Effective Dec. 1, 2007, all persons throughout the state working at the business of interior plumbing must be licensed plumbers or registered apprentices. No person, firm, or corporation shall engage in the business of installing plumbing nor install plumbing in connection with the dealing in and selling of plumbing material and supplies unless at all times a licensed master plumber or restricted master plumber, who shall be responsible for proper installation, is in charge of the plumbing work of the person, firm, or corporation.

A licensed master or journeyman plumber may perform plumbing anywhere in the state. People with restricted licenses may perform plumbing everywhere in the state except in cities with a population of more than 5,000.

Persons having certification as pipelayers may only perform some plumbing installation outside of buildings, if properly bonded.

All plumbing plans, including site utilities, that are submitted to DLI for projects located in cities which have a population over 5,000, must be signed by a master plumber or a registered engineer. Plumbing plans, including site utilities, that are submitted to DLI in cities which have a population less then 5,000 must be signed by a master plumber, a restricted master plumber, or a professional engineer that is registered in the State of Minnesota. Site utilities may be prepared by a certified pipelayer. When a plumber or a certified pipelayer has designed the plans, only the individual who has prepared the plans may use the plans for construction. If another plumber or certified pipelayer is contracted to install the plumbing, they must submit their own plans and specifications for the project.

Visit the Web site of the U.S. Census Bureau for population data.