2021 IBC Chapters 4 and 5
Real buildings rarely hold a single use. A church has a fellowship hall, a kitchen, and a furnace room; a clinic sits over a pharmacy; an apartment house has a parking garage below. Section 508 gives three ways to handle a building with more than one group (accessory, nonseparated, or separated occupancies), and Section 509 handles support rooms. This lesson also covers the special rules GCHS meets most often in Chapter 4: dwelling and sleeping units, parking garages, and pointers to atriums and high-rise buildings.
Section 508.1 requires each portion of a building to be classified individually under 302.1, then lets the designer choose among accessory occupancies (508.2), nonseparated occupancies (508.3), separated occupancies (508.4), live/work units (508.5), or a combination. The choice is a trade: less separation usually means stricter limits on the whole building, and more separation means each use is judged on its own. Special provisions in Section 510, such as those for stacked uses like an apartment building over a base, give further options; check them when a project stacks uses.
An accessory occupancy is ancillary to the main use, such as an office suite in a store or a dining room in an office building. Each is classified on its own (508.2.1), but its height and number of stories follow the main occupancy (508.2.2), and its area follows Section 506 for the main occupancy. Two limits keep it small: all accessory occupancies together may not exceed 10 percent of the floor area of the story they are in, and each may not exceed the nonsprinklered value for its group in Table 506.2 (508.2.3). No separation is required between accessory and main occupancy (508.2.4). The exceptions are Groups H-2, H-3, H-4, and H-5, which are separated under 508.4, and dwelling and sleeping units in I-1, R-1, R-2, and R-3, which are separated under Section 420. For example, in a one-story 24,000 sq ft Group B building, accessory rooms may total 2,400 sq ft.
In a nonseparated building the groups share space with no required wall between them (508.3.3), again with the H-2 to H-5 and Section 420 exceptions. The price is that the most restrictive Chapter 9 provisions apply to the whole nonseparated area (508.3.1), and allowable height, stories, and area follow the most restrictive allowance of the groups involved (508.3.2). Where the building is a high-rise, the most restrictive Section 403 provisions apply throughout (508.3.1.1).
In a separated building, each group is separated from the others by construction rated per Table 508.4 (508.4.4). Separation is a fire barrier under 707, a horizontal assembly under 711, or both, so the groups are completely divided (508.4.4.1). In return, each group is judged on its own. Height and stories: each use meets the limits for the building's construction type (508.4.3). Area: in each story, the sum of the ratios of actual area to allowable area for each occupancy may not exceed 1 (508.4.2). The table's ratings depend on whether the building is sprinklered throughout under 903.3.1.1; sprinklers usually cut the rating by one hour.
| Pair of uses | Sprinklered | Not sprinklered |
|---|---|---|
| A or E next to A or E | None | None |
| A or E with B, F-1, M, or S-1 | 1 hr | 2 hr |
| A or E with F-2, S-2, or U | None | 1 hr |
| A or E with R | 1 hr | 2 hr |
| R with B, F-1, M, or S-1 | 1 hr | 2 hr |
| R with F-2, S-2, or U | 1 hr (see note) | 2 hr |
| B, F-1, M, S-1 with each other | None | None |
| F-2, S-2, or U with B, F-1, M, or S-1 | 1 hr | 2 hr |
| I-1, I-3, or I-4 with R | 1 hr | Not permitted |
| A or E with I-2 | 2 hr | Not permitted |
The note on R with S-2 or U sends you to Sections 406.3.2 and 406.6.4 for garage rules. Group H and other I rows are longer; read them in the adopted code.
Worked example. A one-story, sprinklered (903.3.1.1) Type IIB building holds a 3,800 sq ft A-2 restaurant and 46,000 sq ft of B offices. Ignore any frontage increase (If = 0). The S1 values in Table 506.2 are 38,000 sq ft for A-2 and 92,000 sq ft for B. As nonseparated: the most restrictive value, 38,000, governs the whole building. The actual total is 3,800 + 46,000 = 49,800 sq ft, which is more than 38,000, so it fails. As separated: the ratios are 3,800 / 38,000 = 0.10 and 46,000 / 92,000 = 0.50. The sum is 0.60, which is not over 1, so it passes. Table 508.4 asks for a 1-hour separation between A-2 and B in a sprinklered building. The separation is the price of the larger area. Check taller buildings against the adopted code.
Section 509 treats support rooms, such as boiler and furnace rooms, as part of the building's occupancy, not as a separate group (509.2). Incidental uses may not exceed 10 percent of the building area of the story (509.3), and each must be separated, sprinklered, or both, as Table 509.1 says (509.4). Uses inside and serving a dwelling unit are exempt (509.1). Where the table allows sprinklers instead of a barrier, the room still needs smoke-resistant construction: self-closing doors, no air transfer openings, and limited door undercuts (509.4.2); only the incidental space needs sprinklers (509.4.2.1). Barriers are fire barriers under 707 or horizontal assemblies under 711.
| Incidental use (Table 509.1) | Protection |
|---|---|
| Furnace room, any unit over 400,000 Btu/h input | 1 hr, or sprinklers |
| Boiler room, largest unit over 15 psi and over 10 hp | 1 hr, or sprinklers |
| Refrigerant machinery room | 1 hr, or sprinklers |
| Laundry room over 100 sq ft | 1 hr, or sprinklers |
| Waste and linen collection room over 100 sq ft (most groups) | 1 hr, or sprinklers |
| Paint shop (not H, in other than F) | 2 hr, or 1 hr and sprinklers |
| Incinerator room | 2 hr and sprinklers |
Section 420 applies to Groups I-1, R-1, R-2, R-3, and R-4. Walls between dwelling units, between sleeping units, and between those units and other contiguous occupancies are fire partitions under Section 708 (420.2), and floors between them are horizontal assemblies under Section 711 (420.3). Group R must be sprinklered under 903.2.8, and alarm requirements follow 907 (420.4, 420.5). Lesson 6 covers the ratings.
A public parking garage is any garage that is not a private garage, and it is classified S-2 whether open or enclosed (406.4, 311.3). Clear height is at least 7 ft at each floor level in vehicle and pedestrian traffic areas (406.2.2). An open garage relies on openings for natural ventilation: uniformly distributed on two or more sides, at least 20 percent of the total perimeter wall area of each tier, and at least 40 percent of the tier perimeter in length (406.5.2). Private garages and carports are Group U (406.3). An atrium is a vertical space closed at the top that connects two or more stories in I-2 and I-3, or three or more in other groups; Section 404 governs and prohibits atriums in Group H. A high-rise has an occupied floor more than 75 ft above the lowest level of fire department vehicle access, and Section 403 adds requirements; open parking garages are not subject to it.
In the field
A GCHS client wants a two-story building with a ground-floor thrift store (Group M), an apartment above (R-2), and a small laundry room for residents. Three things follow from this lesson. Apartments over retail need at least 1 hour between them where the building is sprinklered throughout, as a new Group R building must be (903.2.8); the 2-hour value applies only to a nonsprinklered existing building. The laundry room over 100 sq ft needs a 1-hour barrier or sprinklers. And each apartment's walls and floors follow Section 420. Ask early whether the design is separated or nonseparated, because that sets both the walls to inspect and the area limit.
1. A one-story Group B office building has a 24,000 sq ft floor. Under Section 508.2.3, what is the greatest aggregate area its accessory occupancies may occupy on that story?
2. A Group M store has a small Group B office that qualifies as an accessory occupancy. Under Section 508.2.4, the separation required between the office and the store is:
3. A designer combines a Group A-2 restaurant and Group B offices in one building without a separating wall and calls the building nonseparated. Under Section 508.3.1, which Chapter 9 requirements apply?
4. A one-story, sprinklered Type IIB building holds 3,800 sq ft of Group A-2 and 46,000 sq ft of Group B, with no frontage increase. The S1 values are 38,000 sq ft for A-2 and 92,000 sq ft for B. Designed as a nonseparated building, does it comply with area limits?
5. The same building is redesigned as separated occupancies. Using 3,800 / 38,000 for A-2 and 46,000 / 92,000 for B, what is the sum of the ratios under Section 508.4.2?
6. A sprinklered building (903.3.1.1) is designed as separated occupancies with a Group A-2 restaurant next to Group B offices. What minimum separation does Table 508.4 require?
7. An existing building that is not sprinklered throughout has Group R-2 apartments over a Group M store, evaluated as separated occupancies. What separation does Table 508.4 require?
8. A sprinklered building has a Group B office beside Group S-1 storage, designed as separated occupancies. What separation does Table 508.4 require between them?
9. A sprinklered apartment building has Group R-2 units above an enclosed Group S-2 parking garage. What minimum separation does Table 508.4 list for R with S-2?
10. In a Group B building, a furnace room contains one unit with 500,000 Btu/h input. Under Table 509.1, the furnace room must be:
11. A Group R-2 building has a shared laundry room of 140 sq ft serving all residents, not within a dwelling unit. Under Table 509.1, the laundry room is:
12. A new public parking garage is planned below a Group B office. Under Sections 311.3 and 406.2.2, which pair is correct?
13. A new apartment building has its highest occupied floor 78 ft above the lowest level of fire department vehicle access. Under the Chapter 2 definition, it is:
14. In a Group R-2 building, what does Section 420.2 require for walls between dwelling units?