From the IRC Commentary for 302.1 This section provides details for issues related to building location on the property, including the fire rating of exterior walls, permitted openings and projections. Table R302.1 is a tabular overview of the requirements of this section.
Concerning exterior wall protection, the IRC assumes that an owner has no control over an adjoining property. Thus, the location of buildings on the owner's property relative to the property line requires regulation. In addition, Section R309, which lists the separation requirements for garages and carports, specifically requires garages located less than 3 feet (914 mm) from a dwelling unit on the same lot to have not less than 1/2-inch (12.7 mm) gypsum board applied to the interior side of the walls. Opening protection for these walls is regulated by Section R309.1.
The property line concept is a convenient means of protecting one building from another as far as exposure is concerned. Exposure is the potential for heat to be transmitted from one building to another during a fire in the exposing building. Radiation is the primary means of heat transfer.
Walls less than 5 feet (1525 mm) from the property line must be of 1-hour fire-resistant construction. The fire-resistance rating also requires the rating exposure to be for both sides. Specific provisions are not found in the IRC for the fire-resistance ratings of walls or other elements, so one may use either the IBC or another manual of fire-resistant assemblies. Projections may not be closer than 4 feet (1219 mm) to the lot line. Projections should not extend more than 12 inches (305 mm) into an area where walls are required to be of 1-hour fire-resistant construction must be protected on the underside with 1-hour fire-resistant construction [see Commentary Figure R302.1(1)].
Unlike the IBC, the IRC does not set a distance from the property line at which openings must be protected. Openings are not permitted in exterior walls where the exterior wall has a fire-separation distance of less than 3 feet (914 mm) from the lot line. Openings in a wall located at a distance in excess of 3 feet (914 mm) from the lot line can not exceed 25 percent of the maximum wall area (see Commentary Figures R302.1(2) and (3)]. Openings, projections or penetrations that are 90 degrees (1.57 rad) (perpendicular) to the line used to determine fire separation distance are not prohibited. Section R317.3 describes through penetrations and membrane penetrations in detail.
See the definition of Fire Separation Distance in Chapter 2. The requirements for protection do not apply to walls that are at a right angle (perpendicular to) the property line (see Commentary Figures R302.1(2) and (3)].