Where water-mains have been placed in a street, and there is sufficient pressure, water shall be introduced in every dwelling, and in every apartment of two or more rooms in a two-family house, and in a tenement: Provided, That, in sparsely populated districts where there is a well upon the premises, and the water therefrom is of standard purity required by the Board of Health, such well may be considered an acceptable substitute for water in dwellings. When one or more apartments consist of but one room, there shall be at least one source of water-supply, in an easily accessible place, for every two such apartments. In all rooming-houses water shall be introduced in an accessible place on each floor: Provided, That where less than three rooms are on each floor of a rooming-house, water may be introduced on every alternate floor. Wherever a water fixture is introduced, a sink, and suitable drain, properly trapped, leading from it, shall also be installed. No such sink in a tenement shall be enclosed with any woodwork.