Becoming a Plumber You must be certified by the Washington State Plumber Certification Program to work in the plumbing trade in Washington. The plumbing trade includes all: Potable water building supply and distribution pipes. Plumbing fixtures and traps. Drainage and vent pipes. Accessories (appurtenances) within the property lines of the premises. Medical gas and medical vacuum systems, and Water heaters. Except for domestic pump and pump and irrigation systems, certification is not required to install plumbing systems outside of a building (e.g., water lines connected directly to municipal water mains, sewer line connections, etc.).