The Ontario Building Code | Exposure Conditions for Rating
3.1.7.3. Exposure Conditions for Rating
(1) floor, roof and ceiling assemblies shall be rated for exposure to fire on the underside.
(2) Firewalls and interior vertical fire separations shall be rated for exposure to fire on each side.
(3) exterior walls shall be rated for exposure to fire from inside the building.
In Plain Language
This article sets which side of an assembly must be exposed to fire during rating tests: floors/roofs/ceilings are tested from underneath, firewalls and interior vertical separations are tested from both sides, and exterior walls are tested from the inside.
This is a plain-language summary of the 2012/2017 Ontario Building Code text above, for general understanding only and not a substitute for the Code. Always confirm current requirements against the 2024 edition (O.Reg. 163/24) or a qualified professional before construction.