Sprinkler systems - what needs to be protected. (what is considered a "conealed space")
"Not every space in a structure is required to be protected by sprinklers and NFPA 13 is very clear on why those spaces need to remain unprotected."
Everyone wants to save money, builders and building owners included. One way they can do that is to omit sprinklers from areas where the codes say it's okay to leave them out. But it's important to know precisely what those areas are.
The majority of the spaces from which sprinklers can be omitted from NFPA 13 designs center around the concept of concealed spaces. Concealed spaces are non-occupied spaces that are created by building construction. These spaces may contain piping and wiring for various building systems, or, in many cases, may be void of any and all combustible material.
Noncombustible concealed space without sprinklers.
Where combustible concealed spaces are formed, there is a greater concern for fire growth in the cavity. There are, however, certain combustible concealed spaces from which sprinklers can be omitted. Due to the impracticability of installation, sprinklers can be omitted in concealed spaces formed by studs or joists with less than 6 inches (15 centimeters) between the inside edges. Similarly, concealed spaces formed by ceilings that are attached directly to, or within 6 inches (15 centimeters) of, wood joists do not require sprinkler protection.
NFPA #13, 2002 Section 188.8.131.52 says that all concealed spaces of combustible construction shall be protected by sprinklers. Section 184.108.40.206 covers situations where sprinkler protection can be eliminated when it may not be physically practical. (Section 220.127.116.11.6 discusses TJIs or composite wood construction)
18.104.22.168.6 Concealed spaces formed by ceilings attached to composite wood joist construction either directly or onto metal channels not exceeding 1 in. in depth provided the joist channels are fire stopped into volumes each not exceeding 160 ft3 (4.53 m3) using materials equivalent to the web construction and at least 3-1/2 in. of batt insulation is installed at the bottom of the joist channels when the ceiling is attached utilizing metal channels, shall not require sprinkler protection.
In a nut shell if there is 6" or more of horizontal flue space (the distance from the top edge of the bottom board and the bottom edge of the top board) like in an open web wood joist for example, there would be enough flue (air room) for a fire to travel horizontally through the space and sprinklers should be provided.
In Alberta the allowance to not sprinkler the attic and combustible concealed spaces has been rescinded for NFPA #13R (See Alberta Building Code 22.214.171.124.2, 126.96.36.199.10d, 188.8.131.52.10g, 184.108.40.206.10j & 220.127.116.11.11). Those spaces have to be protected by sprinklers as per NFPA #13 standards.
Other NFPA code excerpts of interest
18.104.22.168.3 Concealed spaces formed by studs or joists with less than 6 in. (152 mm) between the inside or near edges of the studs or joists shall not require sprinkler protection. (See Figure 22.214.171.124.5.1.)
126.96.36.199.4 Concealed spaces formed by bar joists with less than 6 in. (152 mm) between the roof or floor deck and ceiling shall not require sprinkler protection.
188.8.131.52.5 Concealed spaces formed by ceilings attached directly to or within 6 in. (152 mm) of wood joist construction shall not require sprinkler protection.
184.108.40.206.7 Concealed spaces entirely filled with non-combustible insulation shall not require sprinkler protection.
220.127.116.11.11 Concealed spaces in which the exposed materials are constructed entirely of fire retardant treated wood as defined by NFPA 703, Standard for Fire Retardant Impregnated Wood and Fire Retardant Coatings for Building Materials, shall not require sprinkler protection.
18.104.22.168.13 Concealed spaces below insulation that is laid directly on top of or within the ceiling joists in an otherwise sprinklered attic shall not require sprinkler protection.
The TLJ Team