The building code divides the use of building into separate occupancies classifications. The purpose of this is to provide a means of providing protection to each occupancy classification from an adjoining hazard. Protection of each individual occupancy is obtained by providing a minimum fire resistant rating on the fire separation as required.

Fire resistant rating is determined by the performance of the building materials to withstand the passage of flame and transmission of heat when exposed to fire under specified test conditions. Fire resistant ratings are rated in hours or fractions thereof, and are applied to fire separations as such.

Fire Separations are a constructed assembly that acts as a barrier against the spread of fire. So, fire separations, are the principal means of separating occupancies that provide a hazard, to themselves, or neighbouring occupancies.

Building Occupancies are broken into six main categories; Assembly, Care or Detention, Residential, Business and Personal Services, Mercantile and Industrial Occupancies. There are sub-groups under these categories; call divisions that even further define their use. The building code has a table, which specifies the fire resistant rating of the fire separation required between various occupancies, thus providing protection for each occupancy.

The allowable size of every building is governed by the code specifications that are specific to the occupancy class, number of stories, area, type of construction, sides of building with roads and whether building has sprinkler protection. Part of these specifications would include such items as fire resistant ratings of floors, ceilings and walls.

A good example would be a Group A Division 2 occupancy, a bowling alley and a group C residential occupancy, which would require a 1 hour fire resistant fire separation. Usually for this type of building you would be allowed to build with combination of combustible and non-combustible construction and the floor assemblies would be required to be fire separations and if combustible have a fire resistance rating of 45 min.

For a more detailed description of requirements for occupancies and fire separation s the Canada Wood Council puts out an excellent reference book called Fire Safety Design in Buildings and of course your applicable building code which contains all the information required to design and construct a building.