The 18-inch applies only to areas that have sprinklers installed. Picture a horizontal plane parallel to the ceiling that is 18 inches below the sprinkler heads. Nothing should be in that area between the bottom of the sprinkler heads and the imaginary horizontal plane parallel to the ceiling that is 18 inches below.
A common rule that is followed for obstructions within 18 inches of the sprinkler deflector is the “three times rule”. This requires sprinklers to be positioned away from obstructions a minimum or three times the maximum dimension of the obstruction.
The minimum vertical clearance between sprinklers and material below shall be 18 inches (45.7 cm). Hydraulically designed systems.
1 of NFPA 13, which states that the distance from the sprinkler to the wall can be no more than one-half the allowable distance between sprinklers. The max distance between standard spray sprinklers in light hazard settings is 15 ft. (4.6 m) (with exceptions for light-hazard, combustible concealed spaces).
Current fire safety codes require sprinkler heads always to be a minimum of 6 feet apart from one another to avoid cold soldering.
1.3 lays out the “Three Times Rule” which states that unless specific requirements are met that “sprinklers shall be positioned away from obstructions a minimum distance of three times the maximum dimensions of the obstruction”, up to a maximum of 24 inches (0.6096 m) though the “maximum clear distance does not apply ...
Meaning that spray heads should never be placed further than 15 ft. apart as this would create dry spots (i.e. dead grass) in your lawn. It is always important to install sprinklers with overlap for full coverage. Spray heads have a higher application rate than other sprinkler heads.
In a nutshell, NFPA 25: Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems says that objects that can block a sprinkler's spray must be at least 18 inches away from the deflector.
At different pressures, the sprinkler head and nozzle will consume different amounts of water. For example, at 35 pounds per square inch (PSI) the 5000 Series Rotor using the 3.0 nozzle will use 3.11 gallons per minute (GPM). If your home's water capacity was 10 GPM, you could place 3 heads per zone.
The purpose of the “18-inch rule” is to prevent storage or any other obstruction from interfering with the spray of water from a sprinkler head during a fire. It is intended to prevent a situation in which the obstruction is within 18-inches of the ceiling and might be between the sprinkler head and the fire.
Under unobstructed construction, the distance between the sprinkler deflector and the ceiling shall be a minimum of 1 in. (25 mm) and a maximum of 12 in. (300 mm) throughout the area of coverage of the sprinkler.
OSHA does not impose penalties or require abatement of de minimis violations. Question 1: 29 CFR 1910.159(c)(10) in part states, "The minimum vertical clearance between sprinklers and material below shall be 18-inches (45.7 cm)." Does this apply only to materials placed directly below the sprinkler heads?
Running multiple zones at once is usually not possible due water-pressure and flow limitations at residential properties.
Check for any leaks in the water line. — A leak in the water line can cause low water pressure and keep water from flowing to the sprinkler head. If water doesn't reach the sprinkler head, then it won't pop-up out of the ground.
Debris on the surface/trapped in the nozzle – plant matter or debris that piles on top of the sprinkler may find its way into the nozzle or into the spray head. This may be due to issues like frequent lawn mowing which leaves behind small blades of grass, or mulch that moves or gets piled over the sprinkler.
If you notice one sprinkler head that doesn't seem to be going as far as the others, you may have a clogged line. If the entire system is falling short of what you expected, however, you may have an issue with the backflow preventer or simply low water pressure.
Calcium carbonate obstructions are most common in sprinkler systems supplied from non-potable wells or surface water in places where water supplies are known to be hard. These blockages can't be detected or corrected by normal flushing operations.
Do not install cable trays, heating ducts or other large obstructions closer than 3 times its width, to a sprinkler head. (i.e. if the duct is 2 ft (0.6 m) wide, then it should be 6ft (1.8 m) from the sprinkler head.)
A minimum 3-foot clearance in front of the entire width of the fire sprinkler equipment and 1-foot clearance on the remaining 3 sides shall be provided.
The sprinkler pipe size from the water supply source to a sprinkler shall be not less than 3/4 of an inch (20 mm) in diameter.
The narrower obstruction allows the sprinkler to be further above the bottom than allowed by the beam rule. The three times rule for standard pendent and upright sprinklers requires that objects be located three times the width of the obstruction from the sprinkler up to a maximum of 24 in.