Water supply, sewerage, plumbing and gas fitting work on private property, with the exception of excavating and refilling trenches, outside work in the installation of septic tanks, septic tank cleaning and well drilling, shall be done by a master plumber-gas fitter or by a journeyman plumber under the supervision of a master plumber-gas fitter, both of whom shall be registered as specified in this chapter. Every person or at least one member of every firm or one officer of every corporation, doing or contracting to do plumbing or gas fitting work shall be a registered master plumber or master gas fitter. Every plumber or gas fitter working as a journeyman shall have a journeyman's registration card issued under this chapter. (Mont. Co. Code 1965, § 95-8.)