All wood pressure-treatment chemicals after 2004 contain copper. Copper in contact with the steel in hangers and nails, in the presence of water and oxygen, promotes galvanic corrosion. That corrosion is an electro-chemical reaction. Basically it’s the same thing we see in many other conditions in which different metals are in contact with each other in the presence of moisture (why we look for dielectric unions). The difference is that this time the copper was in the chemical solution forced by pressure into the wood during treatment.
I also see aluminum flashing on decks severely corroded from PT lumber, Great post. That is why a plastic deck ledger flashing is recommended instead of aluminum.
Yep, I also just did an outbuilding where they used PT for the fascia board then some time later vinyl sided the building and used aluminum over the fascia. Very corroded.
The new generation of pressure-treated wood (alkaline copper quaternary, or ACQ, and copper azole, or CA) is more corrosive than the old stuff (chromate copper arsenate, or CCA). Those might have been left over hangers with the thinner coatings that were used before the new P.T. compounds.
The G-60 and G-90 hot-dipped coatings are what we’ve been using until now. But engineers suggest stepping up to the heavier G-185 coatings for hot-dipped galvanized products, and they recommend class ratings of 40 or above when using electro-galvanized fasteners, such as expansion bolts.
Currently, G-185 is the best galvanized protection that you can buy. Simpson’s ZMax line and USP Structural Connectors’ Triple Zinc line both are rated at G-185. As for nails and screws, many of the composite coatings that are currently available are still good for the new treatments (see www.osmose.comto read about fastener recommendations), but you should verify this by carefully reading the label.
I’m glad you posted this. This is a common problem on beach houses. I know hangers are very popular but I still prefer to use ledgers especially in places close to the beach. Another issue with the new treated wood is it’s performance is dismal at best. I’ve seen it rot out after maybe 5 or 6 years even in places (above ground) where only it only received occasional contact with water. Coating it with copper naphthenate helps.
