listed by AWPA, reaching the required penetration and retention levels is very difficult in some of them. The term "treating to refusal" indicates that the wood has retained as much preservative as possible, but not enough to meet standards.

Southern Pine Group
Ponderosa Pine
Red Pine
Hem-Fir Group*
Jack Pine*
Lodge Pole Pine*
Sugar Pine*
White Pine*
Radiata Pine
Caribbean Pine
Coastal Douglas Fir*
Western Larch*
Redwood*
Sitka Spruce*

*In order to secure penetration of preservative in these species, incising (puncturing the lateral surfaces of the wood) is required.  

Grades

Lumber Grades

Originally, lumber grades were established to categorize the structural strength of wood. A 2x6 with large knots and long cracks is not as strong as a 2x6 of the same species without knots or cracks.

Under the auspices of the American Lumber Standard Committee (composed of lumber producers, distributors, and consumers appointed by the Secretary of Commerce), standards are established for the various grades. For western species this is done by the Western Wood Products Association.

In general, higher grades have fewer knots and smaller imperfections than lower grades. In the minds of most consumers, the same characteristics that reduce the strength of lumber also reduce its visual appeal (i.e., knotty wood isnĖt as pretty as clear wood) so the grades are also a measure of beauty.

Over the last decade, WWPA has also created some grades based on appearance rather than strength. Most notably are the appearance grades for radius edged patio decking.

Grading rules are quite detailed. For example, the western lumber rules fill more than 200 pages of a booklet and cover allowances for factors such as knots, bow, crook, skips, splits, grain slope, and wane. Standards are written by industry representatives who may discuss subtle differences for months; grades are assigned to individual pieces of lumber at sawmills by graders who have one or two seconds to evaluate and mark the piece as it passes by on a conveyor. The grade is imprinted on the wood with an ink stamp.

Graders work for the mills but must be trained and certified by an ALSC-accredited inspection agency, which also checks graded lumber to make sure that the graders are accurate. Because of the difficulty of quickly judging each piece of wood, some variation in judgement is expected. A five per cent variation in grade is considered acceptable. Some mills instruct their graders to grade more conservatively than the rules allow.

Treating the wood does not change its grade. The treating process just makes wood last longer outdoors - it makes low grade lumber last longer and it makes high grade lumber last longer.

Plywood Grades

Plywood grades are produced under the provisions of the U.S. Product Standard PS 1-83 for Construction and Industrial Plywood, a voluntary commodity standard of the Department of Commerce and the plywood industry.

The term "grade" may refer to a veneer grade or a panel grade.

Veneer grades define veneer appearance in terms of natural unrepaired growth characteristics and allowable number and size of repairs that may be made during manufacture. The highest quality veneer is "A" and the lowest is "D." The minimum grade of veneer permitted in Exterior plywood is "C." "D" grade veneer is used only in panels intended for interior use or for applications protected form permanent exposure to the weather.

Panel grades are generally identified in terms of the veneer grade used on the face and back of the panel (e.g., A-B, B-C, etc.), or by a name suggesting the panelĖs intended end use (e.g., APA Rated Sheathing, Underlayment, etc.).

Panels are produced in four exposure durability classifications. "Exterior" panels have a fully waterproof bond and are designed for applications subject to permanent exposure to the weather or to moisture. "Exposure 1" panels are designed for applications where long construction delays may be expected prior to providing protection, where high moisture conditions may be encountered in service, and where exposure to the outdoors is on the underside only (e.g., roof overhang). "Exposure 2" panels are intended for protected construction applications where only moderate delays in providing protection may be expected. "Interior" panels are intended for interior applications only.

Hierarchy of grades

Structural joist and planks

Select Structural

#1

#2

#3

Economy

Western patio decking

Patio 1*

Patio 2*

*appearance grades

Model specification for CCA-treated wood

1. GENERAL

1.01 REFERENCES

1. American Wood-Preservers' Association (AWPA):
   1. Standard C1, All Timber Products - Preservative Treatment by Pressure Process.
   2. Standard C2, Lumber, Timbers, Bridge Ties and Mine Ties.
   3. Standard C3, Piles.
   4. Standard C9, Plywood.
   5. Standard C14, Wood for Highway Construction.
   6. Standard C15, Wood for Commercial - Residential Construction.
   7. Standard C18, Marine Construction.
   8. Standard C22, Lumber and Plywood for Permanent Wood Foundations.
   9. Standard C23, Round Poles and Posts Used in Building Construction.
   10. Standard P5, Waterborne Preservatives.
   11. Standard E13, Standard Method of Testing to Determine Water Repellents in Pressure Treated Lumber.
   12. Standard M4, Care of Preservative- Treated Wood Products.
2. National Institute of Standards and Technology (NIST):
   1. PS 1, U.S. Product Standard for Construction and Industrial Plywood.
   2. PS 20, American Softwood Lumber Standard.
3. Western Wood Preservers Institute
   1. Best Management Practices for the Use of Treated Wood in Aquatic Environments.

1.02 QUALITY ASSURANCE

1. Qualifications:
   1. Treatment Facility: Provide treated materials that have been produced under an ALSC recognized quality assurance program.

1.03 DELIVERY, STORAGE, AND HANDLING

If drying after treatment is selected in part 2, retain the two paragraphs below.

1. Packing and Shipping:
   1. Provide waterproof covers for preservative treated wood during shipment.
2. Storage and Protection:
   1. Store preservative treated wood off the ground and protected from the weather.

2. PRODUCTS

2.01 MANUFACTURERS

1. Preservative: [Specific type]

2.02 MATERIALS

Lumber for preservative treatment must conform to the following specifications.

1. Lumber: In accordance with NIST PS 20 and [specify grade, species, surfacing, and moisture content].

Plywood for preservative treatment must conform to the following specifications.

2. Plywood: In accordance with NIST PS 1 and [specify panel grade, exposure durability, species group, and structural rating].
3. Preservative: CCA Type C in accordance with AWPA P5.

2.03 PRESERVATIVE TREATMENT

Select required end uses below.

1. Pressure Treatment: In accordance with the requirements of AWPA Standard C1 and in accordance with the following standards for indicated end uses:
   1. Lumber: C2.
   2. Piling: C3.
   3. Plywood: C9.
   4. Highway Construction: C14.
   5. Permanent Wood Foundations: C22.
   6. Building Poles and Posts: C23.
      

Select required applications below.

2. Preservative Retention: In accordance with the specified standard, determined in the specified zone, and as follows:
   1. Above Ground: 0.25 pcf, minimum.
   2. Ground or Fresh Water Contact: 0.40 pcf, minimum.
      1. Wood Foundation or Structural Poles: 0.60 pcf, minimum.
      2. Salt Water Immersion: 2.50 pcf, minimum.
3. Moisture Content: Drying after treatment is not required.

Select Above or Below.

4. Moisture Content: Dry after treatment as follows:
   1. Lumber: 19%, maximum.
   2. Plywood: 15%, maximum.

Retain below if fixed preservative is required for aquatic environments.

5. Pressure Treatment of Materials for Aquatic Environments: In accordance with the Best Management Practices published by the Western Wood Preservers Institute.

2.04 SOURCE QUALITY CONTROL

1. Inspection:
   1. Untreated Material:
      1. Lumber: Provide lumber that has been inspected and graded for treatment by an ALSC recognized grading agency.
      2. Plywood: Provide plywood that has been inspected and graded before treatment by a code-recognized inspection and testing agency.
   2. Treated Material: Provide treated material that bears the quality mark of an ALSC-recognized agency which maintains supervision, testing, and inspection of the quality of the product. Provide quality marks that are affixed to each piece and that include the following:
      1. Identification of the inspection agency.
      2. Identification of the standard to which the material was treated.
      3. Identification of the treating facility.
      4. Identification of the preservative and retention.
      5. Identification of the end use for which the product is suitable.

3. EXECUTION

3.01 INSTALLATION

1. Surface Treatment of Field Cuts: Treat field cuts on members that provide structural support to a permanent structure in accordance with AWPA Standard M4.