Code Differences Between IRC and IBC for Residential Buildings

Last updated April 1, 2025
By Ian Story

This articles covers code differences for residential structures built under the IRC vs the IBC. We will stick to R-2 and R-3 occupancies that allow a single exit (this article does not cover apartment buildings with a central corridor). This discussion will also include energy code differences under the Washington State Energy Code, even though those aren’t technically part of the building code.

To compile this list, we reviewed Chapters 3 through 15 of the IBC, skimming all of the sections to find potential differences. While we believe this list to be reasonably comprehensive, it is possible that we missed some more detailed differences. The purpose of this list is to point out areas of difference for further review. It does not cover what the differences are in depth or their implications for a project. Review this list as a starting point, then consult the relevant code sections to more precisely identify the differences.

Thresholds

The IRC covers one- and two-family dwelling units with up to 3 stories above grade, and townhouses. These structures are typically classified as R-3 occupancies under the building code. R-2 occupancies (3-plexes, 4-plexes, and apartment buildings) require using the IBC, as do single-family structures that are 4 stories tall.

For the energy code, most residential structures will use the residential provisions. The commercial provisions are required for buildings with more than two dwelling units that meet either of the following criteria:

• Designated R-3 and more than 3 stories above grade.
• Designated R-2 and either more than 3 stories above grade or not all units are accessed directly from the interior.

Occupancy Classification

Many of the code requirements distinguish between R-2 and R-3 occupancies. Generally speaking, R-3 occupancies are single-family residences, true townhouses, and duplexes. If there are more than 2 dwelling units in the building and these units are not townhouses (attached side by side with no stacked units), then the structure is considered an R-2 occupancy and needs to comply with the requirements for apartments.

Note that a triplex or 4-plex can be considered an R-3 occupancy if units are separated into pairs by fire walls (fire walls are a higher grade of separation than fire partitions – think party walls where one side can completely burn down and leave the other side intact). They would still be regulated by the IBC though.

General Notes

• In many places, the code distinguishes between elements within a dwelling unit and those outside. Within a dwelling unit, the IBC requirements are generally very similar to the IRC. Outside of a dwelling unit, however, the IBC treats many elements as though they were part of a large apartment building (especially for R-2 occupancies). These spaces require full egress considerations, just as though they were part of a commercial building. This includes private-use components located outside of a dwelling unit, like steps to access the front door of the unit. (see discussion here)

Construction Type

IRC structures are almost always Type VB (non fire-rated) construction. When pushing the limits with the IBC, it may occasionally be worth considering Type VA construction or providing fire walls to subdivide the units in order to switch to an R-3 occupancy. In most cases, however, Type VB will still be preferred because it qualifies for a lot of exceptions that simplify the detailing for the building.

Stories and Height Limits

IBC structures have limits on the number of stories and overall height of the building, depending on the fire sprinkler type (13D, 13R, etc.) and whether construction is Type VA or VB. The height limit ranges from 40-50 feet to 60-70 feet, and the story limit ranges from 3 to 4. See IBC Tables 504.3 and 504.4.

Fire Sprinklers

Under the IBC, all residential structures require fire sprinklers, with no exceptions (note: it might be possible to avoid sprinklers when converting an existing building, using the performance approach in the IEBC). See IBC 420.4.

There are two main types of sprinkler systems used for residential structures: NFPA 13D (IBC 903.1.3) and NFPA 13R (IBC 903.1.2). R-3 occupancies can use 13D (D = domestic, essentially single family), while R-2 occupancies have to use 13R (R = residential multifamily; can alternatively use NFPA 13: the full commercial version). The difference between 13D and 13R is primarily a question of capacity: 13R requires more simultaneously operating sprinkler heads and therefore a higher fire flow. Also, 13D does not require a fire department connection (FDC).

Accessibility

Structures with 4 or more dwelling units in a single structure (regardless of fire walls or R-2 vs. R-3 designation) need to provide at least some Type B accessible units. See IBC 1107.6.2.2.2 and IBC 1107.6.3.

For conversions, accessibility compliance is usually required to the extent technically feasible (see IEBC).

The following requirements apply if any Type B units are required:

For buildings without an elevator, pick one story (typically the ground story). Only the units on that story need to be Type B units and have an accessible access route. However, for buildings that have multiple stories with near-grade access (such as buildings built into the side of a hill), all stories that can feasibly provide an accessible access route (based on code criteria, namely less than a 10% grade from site access to story entrance) need to have Type B units. See IBC 1107.7.

Multistory Type B units have to have a living area, kitchen, and toilet on the accessible floor, but the upper stories don’t need to be accessible. See IBC 1107.7.2.

See ANSI A117.1 for requirements for Type B units. Type B units are considered “adaptable” units – they don’t need to be full accessible, but they do require things like an accessible route to the front door, doors wide enough for a wheelchair to pass, parallel approaches at sinks and vanities, removable base cabinets, and blocking for future grab bar installations.

The site will require an accessible route from the site access point and accessible parking space to the front door of all Type B units. See IBC section 1104.

One accessible van parking stall is required (assuming there are less than 26 total parking spaces required). See IBC section 1106.

Egress, Exterior Doors, and Stairs

This article applies to residential structures that require only a single exit. Be sure to check the IBC thresholds to determine whether a second exit is required (this will typically force the design to be a central corridor apartment building). See IBC section 1006.

General Requirements – Including within a Dwelling Unit

Within a dwelling unit, the egress requirements are almost identical between the IRC and IBC. Here are the differences we found:

• Under the IBC, stair landings have to have a minimum depth of 48″, door swings can obstruct no more than half the required landing width, and fully-open doors need to sit mostly outside of the landing area (see IBC 1011.6).
• The maximum stair run height without a landing is 12′-0″, compared to 12′-7″ under the IRC (see IBC section 1011.8).
• Egress doors need to be min. 2′-10″ panels (clear width 32″ with door in open position). For type B units, this requirement applies to all doors in the unit, other than reach-in closets. See IBC 1010.1.1.

Requirements that Only Apply Outside Units

Outside of a dwelling unit, the differences are more significant. We will assume for this discussion that there are no internal shared corridors; all units exit either directly into an exit staircase or directly to the outdoors (either at grade or to a private flight of stairs). Requirements for exit access components with a dwelling unit do not differ from basic IRC requirements. This discussion therefore skips over requirements for corridors and exit access components. See IBC section 1021 for egress balcony requirements, if applicable.

If a shared exit staircase is required, it needs to meet the criteria for IBC 1022 through 1027, including fire rated walls, opening protectives, and discharging directly to the exterior of the building. Note that exterior exit stairways need to have a minimum fire separation distance of 10 feet (relaxed to 5 feet for R-3 occupancies). The distinction between exterior exit stairways and exterior stairways within the exit discharge can be critical here, since the fire separation distance requirement only applies to exterior exit stairways. See discussion: Exit Code Definitions.

Accessible spaces need to be provided with an accessible means of egress. See IBC section 1009. The minimum stairway width for accessible means of egress is 44 inches clear between handrails, with exceptions if there is a 13R sprinklers system (IBC 1009.3.2). But note that areas of refuge are not required for R-2 occupancies (see IBC 1009.3.3).

Note that even though residential structures typically have low occupancies, it may still be necessary to provide a code plan documenting occupant counts for egress calculations.

The following additional requirements apply only for R-2 occupancies (not for R-3 occupancies):

• All stairs outside of dwelling units (including private steps up to the front door) have a maximum riser height of 7 inches and a minimum tread depth of 11 inches. See IBC 1011.5.2.
• If there is a single step at the entry door, handrails must be provided on both sides of the step.
• Fire protection required for exterior exit stairways serving a single dwelling unit.
• Narrow egress courts with 10 occupants need a 1-hour fire rating. See IBC 1029.3.
• Need to provide an emergency guide for each dwelling unit. See IBC 1002.2 and IFC 403.9.2.2.

Fire-Rated Assemblies

Under the IBC, floor and walls separating dwelling units need to be 1/2-hour rated assemblies (more if sprinklers are somehow omitted), not just prescriptive 1/2″ gypsum board. See IBC 420.2, 420.3, 708.3, and 711.2.4.2. Partition walls need to extend down to the foundations to separate the crawl space between units (IBC 708.4). Fireblocking or draftstopping may also be required to separate attic spaces and floor spaces (IBC 708.4.2).

IBC buildings need to include specifications and detailing for joints and penetrations through rated assemblies, per sections 714 and 715. Note specifically that electrical outlet boxes need to be offset.

For structures and projections within 5 feet of the fire separation distance line, the requirements are more strict for IBC buildings. Within the 3 to 5 feet separation distance range, projections are more strict under the IBC. Also, IBC restricts the allowable openings in exterior walls as a percentage of the façade for R-2 occupancies (there is a footnote that exempts R-3 occupancies with at least 5′ FSD). This may require some openings to be fire protected. See IBC Tables 705.2 and 705.8.

Sound-Rated Assemblies

The IBC requires sound-rated assemblies between units. The minimum is STC 50 for walls and floors, and IIC 50 for floors. See IBC 1206.2 and 1206.3.

Ceiling Height

The IBC minimum ceiling height is 7′-6″, compared to 7′-0″ under the IRC. This can be reduced to 7′-0″ in hallways. The IBC also eliminates exceptions for beams and girders hanging down below the ceiling in R-2 occupancies. See IBC 1208.2.

Ventilation

R-2 occupancies are required to use a balanced supply/exhaust system for whole-house ventilation. They can’t rely solely on exhaust fans. This typically means an R-2 occupancy will use an HRV/ERV. See WA State amendments to IMC 403.4.4.1)

Structural Considerations

We already engineer buildings to IBC standards rather than IRC, so these changes aren’t relevant to us. For conversion projects, review IEBC to determine which structural upgrades are required (in general, check 5%/10% rules for gravity and lateral systems, and substantial alteration rules for lateral upgrade thresholds).

Energy Code

This section is incomplete. None of the residential buildings we have worked on have been required to use the commercial energy provisions yet. We will complete this section when we encounter a project that needs it.

Additional Considerations

The following additional differences between the IBC and IRC affect detailing and specifications, but shouldn’t significantly affect the design or planning of the building:

• Ducts that penetrate walls and floors may need to be detailed as penetrations and may require specially rated fire and/or smoke dampers (IBC section 717).
• Insulation materials need to meet specifications for flame spread index and smoke-developed index (IBC 720.2). Spray foam also needs to be labeled (IBC 2603.2), and may be subject to additional restrictions and documentation requirements in construction types other than Type V (IBC 2603.5).
• When using foam insulation as part of a roof assembly, the whole assembly needs to be tested as a Class A, B, or C roof assembly and installed over plywood sheathing (testing per NFPA 276 / UL 1256, or ASTM E108 / UL 790) (IBC 1508.1 / 2603.6).
• Interior wall and floor finish materials need to be tested for flame spread index and smoke-developed index, with exceptions (IBC sections 803 and 804). Interior trim is limited to covering 10% of a given wall or ceiling section (IBC 806.7).
• R-2 occupancies are required to have portable fire extinguishers in each dwelling unit (IBC 906.1).
• Guardrails need to be 42″ in the following cases: buildings over 3 stories in height; outside of dwelling units for R-2 occupancies; and all locations for R-2 occupancies sharing means of egress. The previous requirement are except for stairs within dwelling units, which only need to be 34″ (IBC 1015.3).
• There are limitations on how far elements can protrude into stairways, hallways, and other egress pathways. These areas also have to meet specifications for slip-resistant surfaces (IBC 1003 and 1004).
• For routes designated as accessible, protruding objects are not allowed to protrude into the minimum clear width of the route (IBC 1003.3.4).
• Areas with low headroom (particularly under stairs) need a detectable barrier to prevent visually-impaired individuals from hitting their head (IBC 1003.3.1).
• Outside of dwelling units, all stairs require handrails on both sides of the stairs. All steps require handrails, with the sole exception of single-riser steps with a full landing on both sides (IBC 1011.11).
• Exterior egress doors may be required to have an accessible threshold (1/2″ max rise). The code on this is confusing and potentially conflicting (IBC 1010.1.6, but also 1010.1.4 and 1003.5).
• One- and two-riser stairs along the path of egress have different rise and run requirements, and require handrails (IBC 1003.5).
• The entire means of egress outside of the dwelling unit, including any shared stairs, exterior, steps, and walkways, needs to be permanently illuminated (IBC 1008.1). Emergency backup power is not required for buildings that only need one exit.
• The space under exterior stairs cannot be used for any purpose unless it is enclosed in 1-hour fire rated enclosure (IBC 1011.7.4).
• Operable windows with sills less than 36″ may require fall protection devices (IBC 1015.8).
• Egress windows from basement bedrooms may be more lenient under IBC than under IRC (IBC 1031.2).
• In some cases (nonsprinklered R-2, or less than 5′ FSD), the roof may require additional fire protection to avoid the need for parapet walls (IBC 708.11).
• Roof coverings generally need to be classified (Class C for VB construction). Wood shakes and shingles are only allowed when meeting minimum fire separation distances (IBC Table 1505.1).
• Ice barriers may be required within 24 inches of eaves (IBC 1507.1.2).