Fire Area

[Tjac]

2018 IFC Definition of Fire area. Is the fire area measured inside the walls of the building, or is it measured from outside wall to outside wall such as one would measure the square footage of the building?

BF] FIRE AREA.​

The aggregate floor area enclosed and bounded by fire walls, fire barriers, exterior walls or horizontal assemblies of a building. Areas of the building not provided with surrounding walls shall be included in the fire area if such areas are included within the horizontal projection of the roof or floor next above.

 

[Yankee Chronicler]

The IBC Commentary doesn't directly answer this, but I think the definition alone contains the answer. We know that for building code purposes, the area regulated by the code is measured to the inside face of the exterior walls. The definition of Fire Area says it's the area "bounded by" fire walls, fire barriers, exterior walls, or horizontal assemblies. The term "bounded by" connotes "enclosed within," and consistency with the definition of Building Area would mean that Fire Area is measured within the enclosing construction, but not including the enclosing construction.

For building code purposes, contrary to what is implied in your question, we do NOT measure building area to the exterior dimensions. Zoning codes do that, but look up the definition of Building Area:

AREA, BUILDING. The area included within surrounding
exterior walls, or exterior walls and fire walls, exclusive of
vent shafts and courts. Areas of the building not provided with
surrounding walls shall be included in the building area if such
areas are included within the horizontal projection of the roof
or floor above.

The Commentary goes on to say:

...
A building area is the “footprint” of the building; that is, the area measured
within the perimeter formed by the inside surface of the exterior walls. ...

 

[Paul Sweet]

The tricky part is "Areas of the building not provided with surrounding walls shall be included in the fire area if such areas are included within the horizontal projection of the roof or floor next above." It is usually applied to portes-cocheres or parking below an upper story, but an overly strict reading could say it includes the area under the roof overhang.