sprinklernotes
Unsprinklered Combustible Concealed Spaces and the 3000 sq. ft. design area
increase
You have asked the following questions: “We have a flat roof framed with solid
wood joists filled with insulation and with a gypsum board attached to the
joist. Directly above the joists, we have crickets formed by solid wood members
and divided into 160 cu. ft. spaces by the solid wood members. The cricket area
is separated from the joist below with a solid layer of plywood. Above the
cricket is another layer plywood with a layer of rigid insulation and
thermoplastic polyolefin (TPO) roofing membrane on top. Sprinklers are not
required in the crickets per Section 8.15.1.2.5, however, the AHJ is
questioning whether the design area for the sprinklered space below needs to be
increased 3,000 sq. ft. for the hydraulic calculations per Section
11.2.3.1.5.2(4). In our situation, we have fully insulated areas below and
above the cricket bays. Would that meet the intent of the exception for the
3,000 sq. ft. calculation?” In response to your question, we have reviewed NFPA
13, 2016 edition as the applicable standard. Our informal interpretation is
would not require the 3,000 sq. ft. design area increase.
Even though these crickets do not exactly meet one of the 10 unsprinklered
combustible concealed spaces that do not require an increase to the design area
listed in Section 11.2.3.1.5.2, they do meet the intent of the section. The
intent of Section 11.2.3.1.5.1 is to prevent a fire that could start in the
unprotected space from breaking-out in multiple locations in the protected
area. This could cause the sprinkler system to be overwhelmed and the fire
would not be controlled (the purpose of the sprinkler system is to control the
fire.) The intent of increasing the design area, thus the available water to
the sprinkler system, is to still be able to control the fire if this were to
happen. The 10 unsprinklered combustible concealed spaces provided in Section
11.2.3.1.5.2 are situations where the fire propagating around the 1,500 sq. ft.
design area is extremely unlikely due to the lack of fuel load to allow the
fire to propagate, low probability of ignition, and/or low probability of the
fire breaking out of the concealed space. In the case of exception number 4, a
limited- or non-combustible ceiling is required to reduce the potential for the
fire to readily break-out of the concealed space. This particular exception
aligns very well with your installation though it does require one to look at
the entire assembly and not just the membrane immediately bounding the bottom
of the concealed. In a typical case it is only a layer of gypsum separating the
concealed space from the protected area beneath it. In this case, there is a
layer of gypsum with additional layers of insulation and plywood. This makes it
even less likely for the fire to break-out of the concealed space, which is the
intent of Section 11.2.3.1.5.2. With this being the case, the cricket assembly
presents an equivalent installation to the defined exception number 4.
Equivalencies are allowed per Section 1.5 which states: Nothing in this
standard is intended to prevent the use of systems, methods, or devices of
equivalent or superior quality, strength, fire resistance, effectiveness,
durability, and safety over those prescribed by this standard.
However, this should be discussed with the local AHJ and a written agreement
with this interpretation should be obtained (or at least an email sent to the
AHJ identifying the outcome of the discussion) before proceeding further with
the project.
Another way to view this installation is having the ceiling defined by the top
of the occupied protected area and not the immediate bottom of the concealed
space which would avoid the need for an evaluation of intent of Section
11.2.3.1.5.2. As such, it then immediate complies with the requirement for a
noncombustible material. This could have been simply stated as the
interpretation but a discussion on the intent seemed a much better approach.
