This week’s Insights post discusses how a building’s Fire Separation Distance (FSD) and Frontage are measured from adjacent buildings or structures, lot lines, and public ways. There are various impacts that these measurements have, including the composition of the building’s exterior wall construction, restrictions on unprotected openings, and allowable building area, which are all critical to a building’s façade, geometry, and shape and size. Therefore, it is crucial to appreciate these attributes early in design. For the purposes of this post, the 2015 Edition of the International Building Code (IBC) is referenced as it is the currently adopted model code for the Massachusetts State Building Code (780 CMR).
The IBC defines Fire Separation Distance (FSD) as the distance measured from the building face (at a right angle from the face of the wall) to one of the following (IBC Section 202):
- The closest interior lot line;
- To the centerline of a street, alley, or public way;
- Or, to an imaginary lot line between two buildings on the same lot (note: an imaginary lot line does not have to be equidistantly placed between two buildings)
A demonstration of each of these three conditions are shown in the figures below (IBC Commentary, Figure 202(20) – 202(22)):
Most notably, FSD drives the fire-resistance rating required for exterior walls based on occupancy type(s) and the construction type of the building (IBC Table 602). FSD also dictates the percentage of allowable unprotected openings that may be permitted in the exterior wall (IBC Table 705.8), in order to mitigate the potential of flame spread from one structure to an adjacent one.
Similar in concept to Fire Separation Distance (FSD), Frontage of a building is a measure of the amount of open space or access to a public way around a building’s perimeter. However, this factor drives allowable area increases for the building (IBC Section 506.3). The IBC Commentary expands on this concept to state that “the allowable area of a building is allowed to be increased when it has a certain amount of frontage on streets (public ways) or open spaces, since this provides access to the structure by fire service personnel, a temporary refuge area for occupants as the leave the building and reduces exposure to and from adjacent structures.
In order to qualify for Frontage Increase, a building must possess:
- At least 25% of its perimeter located along a public way or open space (IBC Section 506.3.1);
- At least 20 feet in width from the building’s exterior wall to the public way or open, as measured at right angles to any of the following (IBC Section 506.3.2):
- The closest interior lot line;
- The entire width of a street, alley, or public way;
- Or the exterior face of an adjacent building on the same property.
An important distinction for a portion of the building’s perimeter to qualify for Frontage is the need for the wall to be accessible for the fire department by means of a street or fire lane. The IBC Commentary expands on fire department access with: “for instance, if the back side of a building on a narrow lot cannot be reached by means of a fire lane on one side of the building (and there is no alley or street at the back), that portion of the perimeter is not considered open for purposes of frontage increase, even if there is actual open space exceeding 20 feet in width.” For example, the north exterior wall in the figure below can be accounted for Frontage since a fire lane is provided on the adjacent side (IBC Commentary Figure 506.3.2.(1)).
Please note that the content of this blog is relative to a new building with respect to existing or known site conditions on its lot. This does not account for any requirements (in terms of fire-resistance rating or proximity from a building), for select equipment such as electrical transformers, emergency generators, chemical or gas bulk storage tanks, dumpsters, sheds, etc. Please refer to the codes and standards specific to these types of equipment or features for any additional separation requirements from adjacent structures.
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