sprinklernotes Design Areas and the 30% Increase Requirements
You asked the following questions: “We have a space similar to the space described in section 8.15.1.2.16 and A.8.15.1.2.16, and the space above the insulation is an unheated unoccupied wood truss space with a slope greater than 2/12. Using the Density/Area Method the resulting Density/Area would begin with 0.1 at 1,500 per 11.2.3.1.1. The design area would be increased by 30 percent per 11.2.3.2.4 and 11.2.3.2.5 resulting in a Density/Area of 0.1 at 2,535. And lastly section 11.2.3.2.7.2 would be applied and the final resulting Density/Area will be 0.1 at 3,000. Is this the correct process to developing the Density/Area for the space above the insulation?” In response to your question, we have reviewed NFPA 13, 2007 and 2016 editions as the applicable standard. Our informal interpretation is you have asked multiple questions which are answered below. To answer your first question, you would have to apply Section 11.2.3.2.4 which tells us for roof slopes that exceed a 2 in 12 pitch you would have to increase your design area by 30 percent from 1500 sq. ft. to 1950 sq. ft. The reasoning for the design area increase for sloped ceilings is that the heat from a fire is likely to activate sprinklers outside the fire area as the heat travels to the high point on the roof which could overwhelm the sprinkler system. This was proven in fire tests conducted by FM Global which was the justification for Section 11.2.3.2.4 to be added to the standard by the technical committee. Additionally, since you stated the area is unheated Section 11.2.3.2.5 applies which is another design area size increase for using dry systems resulting in an area of 2,535 sq. ft. Again, this design area increase is due to the potential for the fire to increase in size due to the delay in water being discharged from the sprinklers that have activated. The final question is does Section 11.2.3.2.7.2 tells us to now apply the 3,000 sq. ft.- minimum size due to the area below the insulation not being protected? This is indeed the correct process when an attic-type space has an unprotected concealed space adjacent to it. The primary question though, is the area beneath the insulation considered a combustible concealed space? If you go back to the original proposal that created this criterion (ROP A2006: proposal 13-288), it states the only combustibles in the space are the bottom of the chords and the insulation facing. Unfortunately noting in the text states this except for the restriction on the insulation facing to a heat content of no more than 1,000 btu/sq. ft. This is an existing allowance where a concealed space is treated as noncombustible even when there is combustible insulation facing with this maximum heat content. So, as long as the area beneath the insulation is otherwise noncombustible, sprinklers can be omitted without triggering the 3,000 sq. ft. remote area requirement by Section 11.2.3.1.4.
