A proprietary duct system that has its own inherent fire-resistance rating can be used, such as a metallic duct, either field fabricated or UL certified factory-built designs. Field fabricated is typically made from 16 gauge carbon steel, all welded per local codes, which is then externally treated with fireproofing. Factory-built designs are UL tested to UL 1978 and UL 2221 (for fire rated models) test standards, are made from lighter gauge stainless steel, and offered in single wall and multiple double wall insulated designs. Typical materials used for fireproofing field fabricated designs are: