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.
Where moisture and termites are a concern, 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 applications:
- 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 applications, 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.
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 less thanc19%). 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®.
For more information, refer to the SFPA publication Pressure-Treated Southern Pine.