The fire chief shall have power to modify any of the provisions of the fire prevention code 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 the code; provided, that the spirit of the code shall be observed, public safety secured, and substantial justice done. The particulars of the modification, when granted or allowed, and the decision of the fire chief thereon shall be entered upon the records of the department and a signed copy shall be furnished the applicant.