Mich. Comp. Laws § 339.6109

Sec. 1109. An individual who is applying for a plumbing contractor, master, or journey license is eligible for examination if he or she complies with article 4 and establishes to the satisfaction of the board that he or she meets any applicable experience requirement or an equivalent of that experience requirement for the particular class of licensure, by use of a notarized statement from current and past employers and master plumbers.