Or the Wooden Deck Treated with Alkaline Copper Quaternary Compounds Compatibility Mystery
Case History: New construction inspection, house completed.
On March 15, 2005, I inspected a new Craftsman-style house with a deck across the front and another along the rear. As I inspected the exterior, I pulled off several YellaWood(r) tags from the ends of the decking lumber for future reference.
The yellow tags on YellaWood® (pressure treated pine from Great Southern Wood Preserving, Inc.) have a lot of information of interest to home inspectors, as does the company's Web site, www.greatsouthernwood.com.
On the back of this tag, about the fifth bullet down under the Important Information heading, it reads, "DO NOT USE PRESERVED WOOD IN DIRECT CONTACT WITH ALUMINUM."
Guess what the builder had used for flashing under the decks of this brand-new house? Aluminum-rolls of aluminum!
YellaWood® used for decking is treated with alkaline copper quaternary compounds as a preservative. When metals from near the top of the Electromotive Series, such as aluminum, come in contact with metals on the lower end of the series, such as cooper, there will be rapid corrosion when surrounded by an electrolyte, such as water.
That's why you don't want to put aluminum flashing in contact with wood treated with copper. When the materials get wet, holes will be eaten in the aluminum flashing due to corrosion.
Why is it a problem?
Code requires flashing on decks so water will not migrate under the siding and cause rot/decay in the untreated house framing. If the material used for flashing develops holes, the water will pass through it. This is what will happen once the copper eats holes in the aluminum flashing.
The building superintendent stopped by near the end of this inspection to ask if there was anything that needed fixing. I handed him one of the YellaWood tags and asked him if he had ever read one? He said that he had been building decks for years and didn't need any instructions. When I told him that because he hadn't bothered to read the tags, he might have to remove most of the front and rear decks to repair the flashing problem, he was angry.
In my report, I recommended to my client that she ask the builder to repair the flashing. I quoted product installation information from the Great Southern Wood YellaWood® Web site to support the recommendation. The report was sent to the selling agent, who didn't take it seriously, saying the flashing was not a problem.
When the builder/selling agent refused to make the repairs to the flashing, we called Great Southern Wood Preserving, Inc. (YellaWood®). The sales department put us in touch with the engineering department and with the communications manager for their chemical preservative supplier.
We summarized what we had found in the field and asked for their recommendation for repairing the aluminum flashing that was in contact with the copper alkaline-treated wood decking.
The communications manager responded as follows:
"I understand that you have examined decks built with NatureWood(r) (ACQ)-treated wood products, which were built using aluminum flashing. Aluminum is not recommended for use with copper-based products such as NatureWood®-treated wood. The aluminum will corrode. The aluminum flashing should be replaced with a compatible flashing or a physical barrier such as 10 mil poly plastic should be installed between the aluminum flashing and the treated wood."
The engineer from YellaWood® concurred with the communications manager's recommendation and added his own:
"I would further recommend that any physical barrier resist corrosion as well as abrasion, sunlight, and have a good weathering characteristic."
My client and the buyer's agent presented this information to the builder/sales agent along with the engineer's additional recommendation that the deck boards be pulled up and approved flashing be installed. My client requested the correct flashing be installed per information on the back of the YellaWood® tag. The builder agreed to replace the flashing.
Case of the yellow tags closed.
Download the Yellawood® Fastener Information Sheet.