What is a Fire Area versus a Building Area?

MeyerFire University | LS103.05

By Chris Campbell, PE

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• Notes Page

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TRANSCRIPT

What is the difference between a fire area and building area? 

In the IBC, Building Area is defined as “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.” 

Essentially, this means that the building area is the area located within the exterior walls. Any interior walls, even if they are fire partitions or fire barriers, have no impact on the building area calculation. If a fire wall is provided, the two structures on either side of the fire wall are considered separate buildings from an area standpoint, so you would have two separate building areas in that case. 

The second sentence of this definition means that any areas of the building that are located directly under a roof or floor above but are not surrounded by exterior walls are still included in the building area calculation. 

Fire Area is defined as “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.” 

So where building area does not consider fire barriers, the fire area does. A designer can therefore provide one or more fire barriers in the building to create separate fire areas. The fire resistance rating requirement of the fire barriers depends on the occupancies involved.  

IBC Table 707.3.10 gives the fire resistance rating requirement for fire barriers used in this situation – the requirement is 2 hours for most occupancies.  

In multi-story buildings, the fire area also depends on horizontal assemblies, such as floors or ceilings. If an unrated floor/ceiling assembly is provided between two stories, the area of both of those stories would need to be added to calculate the fire area. A rated horizontal assembly can be used to separate a space into multiple fire areas as long as the horizontal assembly has a fire resistance rating per Table 707.3.10. 

Note that the fire area separation can be provided by a combination of these assemblies. So a designer could use a fire wall, fire barrier and horizontal assembly in conjunction to create a fire area separation. 

Why do fire areas matter? 

Quite simply, they often determine sprinkler requirements. Per IBC Chapter 903, the requirements for sprinkler protection are triggered in Group A, E, F-1, M, S-1, and S-2 parking garages occupancies when the fire area exceeds a certain threshold. Similarly, requirements for sprinkler protection are triggered when the fire area is located on a certain level in the building. 

As an engineer or designer, both the building area and fire area are important concepts to understand. The requirements for allowable area, found in IBC Chapter 5, are based on the building area. The requirements for sprinkler protection, found in IBC Section 903, are often triggered by the size and location of the fire area. 

I'm Chris Campbell, this is MeyerFire University.