Where the local authority has made by-laws providing for the examination and licensing or plumbers and drainlayers, no person other a plumber or drainlayer, licensed under such by-laws, shall undertake and plumbing or sewerage work as the case may be other than replacement, rewashering and repacking of taps and ball-valves, cleaning the waste-pipe of a sanitary fitting and the clearance of obstructions in private sewers.