Choosing Pressure-treated Lumber: Which Preservative Level is Best?
Lumber traders and construction professionals throughout the Caribbean know fully that pressure-treated lumber is a must, particularly given the harsh tropical climate in the region. On a per-capita basis, buyers in the Caribbean choose pressure-treated Southern Pine over non-treated more than anywhere else in the world.
When choosing pressure-treated wood, an important consideration is the preservative level needed for the application.
So, which preservative level is best?
Preservation levels are categorized by “use categories” ranging from UC1 (minimum preservative) to UC4B (maximum preservative). For example, UC1 is suitable for areas that face little moisture or risk of decay, such as interior trim and finishing carpentry. UC4B is used in areas facing extensive moisture and potential exposure to termites, such as wood used in contact with the ground or exposed to saltwater splash.
Given the tropical conditions in the Caribbean, the Southern Forest Products Association recommends using UC3B and above.
The two most common service conditions for pressure-treated Southern Pine lumber are Above Ground, Exposed (UC3B) and Ground Contact, General Use (UC4A); the term “Ground Contact” also includes contact with fresh water. Specifying the proper end-use application is very important for long-term performance because higher retention levels are typically required for Ground Contact (UC4A) as compared to Above Ground (UC3B).
TIP: Use Ground Contact for Long-Term Performance
In addition, different preservatives may be used for Above Ground (e.g. carbon-based preservatives) as compared to Ground Contact (e.g. copper azoles & quats). The most common misapplications occur when lumber treated for Above Ground Use Only is used where the actual service condition simulates a Ground/ Fresh water Contact end use. For longer-term performance, specify and verify Ground Contact (UC4A) for applications such as:
- Deck joists and beams in close proximity (within 6”) to the ground
- Deck decking, joists and beams subject to frequent wetting from moisture sources such as hot tubs or air-conditioning units.
- Joists and beams which are difficult to maintain and are critical to the performance and safety of the entire system.
- Fresh water pier cross bracing.
- Fresh water dock platforms and fresh water pier joists and beams subject to water immersion or frequent wetting from wind, waves, water-level changes or other factors.
- Stair stringers in ground contact or in close proximity to the ground.
- Walkway and boardwalk decking, joists and beams in close proximity to the ground or subject to water immersion or frequent wetting.
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