Q: As lumberyards and building codes begin adapting to the AWPA’s revised Standard U1 for treated lumber (see “Tougher Rules for Treated Wood,” deckmagazine.com), will the higher chemical retention levels found in lumber treated for ground contact result in possibly more-corrosive environments for fasteners? In other words, will deck builders once again need to take extra measures—such as wrapping the ends of joists with self-adhering flashing tape—to isolate joist hangers and other metal hardware from joists and beams?
A: Robert Leichti, an engineering manager in the Fastening Systems department at Simpson Strong-Tie, responds: Even though changes to AWPA U1 and AC326 may push up some application specifications one use category—mostly from UC3B (above ground) to UC4A (ground contact), and to a lesser extent from UC4A to UC4B (heavy-duty ground contact)—the actual chemical retention levels for UC3B, UC4A, and UC4B have not changed.
Simpson Strong-Tie tuned its corrosion-resistance classifications to these categories in 2014. For above-ground ACQ-C, ACQ-D, CA-B, and CBA-A treatments without ammonia (western treaters typically use ammonia as an additional carrier since western species are more difficult to treat than southern yellow pine), Simpson recommends HDG hardware with a minimum G185 zinc coating. For PT lumber with retention levels greater than 0.40 (for ACQ and MCQ), 0.41 (for CBA-A), or 0.21 pcf (for CA-B)—the AWPA-specified retention levels for UC4A (ground contact) treated lumber—Simpson continues to recommend stainless steel hardware. Because our recommendations refer to the use categories and their retentions, we don’t expect our corrosion-resistance and hardware recommendations to change, although for materials that push from UC4A to UC4B, the recommendations in UC4B will be stainless steel, just as it is now. Also, critical structural components should already be fastened with stainless steel to mitigate risk.