Every theatre or opera house or other building which may be hereafter erected or substantially remodeled, intended to be use for theatrical or operatic purposes, or for public entertainments of any kind where stage scenery and apparatus are employed, shall be built to comply with the requirements of this section. No building, which at the time of the passage of this act is not in actual use for theatrical or operatic purposes, and no building hereafter erected not in conformity with the requirements of this section, shall be used for theatrical or operatic purposes, or for public entertainments of any kind where stage scenery and apparatus are employed, until the same shall have been made to conform to the requirements of this section, and no building hereinbefore described shall be opened to the public for theatrical or operatic purposes or for public entertainments of any kind where stage scenery or apparatus are employed, until the bureau of building inspection and the bureau of fire, respectively, shall have approved the same in writing as conforming to the requirements of this section; and no building now being used for theatrical or operatic purposes, for public entertainments of any kind where stage scenery and apparatus are employed, shall be used as such until such buildings are made to conform as nearly as possible to the requirements of this section and the same approved by the bureau of building inspection.