Preservative-Treated Wood Products
Where moisture and termites are a concern, such as in the Southeastern U.S., pressure-treated framing is a prudent choice for floor systems. Treatment with preservatives protects wood exposed to the elements, in contact with the ground, or subjected to high-moisture conditions. Generally, building codes require pressure-treated or naturally durable wood for the following
- Joists or the bottom of structural floors without joists that are within 18″ of exposed soil
- Beams or girders closer than 12″ to exposed soil
- Framing members (including sheathing) which rest on exterior foundation walls and are less than 8″ from soil
- Plates, sills and sleepers on concrete or masonry
- Wood in permanent structures closer than 6″ to soil
- Girders entering exterior masonry or concrete walls without a minimum 1/2″ air space on top, sides and end
- Posts or columns not separated from concrete piers by an impervious moisture barrier
- Wood supports that are embedded in, or in contact with, the ground
Generally, when wood is exposed to the elements, excessive moisture, or in contact with the ground, it is susceptible to fungal decay and insect attack. Four conditions are required for decay and insect attack to occur: wood moisture content in excess of 19%, a favorable temperature range (approximately 50° to 90° Fahrenheit), oxygen, and a source of food (wood fiber). If any one of these conditions is removed, infestation and decomposition will not occur. Pressure treatment eliminates wood fiber as a food source.
When treated lumber or plywood is specified for most residential, commercial, and marine building applications, waterborne preservatives are preferred. These preservative treatments are clean, odorless, and paintable, plus they are EPA-registered for both interior and exterior use.
The American Wood Protection Association (AWPA) has approved several preservative treatments suitable for residential or commercial enclosed (interior) framing applications. For all structural framing uses, pressure-treated lumber must be dried after treatment (air or kiln-dried) to a moisture content of 19% or less before enclosure.
AWPA sets preservative retention levels for treated lumber, based upon intended use. Retention levels refer to the amount of preservative that remains in the cell structure after the pressure treating process is completed. Retentions are expressed in pounds per cubic foot of wood. The higher the number, the harsher the conditions to which the wood may be exposed.
Jobsite fabrication cuts and borings should be field treated with copper napthanate having a minimum 2% metallic solution (1% solution if 2% not available) in accordance with AWPA Standard M4.
Southern Pine has long been a preferred species when pressure treatment with preservatives is required, because of its ease of treatability. The unique cellular structure of Southern Pine permits deep, uniform penetration of preservatives, rendering the wood useless as a food source for fungi, termites, and micro-organisms. For more information, refer to the SFPA publication Pressure-Treated Southern Pine.
Treated Structural Panels
Just like lumber, structural panels (plywood, OSB) can be treated with preservatives. Properly treated according to AWPA standards, treated structural panels maintain their strength and stiffness, thermal properties, workability, light weight, and economy. For more information, refer to the APA publicationPreservative-Treated Plywood, available at www.apawood.org.
Design values published in the SPIB Standard Grading Rules for Southern Pine Lumber apply to both treated and untreated Southern Pine. Also, design value adjustment factors in the National Design Specification® for Wood Construction (NDS®) apply to both treated and untreated lumber, with the exception that load duration factors, CD, greater than 1.6 shall not apply to structural members pressure treated with waterborne preservatives.
Design values for dimension lumber are based on normal use conditions (moisture content ≤ 19%). They are intended for use in covered structures or where the moisture content in use does not exceed 19% for an extended period of time. For applications where the moisture content will exceed 19% for an extended period, tabulated design values must be multiplied by the appropriate wet service factor, CM. For additional information about design values and design value adjustment factors, refer to the SFPA publication Southern Pine Use Guide or the National Design Specification®.
Today’s Wood Preservatives
For more than 70 years, Chromated Copper Arsenate (CCA) has served as the leading waterborne wood preservative in the United States and throughout the world. It’s availability and widespread uses, including hundreds of applications ranging from decks and patios to wood-frame homes to marine structures, made it the preferred choice for preserved wood products. However, changing perceptions and consumer interest in alternative preservatives has transformed the market, offering more choices for the builder and homebuyer.
Over the past decade, the major wood preservative manufacturers have developed and refined a number of new and highly effective waterborne wood preservatives. After discussions with the U.S. Environmental Protection Agency (EPA), and in light of the growing interest in new products, key preservative manufacturers have voluntarily agreed to make a transition to alternative wood preservatives for the consumer and residential market. The EPA continues to support the use of CCA-treated wood products for a variety of industrial and commercial applications. Table 5 lists the leading waterborne preservatives currently available, suitable for the structural framing components of residential or commercial buildings.
|Table 5 Today’s Wood Preservatives|
|Preservative Types||Brand Names||History and Uses|
|Alkaline Copper Quat (ACQ)||Viance Preserve®
Preserve® with water repellent
|ACQ-treated wood was first introduced in the United States more than 10 years ago. It has been successfully used in Europe, Japan, New Zealand, Asia, and Australia for the last 15 years.Uses: ACQ is a fixed preservative approved for full exposure to above ground, ground contact, and freshwater applications.|
|Copper Azole (CA)||Lonza Wood Protection
Wolmanized® with water repellant
Thompsonized® Wood with water repellant
|Wood products treated with Copper Azole have been used effectively around the world since 1992.Uses: Copper Azole is a fixed preservative approved for full exposure to above ground, ground contact, and freshwater applications.|
|Sodium Borates (SBX)||Lonza Wood Protection
|Wood products treated with Borates were initially established in New Zealand in 1950. Before being introduced into the United States more than 10 years ago, Borates were widely used in New Zealand, Europe, and Southeast Asia.Uses: Borates are a diffusible preservative approved only for above ground applications that are continuously protected from liquid water, such as sill plates and other enclosed structural framing.|
* This table represents several of the leading preservative types and popular brand names available to date.