Twenty-Inch Bays, Eight Detectors, One Diffuser
A tenant improvement converts 6,000 sq ft of a single-story office building to open-plan workspace. The ceiling is the exposed roof deck at 12 ft. Steel beams 24 in deep run both directions on 20-ft centers, forming 20 × 20 ft bays. The contractor submits eight spot-type photoelectric smoke detectors laid out 30 ft on center on a smooth-ceiling grid, citing 900 sq ft per detector. On the walk you also find one detector mounted 8 in from the edge of a 4-ft linear supply diffuser fed by a roof-mounted unit, and another mounted 3 in from a partition that terminates 6 in below the deck. The building is fully sprinklered, the panel is addressable, and the engineer of record has not yet reviewed the layout.
1. Is the 30-ft smooth-ceiling layout acceptable in this space? Determine what the layout should be.
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No. Beams projecting more than about 18 in below the deck divide the ceiling into separate areas: a detector cannot be credited for a bay it cannot see over, and the 900 sq ft allowance is applied within a bay, not across a 24-in obstruction. (Confirm the 18-in threshold against the edition adopted in your jurisdiction before citing it in a report.)
Each bay here is 20 × 20 = 400 sq ft, well inside what one detector covers, so one detector centered in each bay is sufficient - but every bay needs one. The floor is roughly 6,000 ÷ 400 ≈ 15 bays, so the layout needs about 15 detectors, not 8. The submitted grid leaves whole bays with no coverage at the floor.
Location within each bay also matters: keep detectors out of the dead air at beam faces and roughly centered in the pocket, not perched on a beam line.
2. Evaluate the two placement defects you found during the walk.
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8 in from the supply diffuser. Supply air washes across the detector, diluting smoke and delaying response - sometimes preventing it entirely at low smoke levels. Detectors should be kept at least 3 ft (36 in) from supply outlets; return openings follow a different rule. Relocate the detector out of the diffuser's throw.
3 in from the partition. The wall/ceiling juncture is dead air; smoke does not readily enter the narrow band at the perimeter. A detector must be at least 4 in from the sidewall, and 3 in is inside the dead zone.
Note also that a partition stopping 6 in below the deck is not a smoke barrier - it does not create a separate compartment - but it does disturb airflow locally, so the final location must satisfy the diffuser separation as well.
Both defects typically pass a 'we sprayed it and it alarmed' test and still fail in a real fire, so they belong on the punch list, not in the record of completion.
3. What downstream checks must be completed before the revised layout is issued?
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- SLC loading and addressing. Adding roughly seven devices changes the point count; confirm the loop is within its rated device count and its power/voltage budget.
- Standby current and battery calculation. Every added detector raises standby draw; re-run the battery sizing, especially if it was already marginal.
- Panel and annunciator capacity. Point count against panel capacity, plus device labels and zone/floor assignments that responders will read.
- Plan review. A change of this size is a design change - the engineer of record, and in most jurisdictions the AHJ, should see the revised drawing before installation.
- As-builts and record of completion updated to reflect the installed layout, with the coverage rationale documented so the next inspector does not repeat the analysis.