The Ontario Building Code | Fire Blocks in Horizontal Concealed Spaces
3.1.11.5. Fire Blocks in Horizontal Concealed Spaces
(1) Except for a crawl space conforming to Sentence 3.1.11.6.(1), a horizontal concealed space within a floor assembly or roof assembly of combustible construction, in which sprinklers are not installed, shall be separated by construction conforming to Article 3.1.11.7. into compartments not more than,
(a) 600 m2 in area with no dimension more than 60 m, if the exposed construction materials within the space have a flame-spread rating not more than 25, and
(b) 300 m2 in area with no dimension more than 20 m, if the exposed construction materials within the space have a flame-spread rating more than 25.
(2) A concealed space in an exterior cornice, a mansard style roof, a balcony or a canopy in which exposed construction materials within the space have a flame‑spread rating more than 25, shall be separated by construction conforming to Article 3.1.11.7.,
(a) at locations where the concealed space extends across the ends of required vertical fire separations, and
(b) so that the maximum dimension in the concealed space is not more than 20 m.
(3) Except as provided by Sentence (4), a horizontal concealed space within a floor assembly or roof assembly of combustible construction in a building within the scope of Article 3.2.2.43A. or 3.2.2.50A. shall be separated by construction conforming to Article 3.1.11.7. into compartments not more than,
(a) 600 m2 in area with no dimension more than 60 m, if the exposed construction materials within the space have a flame-spread rating not more than 25, and
(b) 300 m2 in area with no dimension more than 20 m, if the exposed construction materials within the space have a flame-spread rating more than 25.
(4) Sentence (3) does not apply if the horizontal concealed space within the floor assembly or roof assembly is entirely filled with noncombustibleinsulationhttp://thehandyforce.com/insulation/" title ="Toronto home insulation">insulation such that any air gap between the top of the insulationhttp://thehandyforce.com/insulation/" title ="Toronto home insulation">insulation and the underside of the floor or roof deck does not exceed 50 mm.