A creak here, a groan there—the familiar orchestra of an aging deck. Though in most cases these noises are innocent, some may betray a deeper problem. Beneath your feet could be impending disaster, a backyard platform poised to plummet to the ground below—even if the wood comprising it is completely sound.
Corroded metal fasteners have been responsible for several deck collapses across the country, and tragically, decks seldom fail when they are unoccupied. Researchers at the Forest Products Laboratory (FPL), in cooperation with the United States Department of Transportation and the Federal Highway Administration, have been investigating metal fasteners, and their corrosion problems, for years. They found that although wood is generally not corrosive, copper-based wood preservatives can react with the metal components of the deck, and lead to compromised structural integrity.
Corrosion of a galvanized joist hanger and nails supporting a wood deck treated with a copper-containing wood preservative. This deterioration would be easily spotted during a visual inspection.
In 2004, changes in regulations saw an influx of wood treatments with increased copper content. Although effective at preserving the wooden components of external structures, they increase the incidence of corrosion.
When two dissimilar metals (for example, the nails in a deck and the wood’s copper coating) come into contact with one another, the electron exchange between the two materials begins the corrosion process. In addition to producing unsightly rust, this significantly weakens the metal.
Fortunately, there are several steps you can take to mitigate these hazards. Because dry materials do not react with one another, FPL stresses that, “proper moisture management is the most important thing one can do to reduce corrosion of metals in treated wood.” This includes preventing moisture from seeping in through the ends of wooden components (where it moves into the timber up to 10 times faster than from other directions) and designing roofs and overhangs so that they do not drain onto lower structures. Researchers maintain that, “if the wood is kept dry, both the wood and fasteners can last for centuries.”
Illustration of the importance of roof overhangs for protecting wood from biodeterioration and corrosion. The right side of the beam is protected by the large roof overhang, whereas the left side is exposed to rain.
Isolating the metals from one another is another step one can take. The most common way of doing this is through non-metallic coatings, such as those found on some screws or bolts designed for exterior use. Extreme care must be taken however when using coated metals in construction, as the coatings can be easily damaged during the installation process.
Finally, avoiding metal-on-metal contact altogether is a surefire method to prevent corrosion, but the hardest to implement. Although copper preservatives and metal nails are sometimes unavoidable neighbors in deck construction, being aware of metallic washers used on dissimilar metal bolts, or metal signs hung by metallic screws, can help put a damper on the corrosion process. Using non-conductive washers with metal signs or joist hangers, for example, can significantly decrease the speed of the corrosion and extend the life of the metal by decades.
Preventing corrosion is a multi-billion dollar industry in the United States, with over $100 billion spent in protective coatings alone. It is a problem as old as the material itself and wherever moisture and metal are found, corrosion is sure to follow. By utilizing proper construction techniques, moisture management, and, of course, regular inspections of your deck or home, your fasteners can last a lifetime—and your deck and family can be spared the tragic results of corroded metal fasteners.
For more information, please see the FPL’s Guide for Materials Selection and Design for Metals Used in Contact with Copper-Treated Wood.