Match the Treatment Level of Preserved Wood and Fastening Systems to the Application and Exposure

There are often situations where wood cannot be adequately separated from ground moisture or protected from exterior moisture sources. In these situations, either naturally decay-resistant wood or preservative-treated wood must be used. In some cases, wood alternatives may be considered, such as plastic porch posts with metal pipe inserts, plastic decking, concrete posts or piers, or plastic lumber composites.

For preservative treated wood, the American Wood Protection Association (AWPA) maintains the Use Category System (UCS), first introduced in 1999 and used to define a range of usage categories, or biodeterioration hazard levels, for treated wood products. The UCS covers a wide range of conditions spanning residential to marine construction, with most residential applications generally found in use categories 2 through 4. Table 8–1 below summarizes typical residential Use Categories and Applications, drawn from specification recommendations for Southern Pine.

The table above serves as a quick reference, with conditions 3B and 4A being the most common for pressure-treated Southern Pine lumber. More in-depth information on the Use Categories, preservative retention minimums by product/end use, and specification details are available from AWPA Standard U1 and the Southern Pine Council (see Additional Resources section at the end of the chapter).

When a residential design includes preservative treated wood, there are several basic recommendations to help assure good durability performance.

• Specify Ground Contact (UC4A) pressure-treated lumber for applications which more closely fit this condition, as opposed to Above Ground (UC3B). Typical applications to specify UC4A instead of UC3B are:
  
  • deck joists and deck beams in close proximity to the ground
  • deck decking, joists, and beams subject to frequent wetting from moisture sources like hot tubs
  • stair stringers with ground contact or in close proximity to the ground
• Verify that treated lumber delivered on site matches the specification and the Use Category. For example, if deck joists close to the ground are specified to be UC4A, field verify that this is the Use Category for the lumber, as listed on the lumber’s plastic end tag or the ink stamp.
• Select fasteners and connectors with the appropriate corrosion resistance given the type of preservative treated wood they’re being used with. Corrosion resistance of fasteners and connectors used with treated wood products is not a new issue, but has gained increased emphasis since the phase-out of Chromated Copper Arsenate (CCA) in residential applications in 2004. The alternative treatments have generally been shown to be more corrosive. The building application should define the Use Category, and this information is used to select the preservative treated wood system. Based on the preservative treatment being used, the wood supplier should be able to provide: the chemical retention level, whether or not ammonia was used, and ultimately a connector/coating recommendation. If the wood supplier cannot provide a connector/coating recommendation, the preservative treatment information can be crossreferenced with the fastener/component supplier’s recommendations. For example, Simpson Strong-Tie offers a Technical Bulletin on Preservative Treated Wood which includes recommended coatings as a function of the preservative type and the application’s exposure (see Additional Resources).
  Beyond following this selection and specification process, several other rules of thumb apply for connectors/fasteners used with preservative treated wood:
  
  • Hot-dip galvanized or stainless steel fasteners/connectors are recommended for use with preservative treated wood. Electroplated, electro-galvanized, and mechanically galvanized coatings should not be considered to be hot-dip galvanized.
  • The thicker the galvanized coating, the longer the expected service life of the fastener, connector, anchor, or other hardware.
  • Hot-dip galvanized fasteners (like nails) should be used with hot-dip galvanized connectors (like joist hangers). The same is true for stainless steel. Hot-dip galvanized components and stainless components are dissimilar metals and should not be used with each other.
  • To be more conservative in aggressive environments, specify stainless components. Note that cost and product availability are both significant considerations in the use of stainless components.