The Chief of the Bureau of Fire Prevention shall have power to modify any of the provisions of the Fire Prevention Code adopted by this article upon application in writing by the owner or lessee or his duly authorized agent when there are practical difficulties in the way of carrying out the strict letter of such Code; provided, that the spirit of such code shall be observed, public safety secured and substantial justice done. The particulars of such modification, when granted or allowed, and the decision of the Chief of the Bureau of Fire Prevention thereon shall be entered upon the records of the Fire Department, and a signed copy shall be furnished the applicant.