Section 602.4.2.2.2 permits unprotected mass timber ceilings in Type IV-B construction to equal no more than 20 percent of the floor area , and the code expressly states that attached beams are included when determining that exposed ceiling area.

The floor area is 900 square feet:

20% × 900 = 180 sq. ft. maximum exposed area .

The exposed 12-inch × 18-inch beam extends across the 30-foot room. Its three exposed surfaces consist of the 12-inch-wide bottom and two 18-inch-deep sides:

30 ft × (1 ft + 1.5 ft + 1.5 ft) = 120 sq. ft.

Because the attached beam counts against the ceiling exposure allowance, only:

180 − 120 = 60 sq. ft.

remains available for the exposed ceiling itself.

The exposed ceiling extends across the 30-foot room width, so:

30 ft × X = 60 sq. ft.

Therefore:

X = 2 ft .

Option A is correct. Ignoring the beam surfaces would overstate the permitted ceiling exposure and would contradict the explicit IBC requirement that attached beams be included in the exposed ceiling calculation.

References/Topics: 2021 IBC Section 602.4.2.2.2, Exception 1.1; Type IV-B exposed ceiling and attached-beam area calculations.

===============