System Layout and Design
Detection and notification are separate layouts
For smooth ceilings, spot smoke detector spacing is 30 ft between detectors, 21 ft maximum from walls, and 900 sq ft maximum per detector. Heat detectors use 50 ft, 25 ft, and 2,500 sq ft. These values apply up to a 10 ft ceiling; above that, apply the NFPA 72 spacing tables and reduce coverage for high ceilings and for joists or beams. Duct detectors serve unit shutdown and annunciation, not general area detection. Beam and air-sampling detectors use manufacturer-specific coverage.
Notification: public-mode audibility is 15 dBA above average ambient or 5 dBA above maximum ambient, whichever is greater, measured 5 ft above the floor. Sleeping areas require 75 dBA at the pillow or 15 dBA above ambient. Visible appliances are selected by candela and room size using the NFPA 72 tables; higher mounting or larger rooms require more candela. Corridors, stairwells, elevator lobbies, and high-noise areas need special attention. Elevator recall, stairwell communication, and door release are frequent sequence-of-operations questions.
Traps: confusing "spacing" with "distance to wall"; using 30 ft as the wall distance; forgetting ceiling-height adjustments; treating 15 dBA as an absolute sound level; audible-only notification in sleeping areas; and omitting visible notification where accessibility rules require it.
Worked example: a 60 ft x 30 ft smooth-ceiling room is 1,800 sq ft. At 900 sq ft per detector, two smoke detectors are the minimum. Place them on the long axis 15 ft from each end wall, centered 15 ft from the side walls (all within 21 ft), and 30 ft apart. Rule of thumb: lay out the spacing grid first, then verify wall distance, ceiling height, and airflow.