Before a piece of treated lumber can quality for a UC, it must meet the primary quality specifications of the AWPA. The lumber will then receive a Standard U1, which indicates that the manufacturer has completed the lumber treatment process properly.
According the University of Florida Institute of Food and Agricultural Sciences, the UC a treated lumber receives depends heavily on the amount of preservative chemical it contains in relation to the lumber's sizes. For example, treated lumber with a UC2 classification has a chemical retention level of 0.25 pounds per cubic foot of lumber, which is not very high. For this reason, the AWPA recommends using UC2 treated lumber for interior applications only. While the lumber in this classification is somewhat weather-resistant, it is not suitable for high levels of moisture.
Treated lumber with a UC3B rating from the AWPA will contain between 0.25 and 0.40 pounds of preservative chemical for every cubic foot of lumber. As the University of Florida notes, UC3B treated lumber is able to resist the damaging effects of prolonged moisture and exposure to harsh weather. People commonly use the treated lumber for constructing picket fences, decks, deck railing and deck joists.
UC4A treated lumber is ideal for constructing utilities or telephones poles, guardrail and privacy fence posts, and exterior structural elements. The lumber can tolerate contact with groundwater and is resistant to all types of weather. According to the University of Florida, the UC4A classification requires a chemical retention level of 0.40 pounds per cubic foot of lumber.
UC4B lumber is more resilient than its "A" counterpart, as it contains 0.60 pounds of preservative chemical for every cubic foot of lumber. The treated lumber can tolerate harsh weather, as well as severe exposure to fresh and salt water. These qualities make UC4B lumber ideal for constructing permanent foundations, horticultural posts and utility posts.
As the University of Florida notes, people use treated lumber with a UC4C classification for constructing foundation pilings, both on land and in water, and for constructing utility poles in high-moisture, high-decay areas. The lumber has a preservative retention level of 0.80 pounds per cubic foot of lumber, making it extremely resilient to moisture and weather exposure.