The principal officer of said bureau shall be known as the superintendent of building inspection and shall be either an architect, a practical builder, civil engineer or carpenter, and the other officers of the said bureau shall consist of an assistant superintendent and such number of inspectors, clerks and messengers or other employés as the said director of the department of public safety shall deem necessary, subject to the approval of the city councils. The assistant superintendent and all of the inspectors shall be either practical builders, civil engineers, carpenters or bricklayers, but shall not all be of the same occupation. None of the aforesaid officers shall be employed or engaged in any other business or be interested in any contract for building or furnishing materials to be used for building in the said cities. The city councils shall from time to time fix their compensation.