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In other words, if a diagonal through the center of the room
is no greater than the diameter of the circle, or 42.4 feet
(12.8 meters), one detector can be used under ideal condi-
tions. Figure 21 shows how a length of hallway can be pro-
tected by only two detectors under ideal conditions.
Special Spacing Problems
The ideal conditions upon which code guidelines are based
do not exist in the majority of buildings. Detector installers
usually have to deal with a variety of problems, such as
uneven ceilings or ceilings crossed by beams and joists;
storage racks and partitions that obstruct the path of smoke
toward detectors; air stratification due to uninsulated roofs,
peaked or sloped ceilings, or localized heating or cooling
from heating, ventilating, and air conditioning systems;
and extensive variability in the value and combustion char-
acteristics of building contents. The following are suggest-
ed techniques for dealing with some of the special detector
spacing problems:
• Solid joist and beam construction. Per NFPA 72-1996,
solid joists are to be considered equivalent to beams
for smoke detector spacing guidelines. For ceiling
heights of 12 ft. (3.66 m) or lower and beam depths of
1 ft. (0.3 m) or less, smooth ceiling spacing running in
the direction parallel to the run of the beams is to be
used, and 1/2 the smooth ceiling spacing is to be used
in the direction perpendicular to the run of the beams.
Spot-type detectors may be located either on the ceil-
ing or on the bottom of the beams. For beam depths
exceeding 1 ft. (0.3 m) or for ceiling heights exceeding
12 ft. (3.66 m), detectors are to be located on the ceil-
ing in every beam pocket. If the beamed ceiling is also
sloped, use the spacing determined for flat beamed
ceilings. Use the average height over the slope as the
ceiling height in such cases. Note that, by definition in
NFPA 72, ceilings are to be considered smooth unless
the beams or joists are more than 4 in. (0.1 m) in
depth.
• High storage racks. Multi-level storage racks present
special problems for early fire detection. Developing
fires, especially smoldering fires, on the lower levels of
the racks may not be sensed rapidly by ceiling mount-
ed detectors. Upward convection of smoke can be
slowed or blocked by goods stored on the upper levels
of the racks. Multi-level fire detection is required.
Detectors should be installed on the ceiling above each
aisle and on intermediate levels of the racks adjacent
to alternate pallet sections, shown in NFPA 72-1999,
A-2-3.6.5. A consultant’s judgement may be required
for specific installations.
•Partitions. Partitions and many types of large, tall
equipment standing on the floor can block the flow of
smoke toward detectors. Any partition or similar
obstruction that is less than 18 inches (45 cm) from
the ceiling should be treated as a side wall dividing the
area protected.
• Air Stratification. Air stratification in a room may keep
air containing smoke from reaching ceiling-mounted
detectors. Three conditions are known to accentuate
air stratification: when a layer of hot air exists under a
poorly insulated roof heated by the sun, cooler air will
stratify the hot air layer at the ceiling; when a layer of
cold air exists under a poorly insulated roof cooled
from the outside by cold air, the heated air is cooled as
it reaches the cold air layer; or when a heating, venti-
lating, or air conditioning (HVAC) system creates arti-
ficial hot or cold air layers in a room, the layers may
affect the flow of smoke to the detectors.
• Uninsulated Roofs. Uninsulated roofs present special
placement problems. Air movement toward ceiling
detectors is not impeded when the outside tempera-
ture is cool, but stratification can occur when outside
temperature is warm or hot, or when the roof is heat-
ed by the sun on bright, sunny days. Although true
thermal barriers are not present in many installations,
smoke tests should be run in factories or warehouses
with metal roofs on warm sunny days to determine
whether such a thermal barrier exists.
•Peaked or Sloped Ceilings. Peaked or sloped ceilings
can foster air stratification. Codes may specify spacing
detectors by using horizontal spacing from the peak of
the roof or ceiling. For instance, if the specified dis-
tance from the peak is 3 feet (1 m), the dis-
tance is measured on the base of the
right triangle formed by a vertical line
dropped from the peak of the roof,
with the roof incline as the
hypotenuse. Additional detec-
tors are installed on the
selected spacing, using
the horizontal distance,
not the distance
along the incline of
the ceiling. (See
Figure 22.)
Rectangles
A 10 ft. × 41 ft. = 410 sq. ft.
B 15 ft. × 39 ft. = 585 sq. ft.
C 20 ft. × 37 ft. = 740 sq. ft.
D 25 ft. × 34 ft. = 850 sq. ft.
E 30 ft. × 30 ft. = 900 sq. ft.
21 ft.
21 ft.
21 ft.
21 ft.
41 ft.20.5 ft. 20.5 ft.
82 ft.
10 ft.
Figure 20: Detector Coverage Patterns
Figure 21: Detector Placement in Hallways
Detector may be placed
anywhere in shaded area.
S = Detector Spacing
S/2 S S
3 ft. 3 ft.
S/2
Figure 22: Detector Spacing Layout – Sloped Ceilings (peaked type)
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