Wood Truss Construction with Less than 6-in Gap Between Top of Bottom Chord and Bottom of Top Chord

[Haldorson]

[NFPA 13 (2013)] A combustible concealed spaced is created by fastening drywall tight to the bottom of a wood truss system. The wood truss has less than a 6-inch gap between the top of bottom chord and bottom of top chord.

Are sprinklers required within this combustible concealed space, or does 8.15.1.2.3 and the 6-inch limit apply to both 'double joist" construction and wood truss construction?

If sprinklers are required, I note the combustible concealed space sprinklers - ie Tyco Model CC3 or Reliable KFR-CCS 56 - have a minimum gap between the top of the bottom chord and bottom of top chord of 6-inches. These sprinklers are not listed for this case where it is less than 6-inches, so are we limited to standard coverage sprinklers in every channel?

 

[SprinklerDesigner2]

The way you describe it sprinklers are not required.

From the 2019 Edition of NFPA 13:

9.2.1* Concealed Spaces Not Requiring Sprinkler Protection.

A.9.2.1 Paragraphs 9.2.1.3, 9.2.1.4, and 9.2.1.5 do not require sprinkler protection because it is not physically practical to install sprinklers in the types of concealed spaces discussed in these three exceptions. To reduce the possibility of uncontrolled fire spread, consideration should be given in these unsprinklered concealed space situations to using 9.2.1.7, 9.2.1.11, and 9.2.1.13. Omitting sprinklers from combustible concealed spaces will require further evaluation of the sprinkler system design area in accordance with 19.3.3.1.4.

9.2.1.3 Concealed spaces formed by studs or joists with less than 6 in. (150 mm) between the inside or near edges of the studs or joists shall not require sprinkler protection. (See Figure 10.2.6.1.5.1.)

This has always been in NFPA #13 at least back to 1975.

 

[Haldorson]

But doesn't 9.2.1.3 only apply to wood joist construction and not trusses? Also, Figure 10.2.6.1.5.1 is not a concealed space.

 

[TravisMack]

I agree with SD2. Sprinklers don't appear to be required. However, I would look and confirm to see if you are going to trigger the 3000 sq ft area.

Travis Mack, SET, CWBSP, RME-G, CFPS
MFP Design, LLC

http://www.mfpdesign.com