Treated lumber is a popular building material that plays a key role in the construction of many structures in the Pacific Northwest. Our good friend, Dennis McWhirter of Exterior Wood, is an expert on pressure-treated wood, with nearly three decades of experience in the industry. In this series, Dennis provides answers to some common questions and reminds us why treating lumber is one of the best things we can do to help preserve our greatest renewable resource.
In today’s episode, Dennis explains how pressure-treated lumber interacts with metal flashing and fasteners—and how to prevent metal corrosion.
How does pressure-treated lumber interact with metal?
When two opposing metals are placed together, a process called electrolysis happens, in which the harder metal attacks and erodes the softer metal. Pressure-treated lumber is often treated with one of two chemicals: copper azole (CA-C) or alkaline copper quaternary (ACQ), both of which contain copper (a harder metal). If the treated wood is situated next to flashing or fasteners made out of a metal softer than copper—like aluminum—the copper in the treated wood will attack the softer metal.
How much do the metals interact?
The degree to which treated lumber interacts with the metals around it depends on the treatment process of the lumber itself. CA-C coating requires 0.15 pounds of chemical treatment per cubic foot to achieve ground contact (treated to the point that the wood can withstand rot and decay even if it’s touching the ground or leaves), whereas ACQ requires 0.4 pounds of chemical treatment per cubic foot. Since ACQ has more copper, it’s more corrosive than CA-C.
To prevent treated lumber from attacking flashing and fasteners, you’ll need to make sure they're made with a harder metal than copper. Hot-dip galvanized or triple-zinc coated metals are great options. And remember: Because of its lower copper content, CA-C is a more user-friendly treatment material.
We hope this video will help you understand more about pressure-treated lumber so you can decide on the best materials for your next project. For more on treated lumber, check out this post about how pressure-treated lumber is made, the history of the chemicals used in treated lumber, and this breakdown of the differences between professional- and premium-grade treated lumber. And be sure to stay tuned for the rest of our series with Dennis.