The American Wood-Preservers' Association, AWPA, is the organisation in charge of setting standards and categories for treated lumber. Most building codes in the United States refer to AWPA code when prescribing building standards. If you are planning to build or remodel your home with treated lumber, it is important you understand how this code can help you choose the best type of treated lumber for your project.
UC1 and UC2
The UC1 and UC2 treated lumber types are reserved for interior use. Under no circumstance should you use these types of wood for projects which will be in contact with the ground or foundation. UC1 is particularly suitable as interior furniture and millwork. UC2 treated lumber is also suitable for interior areas, which are subject to high levels of dampness such as interior beams, flooring and sill plates.
UC3 and UC4
UC3 and UC4 treated lumber can be used for exterior construction projects. UC3 lumber is divided in two subgroups: UC3A, UC3B. UC3A is used for exterior, above ground jobs and is coated. UC3B, on the other hand, may receive a further finishing coat.
UC4 lumber is reserved for exterior projects where the wood may be in contact with freshwater. There are three subgroups of UC4 lumber: UC4A, UC4B and UC4C. UC4A is used for areas with a low risk of getting in contact with freshwater, UC4B is reserved for severe environments with a high risk of damage from water and UC4C is for important structural components located in severe environments.
You can use UC5 treated lumber for areas that may be exposed to saltwater, such as piling structures and bulkheads. However, you must match the type of treated lumber you use to the region it will be used in, because levels and types of saltwater exposure change from one area to another. UC5A is designed for North San Francisco and New Jersey, while UC5B is for South Francisco and from New Jersey through Georgia on the East Coast. UC5C is used in the South of Georgia and the Gulf Coast.
UCF-type treated lumber is designed to provide protection from fire. There are two types of fire-protected lumber: UCFA and UCFB. UCFA is used in areas that require fire protection but are shielded from the weather, such as framing structures. UCFB, on the other hand, is used for siding, shakes and exterior stairways, because it provides both weather and fire protection.
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