2021 IBC Chapter 6
Construction type is the code's shorthand for what a building is made of and how long its structure must resist fire. Chapter 6 sorts every building into one of five types, which Table 601 divides into twelve columns, and that one label then drives three things you will use all through this course: the hour rating each element needs (Table 601 and Table 705.5), the height and area the building may reach (Lesson 5), and how much protection an exterior wall needs near a lot line. This lesson teaches the types, the ratings, and exterior walls by fire separation distance. Local adoption governs which edition applies, so confirm the adopted code before relying on any number here.
The 2021 table has twelve columns: IA, IB, IIA, IIB, IIIA, IIIB, IV-A, IV-B, IV-C, IV-HT, VA and VB. The Roman numeral tells you the material story. The letter, in most types, tells you how much fire resistance the structure carries: the A columns carry hour ratings and the B columns carry mostly zeros. Types IIB and VB are 0 hours for every structural element in Table 601, while Type IIIB still needs 2-hour exterior bearing walls. Section 602.1.1 adds a useful rule: a building does not have to meet a higher type than the one it qualifies for, so once the height and area limits are met, a stronger type is not demanded.
Types I and II use noncombustible materials (602.2), apart from the limited exceptions in Section 603. Type III (602.3) has noncombustible exterior walls with interior elements of any permitted material, and fire-retardant-treated wood (FRTW) may be used in exterior walls rated 2 hours or less. Type IV (602.4) is mass timber or noncombustible construction, covered below. Type V (602.5) allows any material the code permits. In small commercial work, a metal building is usually Type IIB, an older brick building with wood floors and roof is often Type IIIB, and a wood-framed office is Type V. Confirm rather than assume: the certificate of occupancy has to state the type of construction (111.2).
Table 601 gives hours for five element groups. Here it is rebuilt for the eight types you will meet most often.
| Element (hours) | IA | IB | IIA | IIB | IIIA | IIIB | VA | VB |
|---|---|---|---|---|---|---|---|---|
| Primary structural frame | 3 | 2 | 1 | 0 | 1 | 0 | 1 | 0 |
| Exterior bearing walls | 3 | 2 | 1 | 0 | 2 | 2 | 1 | 0 |
| Interior bearing walls | 3 | 2 | 1 | 0 | 1 | 0 | 1 | 0 |
| Floor construction | 2 | 2 | 1 | 0 | 1 | 0 | 1 | 0 |
| Roof construction | 1-1/2 | 1 | 1 | 0 | 1 | 0 | 1 | 0 |
Exterior nonbearing walls are not in the body of the table: it sends you to Table 705.5. Interior nonbearing partitions are 0 hours in every type except IV-HT, unless another section requires more (footnote d), as it does for corridors, shafts and dwelling unit separations. Three footnotes matter most. Where the frame or interior bearing walls support only a roof, Types IA and IB may reduce the rating by 1 hour (footnote a). Except in Groups F-1, H, M and S-1, roof members need no fire protection when every part of the roof construction is 20 feet or more above the floor below (footnote b). Heavy timber roof construction is allowed in any occupancy where 1 hour or less is required (footnote c).
Fire separation distance (FSD) is measured at right angles from the building face to the closest interior lot line, to the centerline of a street, alley or public way, or to an imaginary line between two buildings on the same lot. Notice that a street is measured to its centerline here, while the frontage rule in Lesson 5 measures the full width of the public way. Section 602.1 sends exterior walls to Table 705.5, which sets the hours by FSD, occupancy group and type. The three occupancy columns are shown below in hours.
| FSD | Construction type | Group H | F-1, M, S-1 | A, B, E, F-2, I, R, S-2, U |
|---|---|---|---|---|
| Under 5 ft | All types | 3 | 2 | 1 |
| 5 to under 10 ft | IA, IV-A | 3 | 2 | 1 |
| 5 to under 10 ft | All other types | 2 | 1 | 1 |
| 10 to under 30 ft | IA, IB, IV-A, IV-B | 2 | 1 | 1 |
| 10 to under 30 ft | IIB, VB | 1 | 0 | 0 |
| 10 to under 30 ft | All other types | 1 | 1 | 1 |
| 30 ft or more | All types | 0 | 0 | 0 |
A bearing wall must also meet Table 601, so use the higher of the two answers. Some footnotes remove the rating altogether: open parking garages under Section 406, a building that is only a private garage or carport at 5 feet or more, and an R-3 building of Type IIB or VB at 5 feet or more. Openings are covered in Lesson 6.
The 2021 code added three mass timber types and renamed the old Type IV as IV-HT (heavy timber). Type IV-A is fully protected: the exterior face of mass timber walls carries at least 40 minutes of noncombustible protection, and the interior faces of all mass timber carry protection that contributes at least 80 minutes. Type IV-B allows limited exposure: unprotected mass timber ceilings up to 20 percent of the floor area of a dwelling unit or fire area, walls up to 40 percent, or a combination by Equation 6-1, while beams and columns that are not part of walls or ceilings may stay exposed without limit. Type IV-C allows mass timber to be exposed, with the exterior side of combustible walls and the concealed spaces and shafts protected for 40 minutes. Type IV-HT keeps noncombustible exterior walls and heavy timber interiors under 2304.11.
In Table 601 the three new types read alike at the low end: frame 3, 2 and 2 hours, floors 2 hours, and roofs 1-1/2, 1 and 1 hour for IV-A, IV-B and IV-C. The wood cover over connections is inspected before other coverings hide it (110.3.5, from Lesson 1).
A one-story Group M store of Type IIB construction has a side wall 8 feet from the lot line. Table 705.5, 5 to under 10 feet, all other types, F-1/M/S-1 column: 1 hour. Move the building back to 12 feet and the wall falls into the 10 to under 30 foot band, where Types IIB and VB read 0 hours in that column. Four feet of setback removed a full hour of wall rating. Now keep the 12 feet but make the building Type IIIA. The other-types row gives 1 hour, but a Type IIIA exterior bearing wall must also meet Table 601, which says 2 hours, so the wall is 2 hours. The rule is always the higher of the two.
In the field
A client wants a vacant one-story building, brick walls with a wood roof structure, converted to a community office. Before anyone prices a rated wall, read the type from the original certificate of occupancy or plan sheet, then check the real building against Table 601. If it is Type IIIB, the exterior bearing walls need 2 hours, the wood roof and floors need none, and any wall within 30 feet of a lot line is also checked against Table 705.5. Mistaking it for Type VB would miss the 2-hour bearing wall. Conversions run through the existing-building code (Lesson 1), so also ask the building official which path applies.
1. Under the 2021 IBC, the fire-resistance rating of an exterior nonbearing wall is set by fire separation distance in:
2. Table 601 requires the primary structural frame of a Type IIA building to have a fire-resistance rating of:
3. Table 601 requires the floor construction of a Type VA building to have a rating of:
4. An inspector is reviewing a one-story Group B building of Type IIB construction. The nonbearing side wall is 14 feet from the lot line. Table 705.5 requires that wall to be rated:
5. An inspector is checking a Group M building of Type IIIA construction with a nonbearing exterior wall 8 feet from the lot line. Table 705.5 requires that wall to be rated:
6. Fire separation distance is measured at right angles from the building face to:
7. Under Table 705.5, a nonbearing exterior wall 30 feet or more from the lot line, in any construction type and any occupancy group, needs a rating of:
8. Which statement about Type I and Type II construction is correct under Section 602?
9. Which pair of types has 0-hour ratings for the frame, bearing walls, floors and roof in Table 601?
10. An inspector is reviewing a Type IIIB building with masonry exterior bearing walls and wood floor and roof framing. Which element does Table 601 require to be rated 2 hours?
11. Table 601 gives the roof construction of a Type IA building a rating of:
12. Which 2021 construction type allows mass timber to be left exposed, with the exterior side of combustible walls and the concealed spaces protected for 40 minutes?
13. The 2021 IBC renamed the former Type IV heavy timber construction as:
14. An inspector is reviewing a Group B building of Type IIA whose roof construction is entirely 22 feet above the floor immediately below. Under Table 601 footnote b, fire protection of the roof members is: