If the distance between the ceiling and the top of a door is greater than 24 inches, NFPA 72 states that you should have two smoke detectors, one on each side of the door.

If the distance between the ceiling and the top of a door is greater than 24 inches, NFPA 72 states that you should have two smoke detectors, one on each side of the door.

• A. 24 inches
• B. 12 inches
• C. 18 inches
• D. 6 inches

Answer: (A)

The rule hinges on a threshold where the doorway area creates a potential dead zone for smoke detection. When the space between the ceiling and the top of the door is more than 24 inches, smoke can rise and stratify near the ceiling and may not be detected quickly by a single detector on one side of the door. Placing one detector on each side of the doorway ensures that smoke entering or moving toward the doorway is detected promptly from either side, providing faster warning. That is why the 24-inch mark is the trigger: taller clearance requires dual detectors on both sides to maintain effective coverage. If the clearance is 24 inches or less, a single detector on the appropriate side is typically sufficient.

The rule hinges on a threshold where the doorway area creates a potential dead zone for smoke detection. When the space between the ceiling and the top of the door is more than 24 inches, smoke can rise and stratify near the ceiling and may not be detected quickly by a single detector on one side of the door. Placing one detector on each side of the doorway ensures that smoke entering or moving toward the doorway is detected promptly from either side, providing faster warning.

That is why the 24-inch mark is the trigger: taller clearance requires dual detectors on both sides to maintain effective coverage. If the clearance is 24 inches or less, a single detector on the appropriate side is typically sufficient.