Think before you fasten! The following guidelines are provided by the Southern Pine Council to educate building-industry professionals–but the rules apply to DIY’ers, too….
Metal products in contact with pressure-treated wood must be corrosion-resistant. Examples include fasteners (nails, screws and bolts), and all connecting hardware (joist hangers, straps, hinges, post anchors and truss plates).
When selecting fasteners and connectors, you should consider the potential for corrosion in the particular building application. The 2006 International Residential Code states, “Fasteners for pressure-preservative and fire-retardant-treated wood shall be of hotdipped 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 weights in accordance with ASTM B695, Class 55, minimum.”
Where fasteners are concerned, building codes regard preserved wood as one product, regardless of the formulation used for treatment. However, there are differences between copper-based and borate-based preservatives.
Copper-Based Preservatives Hot-dip galvanized or stainless steel fasteners and connectors are recommended for use when lumber is treated with a copper-based preservative. Copper-based formulations may be used in interior or exterior applications and include the traditional Chromated Copper Arsenate (CCA) and advanced products such as Alkaline Copper Quat (ACQ) or Copper Azole (CA-B).
Hot-dip galvanized fasteners and connectors are generally acceptable for above-grade applications. Hot-dip galvanized fasteners should meet ASTM A153. Hot-dip galvanized connectors should meet ASTM A653, Class G185 sheet with 1.85 ounces of zinc coating per square foot minimum. Fasteners and connectors used together must be of the same metallic composition to avoid galvanic corrosion (for example, use hot-dip nails with hot-dip joist hangers).
Type 304 or 316 stainless steel is recommended for maximum corrosion resistance in more severe exterior applications, such as swimming pools and salt-water exposure. Stainless steel fasteners are generally required for below-grade applications such as Permanent Wood Foundations. Stainless steel is also a recommended option for use with ACQ or Copper Azole treated wood at retention levels greater than required for Ground Contact.
Do not use standard carbon-steel or aluminum products in direct contact when lumber is treated with a copper-based preservative. In addition, electroplated galvanized metal products generally have a thinner layer of protection compared to hot-dip galvanized and are typically not accepted by the building codes for use in exterior applications. Spacers or other physical barriers are necessary to prevent direct contact from treated wood when aluminum or electroplated products are used, such as flashing or termite shields. Such barriers should provide complete separation and remain intact for the intended service life of the metal.
Fasteners and connectors coated with proprietary anti-corrosion technologies are also available for use with copper-based preservatives. Consult individual hardware manufacturers for specifics regarding their performance.
Borate-Based Preservatives Borate preserved wood (Inorganic Boron – SBX) is limited to Above Ground interior use in dry or damp applications, continuously protected from liquid water. Borate treated wood is not corrosive, according to information provided by preservative manufacturers and suppliers.1
Arch Wood Protection, Inc. advises: “Code compliant hardware is adequate. While galvanized fasteners and connectors may be preferable, the use of non-galvanized hardware of sizes and types approved by the code is acceptable when attaching joists, studs, or other framing, provided the wood will remain dry in service, protected from weather and water. Under similar conditions, the use of standard galvanized strapping or mild steel anchor bolts 1/2″ in diameter and larger is also acceptable for fastening borate treated wood to foundations.”
Handling and use guidance from Chemical Specialties, Inc. says, “Borate treated wood can be sawn, nailed, drilled, stained and assembled using standard fastener systems typically used in general wood construction practices.”
A consumer article issued by U.S. Borax, Inc. affirms: “Borates don’t corrode. Borates are often used as corrosion inhibitors in paints and serve the same purpose in wood. No special fasteners are required when building with borate pressure treated wood.”
The Osmose, Inc. Legacy Report (NER 648) for its borate preservative states, “The corrosion rates for carbon steel, galvanized steel, stainless steel, aluminum, red brass, and copper are not increased…when the treated wood products are used as recommended by the manufacturer and properly sized for the materials selected. The fasteners used with the product shall be carbon steel, galvanized steel, stainless steel, copper, and silicon bronze.”
(1) Fastener guidance on borate treated wood from Arch Wood Protection, Inc. (Sill-Bor®); Chemical Specialties, Inc. (Timber-Saver® PT); U.S. Borax, Inc.; and Osmose, Inc. (Advance Guard®) per International Code Council, NER 648.
Adhesives A construction adhesive formulated for treated wood may be used for extra holding power at structural joints and under horizontal decking. These adhesives are not a replacement for nails, screws, or bolts. When selecting any adhesive with treated lumber, be sure the product’s label reads “for use with treated lumber.” Follow the manufacturer’s instructions carefully.
For more information, visit www.southernpine.com.
NOTE: This advisory provides a summary of recommendations from a variety of sources. The Southern Pine Council (SPC) does not guarantee the performance of products used in conformance with these recommendations, and does not endorse any type of wood preservative, fastener or connector. The SPC does not attest to the validity of methodologies used to conduct corrosion tests, and does not attest to the validity of the test conclusions upon which these recommendations are based.