Volume XIV, Number 4 June 2004
Most wood will decompose when it is exposed to the weather, subjected to excessive moisture, or in contact with the ground. The decomposition is caused by decay or insect attack. The solution for years has been to use wood that had been pressure-treated with a preservative like chromated copper arsenate (CCA). The result was that familiar “greenish” colored used for backyard decks, fences, and swing sets. However, as of December 31, 2003, CCA was withdrawn for most residential consumer-use treated-lumber applications. Lumber treated with alternative preservatives has been introduced while CCA-treated lumber continues to be allowed for certain industrial, agricultural, foundation, and marine applications.
Users of lumber treated with the alternative preservatives must become aware of the changes in material selection that must be made because of the potential for corrosion of fasteners and connectors that either penetrate or contact the treated lumber.
Presented below is a discussion of pressure treatment alternatives available to consumers and the changes in practice that must be considered when choosing the proper materials.
Pressure treating wood requires that a preservative chemical penetrate and be retained by the wood. Most wood species require that the surface of the lumber be pierced with a series of small slits to facilitate penetration of the wood by the chemical. Southern pine is one of the few species that does not require this surface-preparation step and, as a result, about 85% of all pressure-treated wood is southern pine.
There are three broad classes of wood preservatives used in the pressure-treating process: Waterborne preservatives, Creosote and creosote/coal-tar mixtures, and Pentachlorophenol (called “Penta”). The creosote and creosote/coal-tar mixtures are used for railroad ties, pilings,
and utility poles and Penta is used for industrial applications and utility poles. It is the waterborne preservatives that are used for residential, commercial, marine, agricultural, recreational, and industrial applications.
Waterborne Wood Preservatives
There currently are five waterborne preservative chemicals being used for pressure treating wood available commercially:
? Sodium Borate (SBX) is also called disodium octaborate tetrahydrate (DOT)
? Alkaline Copper Quaternary (ACQ-C, ACQ-D, ACQ-D Carbonate)
? Copper azole (CA-B, CBA-A)
? Sodium silicate borate (SBX with NaSiO) 2
? Ammoniacal copper zinc arsenate (ACZA)
These preservatives appear in the market as pressure-treated wood that has various trade names.
Presented in the following table is a list of these preservatives and some of the trade names used
for the treated wood.
Preservative Treated wood trade name
Sodium borate Advance Guard, SillBor, Tim-Bor, Pac-Bor, TimberSaver PT, Smart
Guard, Hi-Bor, Cal-Bor, DuraBor, DuraSill, Composibor
Alkaline Copper Preserve, Nature Wood, Preserve Plus
Copper Azole Natural Select Sodium Silicate Borate Envirosafe Plus Ammoniacal Copper Chemonite Zinc Arsenate
Source: www.strongtie.com, March 2004.
New Preservatives Corrode Fasteners & Connectors
The introduction of new preservatives has been accompanied by a new problem – corrosion of
the fasteners and connectors that come in contact with the chemical. The solution recommended
by the wood preserving industry is to use either stainless steel or hot-dipped galvanized fasteners
and connectors. The International Residential Code, Section R319.3 states, "Fasteners for
pressure-preservative treated wood shall be of hot-dipped galvanized steel, stainless steel, silicon
bronze or copper.” Although this statement sounds straight forward, there are different types of
stainless steel and different thickness coatings applied by hot-dipping.
In the case of stainless steel, tests conducted by Simpson Strongtie Company, Inc., (see
www.strongtie.com, TECHNICAL BULLETIN: Pressure Treated Wood, January 2004)
concluded that Type 304 or greater stainless steel products corrode substantially less than other
alternatives. It should be noted, however, that stainless steel products cost significantly more
that standard coated products.
The corrosion performance of a galvanized product is directly related to the amount of zinc
deposited on the surface. There are several methods of applying the zinc coating including
electrogalvanized, hot-dipped, and hot-tumbling. Galvanized products that are produced by an
electroplating process are not recommended for use with wood that has been treated with the
alternative preservatives because the zinc coating is not adequate. Rather, hot-dipped galvanized
products are recommended with the guideline that the more zinc on the surface of the product the
better. The third process involves tumbling heated fasteners through zinc powder leaving a thin
zinc coating. Both the electroplating and the heat tumbling provide protection for the fasteners;
but that protection is not considered to be adequate for use with the new preservatives used in
The amount of galvanized coating is specified as a class or grade such as G60 meaning 0.60
ounces of zinc per square foot of product has been applied or as G90 which results in 0.90
ounces of zinc per square foot of product. The Southern Pine Council issued an Advisory in
June 2004 titled Advisory On Fasteners and Connectors for Treated Wood. The Advisory recommends G185 for fasteners used with the new wood preservatives. (see
Another point to consider is that the fasteners and the connectors should be made of the same
material. That is, stainless steel fasteners should be used with stainless steel connectors.
Similarly, galvanized fasteners should be used with galvanized connectors.
According to the NAHB Research Center, Inc., (see www.nahb.org) lumber treated with the non-borate preservatives (e.g., ACQ and copper azole) is more corrosive than CCA-treated lumber.
Lumber treated with borate preservatives is actually considered to be less corrosive.
Unfortunately, lumber treated with borates is not recommended for any use where it will be
either in direct contact with the soil or in prolonged or repeated contact with water. The reason
for this situation is that the preservative chemical will leach out of the wood. Although there is
no environmental or health risk associated with the leaching, loss of the preservative will
negatively affect the preservation of the wood.
Additional information on the types and uses of available alternative wood preservatives, the
introduction of new alternative preservative chemicals or changes in the formulation of the
current preservatives, and the results of performance testing of fasteners and connectors exposed
to the alternative preservatives are available from a variety of Internet web sites. Inquisitive
readers are encouraged to visit the following web sites:
NAHB Research Center, Inc. www.nahb.org American Galvanizers Association www.galvanizeit.org American Wood-preservers’ Association www.awpa.com US Forest Products Laboratory www.fpl.fs.fed.us Southern Pine Council www.southernpine.com Western Wood Preservers Institute www.wwpinstitute.org Simpson Strongtie Company, Inc. www.strongtie.com