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The International Residential Code
(R319.3) and the International Building
Code (2304.9.5) have similar requirements
for fasteners used with treated
wood. The IRC 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."
The codes do not discriminate between types of preservatives and
do not take into account exposure conditions, nor do they contain provisions
for other hardware such as connectors or flashing. Therefore,
clarification is helpful to assist the specifier.
The potential for corrosion of hardware in contact with treated
wood occurs when metals in the preservative (such as copper) are different
from the metals in the hardware (the iron in steel, or aluminum).
In a wet environment these dissimilar metals create a small electrical
current that triggers a chemical reaction resulting in galvanic corrosion.
To select proper hardware, the specifier should first consider the
end-use application and exposure conditions. In damp or wet exposure,
hardware in contact with pressure-treated wood must be corrosion
resistant. Hardware includes fasteners (e.g. nails, screws, and
bolts) and all connectors (e.g. joist hangers, straps, hinges, post
anchors, and truss plates).
Regardless of exposure condition, fasteners and connectors should
be specified in compliance with the hardware manufacturer's recommendations
and the building codes for their intended use.
Copper-Based Preservatives
Copper-based formulations may be used in interior or exterior
applications and include the traditional Chromated Copper Arsenate
(CCA) and advanced products such as Copper Azole (CA) or Copper
Quat (ACQ or Micronized Copper). CA and ACQ have shown a slight
increase in corrosion rates on mild steel compared to CCA and
Micronized Copper.
In damp or wet environments hot-dip galvanized or stainless steel
hardware is strongly recommended in contact with copper-based preservative
treated wood. 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.
Type 304 or 316 stainless steel is recommended for maximum corrosion
resistance in more severe exterior applications, such as swimming
pools or within five miles of salt-water. Stainless steel fasteners are generally
required for below-grade applications such as Permanent Wood
Foundations. Stainless steel is also a recommended option when CA or
Copper Quat formulations are specified at retention levels greater than
required for Ground Contact.
Standard carbon-steel, aluminum, or electroplated products must
not be installed in direct contact with CA or ACQ treated wood. However,
aluminum products may be placed in direct contact with Micronized
Copper treated wood when used in interior applications, or exterior
applications above ground.1 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. 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).
When aluminum or electroplated hardware
must be used in proximity to CA or
ACQ treated wood, spacers or physical
barriers can protect products such as
flashing or termite shields. Barriers should
be non-conductive and remain durable for
the intended service life of the application.
Suitable barriers may include heavy plastic
sheeting, rubber, vinyl, or a high quality,
non-permeable tar bitumen or epoxy.
Hardware coated with proprietary anti-corrosion technologies is also
available. Consult individual hardware manufacturers for specifics
regarding their performance.
Non-Copper Preservatives
Non-copper based preservatives include traditional borates for use
in interior applications and new carbon-based formulations for outdoor
use. These preservatives are less corrosive than their copper-based
counterparts, but exposure conditions must always be a primary
consideration.
Borate preserved wood (Inorganic Boron — SBX) is limited to Above
Ground interior use in dry or damp applications, continuously protected
from weather. According to specifications from preservative suppliers,
borates are non-corrosive.2 The same code approved hardware
used for untreated wood may be used for borate treated wood in weather
protected applications.
Arch Wood Protection, Inc. advises, "Borate treated wood has been
found to be no more corrosive than untreated wood; therefore, code
compliant hardware is adequate." Viance says borate treated wood, "Can
be assembled using standard fastener systems typically used in general
wood construction." U.S. Borax affirms, "No special fasteners are
required." Osmose, Inc. reports, “The corrosion rates are not increased
when [borate] treated wood products are used as recommended."
New carbon-based preservatives are formulated for Above Ground
exterior use. Like borates, they are no more corrosive to hardware than
untreated wood, but end-use and exposure is the key to proper specification.
For example, wood decking treated with a carbon-based preservative
will still be exposed to all weather cycles including prolonged wetting.
ADHESIVES:
A construction adhesive formulated for treated wood may be used for extra holding
power at structural joints and under horizontal decking. These adhesives are not a
replacement for nails, screws, or bolts. When selecting any adhesive with treated lumber,
be sure the product's label reads "for use with treated lumber." Follow the manufacturer's
instructions carefully.
Additional information:
NOTE: 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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