What fire separation distance controls
The closer an exterior wall sits to the lot line, the more the 2018 IRC restricts its fire rating and its openings. There are two tables, and choosing the wrong one is the whole trap.
Non-sprinklered: Table R302.1(1)
- Wall rating: an unrated wall needs at least 5 ft. Closer than 5 ft, the wall must be 1-hour rated.
- Openings under 3 ft: not allowed.
- Openings from 3 ft to less than 5 ft: limited to 25% of the wall area.
- Openings at 5 ft or more: unlimited.
Sprinklered: Table R302.1(2)
For a dwelling with an NFPA 13D or Section P2904 sprinkler system:
- Wall rating: an unrated wall needs at least 3 ft. The table does not permit an unrated wall on the line at 0 ft.
- Openings under 3 ft: not allowed.
- Openings at 3 ft or more: unlimited. There is no 25% band.
Working the opening cap
Opening area is a share of the whole wall. A wall 40 ft long and 9 ft high has 40 × 9 = 360 sq ft. In a non-sprinklered house at 4 ft, 360 × 0.25 = 90 sq ft of openings. The same wall in an NFPA 13D house at 4 ft has unlimited openings. Watch for 180 sq ft, which is 50% and double the cap, and 36 sq ft, which is 10%, a figure neither table uses.
The traps
- The 5 ft unrated-wall threshold is for non-sprinklered houses; sprinklered houses use 3 ft. Applying 5 ft to a sprinklered house is a listed common error.
- The 25% cap exists only in the non-sprinklered table. Carrying it to a sprinklered dwelling is the most common error.
- An unrated wall with a 25% cap mixes the two tables. That combination exists in neither.
Adding NFPA 13D to a house with a wall at 3 ft 6 in changes it from a 1-hour wall with a 25% cap to an unrated wall with unlimited openings.