Sprinkler Obstructions and Discharge Patterns

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Important to fire sprinkler effectiveness is sprinkler discharge pattern development. A fire sprinkler system should be designed in a manner that provides full water coverage from the fire sprinkler system. Inherent building construction or design elements can pose potential obstructions to the full coverage or discharge pattern of the sprinkler system.  However, these buildings and elements must still be protected. NFPA 13, Standard for the Installation of Sprinkler Systems, provides sprinkler coverage guidance for these obstructed areas.

To determine the proper application it must first be determined if the construction is obstructed or unobstructed.  Then it must be determined if the obstruction is continuous or noncontinuous.

Is this obstructed or unobstructed construction?

To be considered obstructed, the construction elements must be composed of “beams, trusses, or other members” that will impede heat flow, water distribution, or otherwise affect the ability of the sprinkler to control or suppress the fire. [13:3.7.1]

Construction types that may create obstructions, include:
  • Beam and girder
  • Concrete Tee
  • Composite wood joist
  • Panel construction
  • Bar joist with fireproofing
  • Steel purlin

Unobstructed construction includes any “beams, trusses, or other members” that do not impede heat flow, water distribution or the ability of the fire sprinkler to control or suppress fire. This generally consists of construction members that are not solid.[13:3.7.2]

Unobstructed construction elements may include:
  • Bar joist
  • Open-grid ceilings
  • Smooth ceilings
  • Standard mill (heavy timber)

Is the obstruction continuous or noncontinuous?

If the obstruction affects the sprinkler discharge pattern of two or more adjacent sprinklers then the obstruction is considered to be continuous. However, if only one sprinkler discharge pattern is affected then the obstruction is noncontinuous.

Different sprinkler head types and orientations have different requirements for designing to protect obstructed areas.  These are addressed in the following sections of NFPA 13.
  • General guidance: 13:8.5.5
  • Pendant and upright sprinklers: 13:8.6.5
  • Sidewall sprinklers: 13:8.7.5
  • ESFR sprinklers: 13:8.12.5

A great reference tool for quickly determining the design criteria and requirements for obstructed areas is this ceiling obstructions reference tool/calculator provided by Meyer Fire.