Which aspect is not included in the calculation of gross floor area?

Gross floor area (GFA) is a measure that is widely used in building design and regulatory contexts, and it typically encompasses the total area of all floors within a building. This measurement includes not only the usable floor space but also the areas occupied by walls, columns, and other structural elements, which are essential components of the building's footprint.

The area of the roof structure, however, is not included in the calculation of gross floor area. This is because GFA is primarily concerned with the interior spaces of the building where people can occupy or which can be used for various functions. The roof structure sits above the floor area and does not contribute to the habitable space below it, hence it does not factor into the GFA calculation. Instead, definitions of gross floor area focus specifically on the areas that present utility or occupancy potential within the building's vertically occupied dimensions, which is why the other options—total area of all floors, area taken up by walls, and area of columns—are included in the GFA measurement.