Fasteners and Connectors
Like all metal used in construction, fasteners and connectors must be properly protected from corrosion. The potential for corrosion of fasteners (e.g. nails, screws, bolts, nuts and washers) and connectors (e.g. joist hangers, straps, hinges, post anchors and truss plates) in contact with pressure-treated wood varies by preservative system and end-use exposure. For example, wood treated with non-copper based waterborne preservatives, such as SBX, PTI and EL2, is less corrosive overall than wood treated with copper-based preservatives. Also, wood treated with micronized copper preservatives, such as MCA and μCA, is less corrosive overall than wood treated with dissolved copper preservatives, such as ACQ, CA and CCA. In addition, interior, dry environments are less conducive to corrosion than exterior, wet environments. Particularly severe environments include swimming pools, below-grade foundations, saltwater exposure, trapped water and other environments that allow for constant, repetitive or long periods of wet conditions.
It is important to follow specific recommendations from preservative and hardware manufacturers, but the following provides some general guidance:
- Model building code requirements are provided in Section 304.9.5 of the International Building Code and Section R317.3 of the International Residential Code.
- In general, the codes require fasteners to be of hot-dipped, zinc-coated galvanized steel in accordance with ASTM A153, type 304 or 316 stainless steel, silicon bronze or copper.
- An exception allows the use of plain carbon-steel fasteners for wood treated with borates and used in an interior, dry environment.
- Connectors should conform with ASTM A653.
- Fasteners and connectors used together must be of the same metallic composition to avoid galvanic corrosion; for example, use hot-dip galvanized nails with hot-dip galvanized joist hangers and stainless-steel nails with stainless-steel joist hangers.
- Aluminum fasteners and aluminum building products (e.g. window and door trim, flashing, siding, roofing and gutters) should not be placed in direct contact with wood products treated with dissolved copper preservatives. Direct contact with aluminum may be allowed for wood products treated with borates, carbon-based and micronized copper preservatives when used for interior and exterior, above-ground applications that do not trap moisture or allow the wood to be exposed to standing water or to water immersion.
- Stainless-steel fasteners are required for Permanent Wood Foundations below grade and other severe environments.
- Electroplated galvanized fasteners and connectors are typically not accepted for use in exterior applications, regardless of the preservative system.
- Other coatings or metal products may be available so follow the hardware manufacturers’ recommendations for compatibility with wood treated with the various preservative systems and end-use exposures.
- Construction adhesives are not a replacement for metal fasteners, but can be added for extra holding power. Be sure the product label states the adhesive is “for use with treated lumber” and follow the adhesive manufacturer’s instructions.