No such gathering or entertainment shall be held or presented in any premises other than a building as described in the preceding section, unless such premises have been examined by the building commissioner not more than three (3) days before the date of such gathering and found to be free from any fire hazard and found to be suitable for the purpose. No structure enclosed by a roof shall be used for such purpose unless the same is constructed of nonflammable materials. Provided that tents or structures covered by canvas or similar materials may be so used if such canvas or other material has been properly treated so as to be rendered fire resistive. (1965 Code, sec. 27-3.02)