Area limits outside Group R
IBC 2021 718.3 limits combustible floor/ceiling assemblies in buildings other than Group R to draftstopped areas of 1,000 sq ft (93 m²). IBC 2021 718.4 limits combustible attics to 3,000 sq ft (279 m²). Swapping the floor and attic limits is a listed common error.
Counting areas
Divide the area by the limit and always round up, because no area may exceed the limit.
- Floor, 7,500 sq ft: 7,500 ÷ 1,000 = 7.5, so 8 areas. Rounding down to 7 leaves areas over 1,000 sq ft. Using the attic limit gives 3 or 2, both wrong.
- Attic, 10,000 sq ft: 10,000 ÷ 3,000 = 3.33, so 4 areas. Three leaves an area over 3,000 sq ft; ten applies the floor limit to an attic; five works but is more than the minimum.
Group R-2 attics
Under IBC 2021 708.4.2, in Group R-2 buildings of 4 stories or fewer and not over 60 ft above grade plane, attic draftstops go at areas not exceeding 3,000 sq ft or above every two dwelling units, whichever is smaller. With 900 sq ft units, two units make 1,800 sq ft, which is smaller than 3,000 sq ft, so draftstops go above every two units. Picking 3,000 sq ft takes the larger area, and 1,000 sq ft is the non-Group R floor limit. These attics do require draftstopping.
The sprinkler waiver
Both 718.3 and 718.4 carry the exception for buildings sprinklered throughout per Section 903.3.1.1 (NFPA 13). An NFPA 13R system under 903.3.1.2 does not get the waiver on its own; it needs more conditions.
Draftstopping materials
IBC 2021 718.3.1 minimums include 1/2 in gypsum board, 3/8 in wood structural panel, 3/8 in particleboard and 1 in nominal lumber. A 3/8 in panel meets draftstopping but not fireblocking, which needs 0.719 in (23/32 in) panels with backed joints. Gypsum board at 1/2 in is on both lists, and 2 in nominal lumber and backed 23/32 in panels serve both.