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The International Residentail Code (R319.3) and the International Building Code (2304.9.5) have similar requirements for fasteners used with treated wood. The IRC states, "Fasteners for pressure-preservative and fire-retardant-treated wood shall be of hot-dipped zinc-coated galvanized steel, stainless steel, silicon bronze or copper. The coating weights for zinc-coated fasteners shall be in accordance with ASTM A153. Exceptions: 1. One-half inch (12.7mm) diameter or greater steel bolts. 2. Fasteners other than nails and timber rivets shall be permitted to be of mechanically deposited zinc-coated steel with coating weight in accordance wih ASTM B695, Class 55, minimum." The codes do not discriminate between types of preservatives and do not take into account exposure conditions, nor do they contain provisions for other hardware such as connectors or flashing. Therefore, clarification is helpful to assist the specifier. The potential for corrosion of hardware in contact with treated wood occurs when metals in the preservative (such as copper) are different from the metals in the hardware (the iron in steel or aluminum). In a wet environment these dissimilar metals create a small electrical current that triggers a chemical reaction resulting in galvanic corrosion. To select proper hardware, the specifier should first consider the end-use applicaion and exposure conditions. In damp or wet exposure, hardware in contact with pressure-treated wood must be corrosion resistant. Hardware includes fasteners (e.g. nails, screws and bolts) and all connectors (e.g. joist hangers, straps, hinges, post anchors and truss plates). Regardless of exposure condition, fasteners and connectors should be specified in compliance with the hardware manufacturer's recommedations and the building codes for their intended use. |
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