Ever since the producers of pressure treated lumber began moving away from using chromated copper arsenate (CCA) as a preservative, concerns have surfaced regarding the higher corrosivity of the next-generation preservatives, in particular alkaline copper quaternary (ACQ) and copper azole (CBA and CA). Fasteners used with ACQ-, CBA-, or CA-treated lumber have been reported to corrode and fail at a faster rate than when used with CCA-treated lumber.
To determine whether the corrosion rates of these preservatives is indeed higher, several fastener and connector manufacturers have conducted accelerated tests on their own products. Findings indicate that ACQ and the copper azoles are nearly twice as corrosive as CCA preservatives.
Most treated-wood producers using the next-gen preservatives point out that their recommendations for fasteners and connectors are the same as what they have always recommended for use with CCA-treated lumber: hot-dip galvanized or stainless steel. However, treated-wood producers and fastener/connector manufacturers now acknowledge that a thicker coating of galvanized zinc provides higher corrosion resistance.
Simpson Strong-Tie, which has conducted the most extensive testing so far, now recommends a minimum G185 hot-dip product and a maximum of type 304 and 316 stainless steel for use with ACQ, CBA, and CA, in accordance with ASTM standards for fasteners and connectors. According to Simpson's test results, type 304 and 316 stainless steel fasteners and connectors have a high corrosion resistance. Manufacturers also stress the importance of using connectors and fasteners made of the same material to prevent galvanic corrosion.
Arch Wood Protection, maker of Natural Select lumber; CSI, maker of Preserve wood; Osmose, maker of NatureWood lumber; USP Structural Connectors; Maze Nails; and the Southern Pine Council also recommend the use of G185 hot-dip galvanized or type 304 and 316 stainless steel fasteners and connectors. Osmose's ProDrive ceramic-coated screws are approved for use with the next-generation wood preservatives. Bostitch recommends its new ThickCoat Galvanized fasteners, which feature a chromate coating over the zinc layers. Swan Secure Products recommends its stainless steel fasteners. Senco is currently conducting fastener tests with independent laboratories and recommends the use of stainless steel fasteners grade 304 or higher.
Using the recommended grade of fasteners and connectors is critical. "If you use the wrong fastener, you'll see corrosion start, [indicated by] rust stains, and eventually the fastener will fail. The ones we recommend are in the building codes, and as ever, it's important to follow the building codes and the manufacturers' recommendations," says Al Heberer, national marketing manager for Osmose.
By using the grade and type of fasteners required by ASTM standards, "remodelers can be assured of proper performance and long-term durability," says Richard Kleiner, director of treated markets for the Southern Pine Council of the Southern Forest Products Association.
These recommendations may change after further research and when treated-wood producers develop even newer wood preservatives. "If these new copper preservatives should happen to be around 30 years from now, the recommendations may turn out to be different," says Huck DeVenzio, manager of marketing communications for Arch Wood Protection. But, "we're definitely looking for the next generation. There will be different kinds of treated wood in the future."