Height and stories under IBC Section 202
Building height is measured from grade plane to the average height of the highest roof surface. Parapets are excluded.
- Measuring to the top of the parapet is a listed common error.
- Measuring to the ridge overstates it, and starting at the lowest finished ground level uses the wrong datum.
A story above grade plane is one whose finished floor is entirely above grade plane, or where the finished floor next above is:
- more than 6 ft above grade plane, or
- more than 12 ft above finished ground level at any point.
A basement whose floor next above sits 7 ft above grade plane at every point is a story above grade plane, because 7 exceeds 6. The 12 ft test is an alternative trigger, not a required minimum. Always run the 6 ft and 12 ft tests; never judge by the basement's own floor or assume every basement counts.
Frontage increase, Section 506.3
Equation 5-5 (Section 506.3.3):
If = [F/P − 0.25] × W/30
- F: perimeter fronting a public way or open space; P: total perimeter.
- W: open space width, at least 20 ft and capped at 30 ft.
Example: P = 400, F = 200, W = 30. If = (0.50 − 0.25) × 1 = 0.25. The traps: 0.50 forgets to subtract 0.25, and 0.75 adds it.
With F = 300 and W = 45 ft, W is capped, so W/30 = 1 and If = 0.75 − 0.25 = 0.50. Using the full 45 ft gives 0.75, the listed error. A width over 30 ft does not disqualify the increase; it is simply capped.
Sprinkler increases: 13 versus 13R
- The Table 506.2 sprinkler area factors require NFPA 13 throughout. Not 13R, 13D or partial sprinklering.
- An R occupancy with NFPA 13R gets the 1-story and 20-ft height increases of Tables 504.3 and 504.4, but cannot exceed 4 stories and 60 ft above grade plane. Its height is never unlimited.