Soundings
Fasteners for Treated Wood:Practical Solutions for Coastal Environments
Q: I know we're not supposed to use regular galvanized fasteners in treated wood, but I'm constantly running into this issue with carpenters. Why exactly is ACQ lumber more corrosive, and are there any alternatives to stainless steel? Stainless can get very pricey when you start speccing framing hardware like joist hangers and post bases.
A: Ted Cushman responds: You're right, the situation with treated wood and fasteners has gotten complicated in the past few years, especially compared with the good old days when treated wood was treated wood, and nails either were galvanized or they weren't.
That's changed significantly since 2003, when the major wood-treating companies struck a deal with the U.S. Environmental Protection Agency (EPA) to remove potentially toxic arsenic from their wood-preservative formulas. These companies replaced their old wood-treating mixtures with new, proprietary branded recipes that don't use arsenic. Almost immediately, companies that manufacture steel framing connectors or galvanized fasteners began to warn about compatibility problems between the new treating formulas and galvanized steel hardware, nails, and screws. Lab testing indicated that the new copper-based treating formulas might eat through nails and hangers at two to — in some cases — five times the rate expected in the previous conditions. Throw salt moisture into the mix, said experts, and the hardware would likely break down even faster (see "The Science of Corrosion," page 20).
Real-World Decisions
But lab tests don't necessarily reflect real-world conditions, so builders still have to base their decisions on their own best guess. Unfortunately, when it comes to guesswork, actual coastal exposures involve all the things that are likely to make the real-world performance even worse than laboratory performance. Salt air near the coast is rough on metal hardware, and in southern coastal states, heat is likely to play a role as well. Like most chemical reactions, the rate of galvanic corrosion roughly doubles for every 20°F increase in temperature. Corrosion in the lab also increases significantly if there are higher levels of the treating chemicals in the lumber — meaning that the wood that is most likely to stand up to rot is also the most likely to tear up your galvanized nails and hangers.
Stainless steel decking nails, such as these 2-inch ring-shank gun nails, are common in many lumberyards these days. Larger hangers and framing clips, however, usually must be special ordered or custom made.
In practice, builders need to look carefully at their project's actual exposure and choose accordingly. Exterior applications within five miles of the ocean present the toughest case.
Stainless is safest. Where salt-air corrosion is likely, stainless steel (usually Type 304 or Type 316) is the only metal proven to stand up to direct contact with copper-based wood treatments. Major steel connector manufacturers Simpson Strong-Tie and USP Structural Connectors both supply at least some of their connectors in stainless steel. Stainless steel nails and screws are also widely available, including gun-driven collated nails. The bad news is that you have to pay a steep upcharge for that durable hardware.
Isolation membrane. Another option is to isolate the hardware from the lumber with an impermeable membrane. While some builders use ordinary black paper, a butyl-based material, such as Grace Vycor Deck Protector, seems to work the best. Butyl stays flexible and won't dry out in extreme temperatures, and because it is self-healing, the fasteners that are driven through it won't leak. This is important because if the hangers and connectors don't touch wet wood, they won't be attacked by the copper-bearing solution. Fasteners will still penetrate into the wood, though, so you'll have to assess whether the membrane will keep that wood dry enough to keep the galvanic process at bay. Even with an isolation membrane, you'll probably want to stick with the hardware and fasteners that have the heaviest protective zinc coating available.
New treatment formulas. A third option is to use one of the treated-wood products that don't contain copper, like Wolmanized L3 Outdoor decking, which is treated with a combination of organic pesticides and water repellents. According to Arch Chemicals, this new material won't damage either galvanized metal or aluminum flashing. At this point, the L3 material is only available in 2x8 or smaller members, so it may not suffice for the undercarriage of a deck or floor system of an elevated coastal home.
Exposure is everything: if pressure-treated framing will be exposed to a salt-air environment, stainless steel will be the safest, but priciest, option for fasteners and hardware.
Setting Priorities
If you're farther from the water, you may not need to always use stainless steel but may instead be able to rely on hardware with the better galvanized coatings. Ever since wood treaters introduced the proprietary brands, metal connector and fastener suppliers have responded with heavier galvanized coatings. Simpson Strong-Tie and USP, for instance, moved quickly to put G90 instead of G60 galvanizing on their entire product lines (a 50% boost in zinc thickness), and they have since brought out G185 coatings (double the usual thickness and three times as thick as the once-typical G60 standard). The heavier coating, in theory, should stand up to ordinary use in a moderate exposure.
When stainless steel hardware is out of the budget, Rhode Island builder Mike Guertin uses a butyl-based flashing material as an isolation membrane to protect galvanized hangers and connectors from the corrosive compounds in the treated framing.
Keep in mind that if pressure-treated wood is dry in service — used where it's not exposed to rain or ground moisture — the risk is largely abated. Galvanic corrosion happens only when the wood is wet. Wood that's out of the weather will stay at 12% moisture content or less, well below the 17% moisture content needed to support fastener corrosion. If that's your situation, you don't need fancy galvanized or stainless steel hardware (though ordinary galvanized products are required in any treated wood by code).
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The Science of Corrosion |
