This Insights post discusses how a story relative to grade is classified from the perspectives of NFPA 30, Flammable and Combustible Liquids Code and the International Building Code (IBC). With the growing laboratory market in the Northeast, more and more tenants are using and storing flammable & combustible liquids, which may trigger compliance with NFPA 30 depending on the jurisdiction. Because NFPA 30 prohibits the storage of flammable liquids in “basements,” it is important to understand the distinction between what qualifies as a basement per NFPA 30 vs. the building code.
What is an NFPA 30 Basement?
From the perspective of NFPA 30, a Basement is defined as “a story of a building having one-half or more of its height below ground level and to which access for fire-fighting purposes is restricted.” This means that if the area of exterior wall below the surrounding ground surface is greater than half of the total area of all exterior walls on the level in question, the story is considered a Basement in accordance with NFPA 30. An example is provided below (NFPA 30 Handbook, Exhibit I.3.2):
What is an IBC Basement?
Stepping over to the IBC, a Basement is a story that is not a Story Above Grade Plane, which is defined as “a story having its finished floor surface entirely above grade plane or where the finished floor of the level above is either: (1) more than 6 feet above grade plane, or (2) more than 12 feet above the finished ground level at any point.” Where a story does not satisfy one of these criteria, it is considered to be a Basement in accordance with the IBC. Two examples demonstrating the application of the IBC definitions are shown in the figure below (IBC Handbook, Figure 202-21).
Note that the term Grade Plane refers to a reference plane representing the average of finished ground level adjoining the building exterior walls, whereas the term grade refers to the surrounding ground level. The determination of grade plane will be a function of the site topography and may require detailed calculations depending on elevation changes.
Can a story be both an NFPA 30 “Basement” & an IBC “Story Above Grade Plane”?
There are cases where a story can be considered a Basement per NFPA 30 and still be classified as a Story Above Grade Plane per the IBC. Because this possibility exists, it is important that both definitions are evaluated as part of the design process. The following is an example:
What Does this Mean for My Project?
The classification of a story will impact the code requirements pertinent to the design of a building, such as the allowable height and area, construction type, and chemical allowances. Since the classification of a story differs depending on the code, it is critical to evaluate a building in accordance with each applicable code to ensure that all relevant code requirements are met.
If you have questions regarding how to apply these requirements to your project, please contact us at firstname.lastname@example.org.