Special Report: Use of treated wood in roof assemblies, February 2005
Treated wood commonly is used in the U.S. construction industry as a component in
roof assemblies. In
The NRCA Roofing and Waterproofing Manual, Fifth Edition,
NRCA specifically recommends the use of decay-resistant, treated wood for blocking
and nailers at roof perimeters and penetrations for fastening membrane and sheet-metal
flashings. Many roof product and system manufacturers also make similar recommendations
for the use of treated wood.
Recent changes in the chemical treatments used in treated wood have resulted in
reports and concerns about corrosion of fasteners and metals that come in contact
with treated wood that use specific, current generation chemical treatments.
In this bulletin, NRCA provides a brief background of this issue and offers specific
interim recommendations intended to address the concern of corrosion relating to
the use of treated wood.
Background
Since the early 1930s, the most widely used chemical treatment for treated wood
has been chromated copper arsenate (CCA) compounds. CCA-treated wood has proven
to perform successfully in many applications, including as components of roof assemblies
where nontreated wood's resistance to insects, micro-organisms and fungal decay
may be a concern.
As of January 2004, wood preservers voluntarily removed CCA-treated wood from U.S.
and Canadian consumer markets as a result of a voluntary agreement with the U.S.
Environmental Protection Agency (EPA). EPA cited the arsenic and chromium contained
in the CCA treatment as being possible environmental concerns in certain exposed-to-the-weather
applications, such as with outdoor furniture and playground equipment.
Wood preservers have introduced a number of CCA-treatment substitutes, including
alkaline copper quat (ACQ-C, ACQ-D, ACQ-D Carbonate), copper azole (CBA-A, CA-B),
sodium borates (SBX/DOT) and ammoniacal copper zinc arsenate (ACZA). These new-generation
treatments contain biocides that do not include arsenic and chromium and are currently
acceptable to EPA.
The long-term performance under various conditions of the current, new generation
of treated wood products still is largely undetermined. Variations in chemical treatments
and other variations that can affect long-term performance including product combinations,
installation techniques and environmental conditions make it nearly impossible to
predict the new products' long-term performances. This problem is compounded by
the fact there are few standards that adequately define the expected performance
of treated wood. For example, no recognized standards exist to measure or define
the corrosion resistance of metal fasteners and connectors used in contact with
treated wood.
Concern of corrosion
NRCA is concerned with the increasing number of reports and bulletins from the treated
wood and fastener industries regarding the increased potential for corrosion when
using the current, new generation of treated wood.
Published reports of testing indicate most new, current generation treatment alternatives
to CCA are more corrosive than CCA. ACQ compounds and ACZA exhibit more than twice
the corrosiveness of CCA, and copper azoles exhibit slightly less than twice the
corrosiveness. The high copper content in ACQ and ACZA treatments fosters a galvanic
reaction. SBX/DOT treatments may be less corrosive than CCA treatments, but SBX/DOT
cannot tolerate exposure to the elements.
A complicating factor is the specific chemical treatment used in treated wood is
not always readily identifiable to users by the wood's appearance, markings or product
labeling.
For the roofing industry, the potential for there to be corrosion-related problems
with the use of the current, new generation of treated wood is a particular concern.
In roof assemblies, treated wood is oftentimes used in as a component that interfaces
with or is in direct contact with metal—metal fasteners, metal flashings and
other metal accessories, or metal roof decks.
NRCA's recommendations
Until more definitive information is available regarding the long-term performance
and corrosion potential of the current, new generation of treated wood, NRCA recommends
taking a conservative approach when using or interfacing with treated wood.
NRCA suggests the following guidelines when encountering the current, new generation
of treated wood:
Carbon steel, aluminum and electroplated galvanized steel fasteners and connectors
should not be used in contact with treated wood. Hot-dipped galvanized fasteners
complying with ASTM A153 and connectors complying with ASTM A653, Class G185, generally
are acceptable. Type 304 or Type 316 stainless-steel fasteners and connectors are
recommended for maximum corrosion resistance.
Fasteners with proprietary anti-corrosion coatings may be acceptable for use with
treated wood. When considering the use of such proprietary coated fasteners and
connectors, consult fastener manufacturers for specific information regarding the
performances of their products in treated wood and any precautions or special instructions
that may be applicable.
Aluminum fasteners, flashings and accessory products should not be used in direct
contact with any treated wood. ACQ-treated wood is not compatible with aluminum.
Uncoated metal and painted metal flashings and accessories, except for 300-series
stainless steel, should not be used in direct contact with treated wood. Metal products,
except stainless steel, may be used if separated from treated wood by a spacer or
barrier, such as single-ply membrane or self-adhered polymer-modified bitumen membrane
material.
Care should be taken in identifying treated wood where it is a component of or interfaces
with roof assemblies. If it cannot be determined whether the wood is not of the
current generation of treated wood, it would be prudent to consider the wood as
the current generation of treated wood and implement NRCA's recommendations.
As a result of NRCA's concerns of corrosion relating to the use of treated wood,
NRCA is revising its recommendation in
The NRCA Roofing and Waterproofing Manual,
Fifth Edition, for use of treated wood as a component of roof assemblies.
NRCA now is of the opinion that the corrosion-related concerns regarding the use
of the current generation of treated wood possibly outweigh the benefits treated
wood provides as a component in roof assemblies. In many instances, the use of nontreated,
construction-grade wood is suitable for use in roof assemblies as blocking or nailers,
provided reasonable measures are taken to ensure the nontreated wood remains reasonably
dry when in service. Where a specific construction detail provides for a secondary
means of waterproofing, NRCA now considers the use of nontreated, construction-grade
wood to be an acceptable substitute for treated wood. Construction details depicting
such a secondary means of waterproofing are provided in the Construction Details
section of
The NRCA Roofing and Waterproofing Manual, Fifth Edition.
NRCA remains of the opinion use of the current generation of treated wood is acceptable
as a component of roof assemblies, provided NRCA's recommendations are followed.
As more information becomes available regarding the current generation of treated
lumber and its corrosion-related issues, NRCA may revise its recommendations and
guidelines contained in this document.
Questions regarding the information contained in this
Special Report should
be directed to NRCA's Technical Services Section by calling (800) 323-9545 or (847)
299-9070.