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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
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
Advisory
on Fasteners and Connectors for Treated Wood
Metal products in contact with pressure-treated wood must be corrosion resistant. Examples include fasteners (e.g. nails, screws, and bolts), and all connecting hardware (e.g. joist hangers, straps, hinges, post anchors, and truss plates).
One selection criteria for fasteners and connectors should be the potential for corrosion in a particular building application. The 2006 International Residential Code, Section R319.3 states, "Fasteners for pressure-preservative and fire-retardant-treated wood shall be of hot-dipped zinc-coated galvanized steel, stainless steel, silicon bronze or copper. The coating weights for zinc-coated fasteners shall be in accordance with ASTM A153. Exceptions: 1. One-half inch (12.7mm) diameter or greater steel bolts. 2. Fasteners other than nails and timber rivets shall be permitted to be of mechanically deposited zinc-coated steel with coating weights in accordance with ASTM B695, Class 55, minimum."
Traditionally, the treated wood industry has recommended hot-dip galvanized or stainless steel fasteners and connectors for treated wood products. Hot-dip galvanized or stainless steel fasteners and connectors continue to be recommended for use with today's modern wood preservatives (e.g. Alkaline Copper Quat ACQ, and Copper Azole), but additional clarification is needed to ensure adequate corrosion protection.
Hot-dip galvanized fasteners and connectors are generally acceptable for above-grade applications. Hot-dip galvanized fasteners should meet ASTM A153. Hot-dip galvanized connectors should meet ASTM A653, Class G185 sheet with 1.85 ounces of zinc coating per square foot minimum. Fasteners and connectors used together must be of the same metallic composition to avoid galvanic corrosion (e.g. use hot-dip nails with hot-dip joist hangers).
Type 304 or 316 stainless steel is recommended for maximum corrosion resistance in more severe exterior applications, such as swimming pools and salt-water exposure. Stainless steel fasteners are generally required for below-grade applications such as Permanent Wood Foundations. Stainless steel is also a recommended option for use with ACQ or Copper Azole treated wood at retention levels greater than required for Ground Contact.
Do not use standard carbon-steel or aluminum products in direct contact with pressure-treated wood. In addition, electroplated galvanized metal products generally have a thinner layer of protection compared to hot-dip galvanized and are typically not accepted by the building codes for use in exterior applications. Spacers or other physical barriers are necessary to prevent direct contact from treated wood when aluminum or electroplated products are used, such as flashing or termite shields. Such barriers should provide complete separation and remain intact for the intended service life of the metal.
Fasteners and connectors coated with proprietary anti-corrosion technologies are also available for use with treated wood. Consult individual hardware manufacturers for specifics regarding the performance of their products with treated wood.
This advisory provides a summary of recommendations from a variety of sources. The Southern Pine Council (SPC) does not guarantee the performance of products used in conformance with these recommendations, and does not endorse any type of wood preservative, fastener or connector. The SPC does not attest to the validity of methodologies used to conduct corrosion tests, and does not attest to the validity of the test conclusions upon which these recommendations are based.
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Pressure-Treated Lumber
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-Preservers' 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. Refer
to Table 4 (retention levels).
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 SPC publications Pressure-Treated
Southern Pine and Termite-Resistant
Structures.
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 publication Preservative-Treated
Plywood, available at www.apawood.org.
Design Values
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 SPC 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 |
Manufacturer's
Popular Brand Names |
History / Uses |
Alkaline Copper Quat
(ACQ)
 |
Chemical
Specialties, Inc.
Preserve®
Preserve® with water repellent
Osmose,
Inc.
NatureWood®
NatureWood® with water repellant |
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)
 |
Arch
Treatment Technologies, Inc.
Wolmanized®
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)
 |
Arch
Treatment Technologies, Inc.
SillBor®
Chemical
Specialties, Inc.
TimberSaver® PT
Osmose,
Inc.
Advance Guard® |
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.
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