Ground Contact (UC4)
Pressure-treated lumber rated for direct contact with soil or fresh water. Requires higher preservative retention (0.40 pcf ACQ) than above-ground rated lumber (0.15 pcf).
In Detail
AWPA Use Category 4 (UC4) lumber is required for any wood in direct contact with soil: fence posts, landscape timbers, structural posts embedded in the ground, and retaining wall members. UC4A is for general ground contact; UC4B is for heavy-duty (critical structural) ground contact at 0.60 pcf retention. UC4C (2.50 pcf) is for salt-water splash zone. Always verify the end tag retention level matches the application.