Fires in Combustible Concealed Spaces

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.