Preservative Treated Lumber II

Pressure-Treated Wood: The Next Generation, Fine Homebuilding January 2004, has more to say about the new wood preservatives coming to market and their higher corrosiveness as discussed here previously. In summary:

• It's the high concentration of copper in alkaline copper quat and copper azole that give these preservatives their potency, and that makes them as much as 5 times more corrosive than traditional CCA-treated lumber.
• FH recommends stainless steel hardware and fasteners wherever possible. Where hot dip galvanized material is used, a G-185 rated coating is recommended. G-185 hot dip galvanized steel has 1.85 ounces of zinc per square foot of metal, compared with 0.90 or 0.65 ounces per square foot for the more commonly available G-90 or G-65 coatings. Electrogalvanized fasteners, such as expansion bolts, should be rated Class 40 or higher (the electrogalvanizing Class scale ranges from a low value of 5 to high value of 110). Polymer coated screws may also acceptable. (Author's note: Simpson Strong-Tie recommends minimum Class 55 for electrogalvanized fasteners.)
• Avoid any contact between these treated lumber products and aluminum sheet metal or fasteners, as they will corrode quickly. Copper sheet metal is OK.
• Special care must be taken at locations such as mud sills, which must be preservative treated lumber, but are not normally associated with corrosion resistant fasteners. Nails at the bottom edge of the wall sheating that are driven into the mudsill, foundation anchor bolts and straps, and toenails fixing rim joists and floor joists to the mud sill are all possibly at risk of accelerated corrosion.
• Lumber is rated by end-use application. In order of increasing preservative concentration, categories are Decking, Above Ground, Ground Contact, and Permanent Wood Foundation.
• New borate based preservative forumulations, which do not rely on copper for their potency, are also coming to market. According to Fine Homebuilding, borates are not corrosive and have a low toxicity.