Fasteners

Fasteners, Connectors and Flashing for Wood Treated With Copper-Based Preservatives

The presence of moisture is a precondition for corrosion of metals. Treated wood is typically used in applications where it may be exposed to moisture for considerable periods so any fasteners and connectors used with treated wood must also be resistant to these conditions. In addition, most wood preservatives designed for exterior use contain copper that may react with the metals used to fabricate fasteners and connectors therefore, it is important to use the right type of fastener and/or connectors. Where treated wood is used in dry environments to prevent damage by wood-destroying insects, including termites, corrosion is of less concern.

Users and specifiers should also be aware that corrosive industrial, or salt air, environments may also require the use of appropriate corrosion resistant metals.

Types of Wood Preserving Treatments

Chromated copper arsenate, type C (CCA-C) was until recently the industry’s standard wood preservative, but alternative treatments such as alkaline copper quat (ACQ®) and copper azole (CA) have replaced CCA-C in products destined for most exterior residential applications. These preservatives contain significantly more copper than CCA and as a result they may be more corrosive to unprotected fasteners and connectors. In addition to these two newer preservatives, sodium borate (SBX) treated lumber is now used for many interior applications. CCA can still be used to treat wood for most industrial, commercial and agricultural uses.

Past Practice

Hot dipped galvanized steel, types 304 or 316 stainless steel fasteners (nails and screws) and copper have always been recommended by the industry for use with treated wood. The thickness of the zinc on hot dip galvanized fasteners and connectors used with CCA treated wood, had never been specified. Zinc coatings conforming to ASTM A653 G60 and G90 grade were commonly used for connectors such as joist hangers and truss plates. Electroplate galvanized steel and aluminum had never been recommended for use in contact with treated wood. When the use of aluminum cannot be avoided, corrosion can be minimized by using spacers or physical barriers to prevent direct contact of the metal with the pressure treated wood.

Current recommendations for galvanized steel connectors and fasteners are similar to past practice, except that by reference to ASTM standards, the minimum requirements for the amount of zinc are higher than those previously used. Note that galvanizing is a sacrificial coating and the presence of some white corrosion product on the surface is normal. Red rust is an indicator of coating failure.

Current Recommendations on Connectors for Treated Wood

As a minimum connectors used for ACQ or CA treated wood must be manufactured from steel either hot–dipped galvanized in accordance with ASTM A653, G185 designation, or hot dipped galvanized after manufacture in accordance with ASTM A123. The service life of these components can be extended by using a barrier membrane between the connector and the treated wood surface. Stainless steel connectors (type 304 or 316) should be used for maximum service life, for high preservative retentions or severe applications such as salt spray environments. High preservative retentions, normally used for ground contact, are over 4.0 kg/m3 (0.25 pcf) for ACQ and over 1.7 kg/m3 (0.1 pcf) for CA These can be recognized by looking at the plastic tags on the ends of the lumber. For borate treated wood used inside buildings, the same connectors can be used as for untreated wood.

Current Recommendations on Fasteners for Treated Wood

As a minimum, nails for ACQ or CA treated wood must be hot-dipped galvanized in accordance with ASTM A153. The zinc coating laid down using mechanical galvanizing may suffer excessive damage during nailing. Stainless steel should be used for maximum service life, for high preservative retentions or severe applications such as salt spray environments. Where appropriate, copper fasteners may also be used. Fasteners used in combination with metal connectors must be the same type of metal to avoid galvanic corrosion caused by dissimilar metals. For example stainless steel fasteners should not be used in combination with galvanized connectors.

Screws for ACQ or CA treated wood must be hot dipped galvanized in accordance with ASTM A153 or, if recommended by the manufacturer and the preservative supplier, high-quality polymer coated. Stainless steel should be used for maximum service life, for high preservative retentions or severe applications such as salt spray environments.

For borate treated wood used inside buildings, the same fasteners can be used as for untreated wood.

Current Recommendations on Flashing for Treated Wood

Flashing used in contact with treated wood must be compatible with the treated wood and be last long enough to be suitable for the intended application. Flashing must also be of the same type of metal as any fasteners that penetrate through them to avoid galvanic corrosion. Copper and stainless steel are the most durable metals for flashing. Galvanized steel, in accordance with ASTM A653, G185 designation, is also suitable for use as flashing.

Other Fasteners, Connectors or Hardware as Recommended by the Manufacturer

There may be additional products such as polymer or ceramic coatings, or vinyl or plastic flashings that are suitable for use with treated wood products. Consult the individual fastener, connector or flashing manufacturer for recommendations for use of their products with treated wood.
Current Recommendations for Condition of Treated Wood Prior to Construction.

Wood treated with copper-based preservatives should be surface dried at the treating plant, in the store or at the job site before attachment of fasteners, connectors, flashing or other hardware.