10.014 Do Metal Fasteners in Contact With Wood Preservative Require Corrosion Protection?

Q Do foundation hold-down bolts, anchor bolts, plate washers, straps, nails, etc., attaching pressure preservative treated wood, have to be hot-dipped zinc coated galvanized, stainless steel, silicon bronze, or copper?

— From the October 2001 Inspection Division Quarterly Newsletter,
City of Santa Clara, CA.

A 2/09 – Response prepared by Terry Egland, a principle at Testing Engineers, Inc., and a registered engineer in California.

The International Residential 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 (12.7 mm) 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 weights in accordance with 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.

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.

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 hangars, straps, hinges, post anchors, and truss plates).

Regardless of exposure condition, fasteners and connectors should be specified in compliance with the hardware manufacturer’s recommendations and the building codes for their intended use.

A conclusion from the above would indicate that shear wall nailing to a pressure-treated sill plate requires galvanized nails.