The Ontario Building Code | Area of Exposing Building Face
220.127.116.11. Area of Exposing building Face
(1) Except as permitted by Sentences (2), and (3), the area of an exposing building face shall be calculated as the total area of exterior wall facing in one direction on any side of a building measured from the finished ground level to the uppermost ceiling.
(2) If a building is divided by fire separations into fire compartments, the area of exposing building faceis permitted to be calculated for each fire compartmentprovided the fire separations have a fire-resistance rating not less than 45 min.
(3) In a building that contains an interconnected floor space, the area of the exposing building face for the interconnected floor space is permitted to be determined by considering each storey as a separate fire compartment notwithstanding openings through the floor assemblies.