2021 IBC Chapter 5
Chapter 5 answers three questions for every new building: how tall, how many stories, and how big. All three come from a lookup that uses the same three inputs: the occupancy group (Lessons 2 and 3), the construction type (Lesson 4), and whether the building has an automatic sprinkler system. This lesson reads Tables 504.3, 504.4 and 506.2, adds the frontage increase, and works one one-story example. Local adoption governs, so confirm the tables in the adopted code.
Building height is the vertical distance from grade plane to the average height of the highest roof surface. Grade plane is the average of the finished ground level at the exterior walls (on a slope, the lowest points between the building and the lot line, or a point 6 feet from the building if the lot line is farther). A story above grade plane has its finished floor entirely above grade plane, or the floor next above is more than 6 feet above grade plane, or more than 12 feet above the finished ground at any point. A basement is any other story, and Section 506.1.3 lets it go uncounted in allowable area when its total area does not exceed what a one-story building is allowed. Building area is measured inside the surrounding exterior walls (or exterior walls and fire walls), excluding vent shafts and courts.
Height is in feet and stories are counted separately. S means sprinklered throughout under 903.3.1.1. One row covers Groups A, B, E, F, M, S and U in Table 504.3.
| Height, ft (Groups A, B, E, F, M, S, U) | IA | IB | IIA | IIB | IIIA | IIIB | VA | VB |
|---|---|---|---|---|---|---|---|---|
| Nonsprinklered | UL | 160 | 65 | 55 | 65 | 55 | 50 | 40 |
| Sprinklered | UL | 180 | 85 | 75 | 85 | 75 | 70 | 60 |
UL means unlimited. Type IV-A sprinklered allows 270 feet, IV-B 180 feet, and IV-C and IV-HT 85 feet. Group R with 13R gets 60 feet in every type. The nonsprinklered R values apply only when evaluating existing buildings, because new Group R must be sprinklered (903.2.8). Section 903.3.1.2 adds its own limit on 13R: four stories or fewer, with the highest floor 30 feet or less above fire department vehicle access.
| Stories | IIA | IIB | IIIA | IIIB | VA | VB |
|---|---|---|---|---|---|---|
| B nonsprinklered / sprinklered | 5 / 6 | 3 / 4 | 5 / 6 | 3 / 4 | 3 / 4 | 2 / 3 |
| M nonsprinklered / sprinklered | 4 / 5 | 2 / 3 | 4 / 5 | 2 / 3 | 3 / 4 | 1 / 2 |
| R-2 sprinklered (NFPA 13) | 5 | 5 | 5 | 5 | 4 | 3 |
Type IA is unlimited for these groups, and Type IB allows 11 stories nonsprinklered or 12 sprinklered. R-2 with 13R gets 4 stories, or 3 in Type VB.
Table 506.2 gives the tabular area At in square feet for one story. NS is nonsprinklered. S1 is a one-story building sprinklered under 903.3.1.1. A separate SM value covers sprinklered buildings of two or more stories and is not shown here.
| Group | IIB NS | IIB S1 | VA NS | VA S1 | VB NS | VB S1 |
|---|---|---|---|---|---|---|
| B | 23,000 | 92,000 | 18,000 | 72,000 | 9,000 | 36,000 |
| M | 12,500 | 50,000 | 14,000 | 56,000 | 9,000 | 36,000 |
| S-1 | 17,500 | 70,000 | 14,000 | 56,000 | 9,000 | 36,000 |
Equation 5-1 gives the allowable area per story for a single-occupancy building up to three stories above grade plane: Aa = At + (NS x If). At is the table value for the sprinkler condition, NS is the nonsprinklered table value even when the building is sprinklered, and If is the frontage factor. For more than three stories, Equation 5-2 sets a total allowable area, and no floor may exceed its Equation 5-1 value. For multistory buildings, confirm which sprinkler column applies with the adopted code and the building official.
A building must adjoin or have access to a public way, and at least 25 percent of its perimeter must be on a public way or open space (506.3.1). The open space may be on the same lot or dedicated to public use, reachable from a street or approved fire lane. The frontage distance W is measured at right angles from the building face to the closest interior lot line, across the entire width of a street, alley or public way, or to the face of another building on the same property (506.3.2). Only open space of 20 feet or more counts, and the increase uses the smallest counted space. In 2021, Table 506.3.3 replaced the old equation.
| Perimeter with open space of 20 ft or more | W 20 to under 25 ft | W 25 to under 30 ft | W 30 ft or more |
|---|---|---|---|
| 25 to under 50 percent | 0.17 | 0.21 | 0.25 |
| 50 to under 75 percent | 0.33 | 0.42 | 0.50 |
| 75 to 100 percent | 0.50 | 0.63 | 0.75 |
Below 25 percent of the perimeter, or with W under 20 feet, If is 0. The largest possible If is 0.75.
In 2021, Section 504.2 is about mixed occupancy: no occupancy may exceed the height and story limits for that occupancy. Sprinklers are a column choice in the tables: S is 903.3.1.1, S13R is 903.3.1.2, and S13D is 903.3.1.3, and the area increase is built into the S1 and SM values. For mixed occupancies, area follows Section 508.3.2 (nonseparated) or 508.4.2 (separated, where the sum of the ratios of actual area to allowable area on each story cannot exceed 1).
A compliant mezzanine is part of the story below. It adds nothing to the number of stories or the building area, although its area counts toward the fire area, and it needs at least 7 feet clear above and below. Under 505.2.1, all the mezzanines in a room together may cover no more than one-third of that room's floor area. The one-half exceptions are narrow: sprinklered Type I or II buildings with emergency voice/alarm communication, and mezzanines inside a dwelling unit of a sprinklered building.
A developer proposes a 45,000 square foot, one-story Group M store, 300 feet by 150 feet, of Type IIB construction. The perimeter is 2 x (300 + 150) = 900 feet. Open space is 80 feet at the front, 30 feet on the right, 25 feet on the left, and 12 feet at the back, which is under 20 feet and does not count. Counted perimeter is 300 + 150 + 150 = 600 feet, or 66.7 percent, the 50 to under 75 percent row. The smallest counted open space is 25 feet, so Table 506.3.3 gives If = 0.42. Table 506.2 gives Group M Type IIB an NS of 12,500 and an S1 of 50,000. Sprinklered: Aa = 50,000 + (12,500 x 0.42) = 55,250 square feet, so 45,000 fits. Nonsprinklered: Aa = 12,500 + 5,250 = 17,750 square feet, so it does not. Height and stories pass either way.
In the field
An owner asks GCHS to estimate a 6,000 square foot addition to a 5,000 square foot, one-story, unsprinklered Group B office of Type VB. The total, 11,000 square feet, is over the 9,000 square foot nonsprinklered base for that group and type. Either generous open space has to carry the frontage increase, or a sprinkler system, which moves the base to 36,000, has to be priced in. Ask the building official which before pricing.
1. Under Table 504.3, the allowable height of a Group B building of Type IIB construction sprinklered throughout under 903.3.1.1 is:
2. Under Table 504.3, the allowable height of a nonsprinklered Group M building of Type VB construction is:
3. Under Table 504.4, a nonsprinklered Group B building of Type IIB construction may be at most:
4. Under Table 504.4, a sprinklered Group M building of Type VB construction may be at most:
5. An inspector measures the average height of the highest roof surface of a sprinklered Group B building of Type VB at 64 feet above grade plane. What is the finding?
6. In Equation 5-1, Aa = At + (NS x If), the term NS represents:
7. To earn any frontage increase, Section 506.3.1 requires at least what share of the building perimeter on a public way or open space?
8. For the frontage increase, open space counts toward W only when it is at least:
9. An inspector reviews a one-story building with open space of 32 feet or more on 80 percent of its perimeter, the smallest open space being 32 feet. Table 506.3.3 gives an If of:
10. A one-story, nonsprinklered Group B building of Type IIB construction has a frontage factor If of 0.25. Using Table 506.2 (NS 23,000), the allowable area under Equation 5-1 is:
11. A one-story, sprinklered Group B building of Type IIB has NS 23,000, S1 92,000, and If of 0.25. The allowable area under Equation 5-1 is:
12. An inspector reviews a Group B room of 6,000 square feet in an unsprinklered Type IIB building. What is the maximum aggregate mezzanine area in that room?
13. A mezzanine that complies with Section 505.2 is treated as:
14. An inspector finds a lower level with its finished floor below grade plane, but the floor next above is 8 feet above grade plane. Under the definitions, that lower level is: