2024 IRC Has New Seismic Bracing Requirements and Recognizes Vertically Stacked Units

Jeffrey M. Hugo, CBO

June 30, 2025

 The 2024 International Residential Code (IRC) introduces several updates relevant to fire protection and construction professionals. Two areas stand out for their impact on sprinkler contractors, layout designers, and code officials: seismic restraint for equipment and appliances, and fire and smoke separation requirements between dwelling units.

Seismic Restraint for Equipment and Appliances

Section R301.2.2.10 establishes requirements for anchoring and bracing fixed appliances and equipment in areas with moderate to high seismic activity. This applies to all dwellings in Seismic Design Categories D0, D1, and D2, as well as townhouses in Category C.

If your project is in one of these seismic zones, any fixed appliances or equipment must be anchored or braced so they cannot tip over or break loose in an earthquake. You have two options to meet compliance: follow the component manufacturer’s recommendations or use the anchorage methods described in Section R301.2.2.10.1.

However, the IRC specifically exempts certain systems from these seismic restraint requirements. You are not required to brace or anchor:

• Suspended mechanical ducts
• Electrical conduit
• Automatic sprinkler systems
• Plumbing systems

This exception is important for fire sprinkler contractors. Sprinkler piping does not need additional seismic bracing under this section of the IRC.

 General Seismic Zones in the U.S.:

The IRC does not include a single seismic map, but here are some general locations where Seismic Design Categories D0 through D2 are found:

• West Coast regions including California, western Oregon, and western Washington
• Western Nevada and Utah
• Most populated areas in Alaska
• Portions of Idaho and Montana
• Intermountain West, including Salt Lake City and Boise
• Central U.S. New Madrid Seismic Zone (Missouri, Arkansas, Tennessee, Kentucky)

Category C areas are often found near moderate seismic sources, such as parts of Colorado, South Carolina’s Charleston area, and some areas along the East Coast.

Always verify the exact seismic design category for your project using USGS seismic design maps https://seismicmaps.org/ or your local building department.

Example of a seismic map

Updated Separation Requirements for Vertically Stacked Units

Section R302.3 has been reorganized to make dwelling unit separation rules clearer and more consistent with the International Building Code. The section now clarifies that separations between units can include vertical elements (walls), horizontal elements (floors and ceilings), or combinations of both.

This is especially relevant where interior stairways are used as the means of egress for the upper dwelling.

Fire-Resistance Ratings:

• Assemblies separating units must be rated for one hour.
  • Ratings must be established by testing to ASTM E119 or UL 263, or through an approved analysis per IBC Section 703.2.2.
• If the building is fully sprinklered with a system meeting IRC Section P2904 (NFPA 13D), the rating can be reduced to a half hour.

New Rules for Stacked Duplexes

The 2024 IRC adds specific requirements for vertically stacked duplexes when the building does not have sprinklers installed in both units.

Section R302.3.5 includes two requirements:

1. Assemblies separating the units, including the interior stairway serving the upper dwelling, must be built to limit the transfer of smoke. This adds an additional level of protection beyond fire resistance.
2. A notification appliance must be installed in each dwelling unit that is activated by smoke alarms in the other unit. This ensures that occupants receive an early warning if smoke spreads from one unit to the other.

The goal is to improve life safety by delaying smoke transmission and giving residents more time to escape. This is especially important when:

• The building does not have a sprinkler system installed in both units, which increases the likelihood that smoke and fire can spread unchecked.
• Occupants of the lower unit are not home or are unable to alert the upstairs residents.
• The upper unit has two or more stories or a habitable attic, which can significantly increase escape distance and time.

References to NFPA Standards

The 2024 IRC references the 2022 editions of both NFPA 13D and NFPA 13R. If you are working on residential sprinkler systems, be sure you are using these current standards for design, installation, and inspection requirements.

Key Takeaways on Seismic Bracing and Stacked Duplexes

For contractors and code officials, the 2024 IRC brings clearer separation requirements and stronger smoke protection measures in stacked duplexes. For seismic design, remember that automatic sprinkler piping and other suspended systems remain exempt from seismic bracing under these provisions, but all other fixed equipment must be anchored where required.

If you have questions about how these changes impact your projects or inspections, contact us today.