We had a fire similar to this one in my neck of the woods. [firefightingnews.com]

The fire here destroyed the whole building after smoldering for 10 hours in the ceiling joists. When someone finally did smell smoke and called the fire department it was too late. Just as the fire fighters entered the building it reached flash-over, severly injuring all seven(?) of them.

Historic buildings are tricky because all of them have combustible concealed spaces. Would sprinklers have stopped that fire from smoldering in those ceiling joists? Probably not. But there is a good chance the fire wouldn’t have reached flashover. Also keep in mind that NFPA 13 has specific requirements for the protection of buildings with unprotected combustible concealed spaces.

From NFPA 13 section 11.2.3.1.8;

(3)* For buildings having unsprinklered combustible concealed spaces, as described in 8.14.1.2 and 8.14.6, the minimum area of sprinkler operation shall be 3000 ft2 (279 m2).

(4) The following unsprinklered combustible concealed spaces shall not require a minimum area of sprinkler operation of 3000 ft2 (279 m2):

(a) Combustible concealed spaces filled entirely with noncombustible insulation.

(b)* Light or ordinary hazard occupancies where noncombustible or limited combustible ceilings are directly attached to the bottom of solid wood joists so as to create enclosed joist spaces 160 ft3 (4.5 m3) or less in volume, including space below insulation that is laid directly on top or within the ceiling joists in an otherwise sprinklered attic.

(c)* Concealed spaces where the exposed surfaces have a flame spread rating of 25 or less and the materials have been demonstrated to not propagate fire in the form in which they are installed in the space.

(d) Concealed spaces over isolated small rooms not exceeding 55 ft2 (5.1 m2) in area.

(e) Vertical pipe chases under 10 ft2 (0.93 m2), provided that in multifloor buildings the chases are firestopped at each floor using materials equivalent to the floor construction. Such pipe chases shall contain no sources of ignition, piping shall be noncombustible, and pipe penetrations at each floor shall be properly sealed.

“Minimum area of sprinkler operation” means that in hydraulic calculations for the sprinkler system you must make sure your available water supply will support the demand of sprinklers working in unison over 3000 square feet. That is double the area of what is usually required in light and ordinary hazard occupancies.

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But it is a whole lot worse then that.

From NFPA #13

11.2.3.2.3.1 Where listed quick-response sprinklers, including extended coverage quick-response sprinklers, are used throughout a system or portion of a system having the same hydraulic design basis, the system area of operation shall be permitted to be reduced without revising the density as indicated in Figure 11.2.3.2.3.1 when all of the following conditions are satisfied:

(1) Wet pipe system

(2) Light hazard or ordinary hazard occupancy

(3) 20-ft (6.1-m) maximum ceiling height

(4) There are no unprotected ceiling pockets as allowed by 8.6.7 and 8.8.7 exceeding 32 ft2

There is a formula y=(-3x/2)+55 where y is the percentage of reduction and x is ceiling height.

If ceiling height 20′ then use 0%.

For a 12′ ceiling height the area of application is reduced by 37% or 945 sq. ft. This is substantial.

I would expect the water requirement for light hazard, covering 945 sq. ft., would be somewhere around 100 to 150 gpm but everyone should recognize there are as many ways to “tweak” something like this, such as using small orifice sprinklers (light hazard only) or tweaking the spacing, as there are technicians.

When not sprinkling the small concealed spaces there isn’t a reduction; the sprinkler design area will be the full 3,000 sq. ft. which will require a whole lot more water and much bigger pipes all the way through the system.