Existing Hotel Corridor: Smoke Detector Spacing After a Remodel
A 1970s three-story hotel is being renovated. Corridors are 72 in (1.83 m) wide with a 9 ft (2.74 m) ceiling. The owner adds a 30 in (0.76 m) deep decorative soffit along one corridor wall for the full 120 ft (36.6 m) length; the soffit projects more than 10 percent of the corridor width. Existing corridor smoke detectors are spaced at 42 ft (12.8 m) on center, centered in the corridor. The contractor proposes to keep the layout because the detector spacing is under the 30 ft (9.1 m) smooth-ceiling limit used for 9 ft ceilings. The AHJ notes the corridor is a required means of egress and asks for a technical basis before accepting the plan.
1. Evaluate whether the proposed 42 ft spacing can be accepted in the corridor as modified.
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No. Two separate issues defeat the proposal.
First, the 30 ft spacing the contractor cites is a smooth-ceiling area-coverage rule, not a corridor rule. In corridors, detectors are normally placed along the centerline and spaced so that no point exceeds the listed spacing, with the first detector within half the listed spacing of the end wall. A 42 ft on-center layout already exceeds 30 ft regardless of the soffit.
Second, the soffit changes the ceiling geometry. A projection deeper than 10 percent of the corridor width (10 percent of 72 in = 7.2 in; the soffit is 30 in) creates a beam-like obstruction that divides the ceiling into separate airflow/ceiling pockets. The detector must be evaluated for the pocket it serves, and spacing is measured within that pocket, not across the full corridor width. The soffit also creates a dead-air zone on the wall side.
The contractor must submit a revised layout using the manufacturer's listed spacing, treating the soffit as a ceiling obstruction, and demonstrate coverage of the pocket adjacent to the soffit. The AHJ should require the revision before acceptance.
2. What documentation should the contractor provide so the AHJ can close the review?
Read the worked answer
At minimum: (1) a reflected ceiling plan showing the soffit depth, corridor width, detector locations, and dimensions from walls and from the soffit; (2) the manufacturer's published spacing and installation instructions, including any beam/obstruction guidance; (3) a coverage demonstration (dimensioned spacing checks or a smoke travel/coverage analysis) showing no point in the corridor exceeds the listed spacing; (4) the design basis citing the applicable code edition and the corridor provisions; and (5) a written statement of the design method used. Without these, the AHJ has no basis to accept the deviation, and the record should show the plan was returned for correction.