Occupancy classification and building use

The first question in any code review is what the building is used for. Occupancy classification sorts buildings and parts of buildings into groups according to the hazard their use presents: how many people are present, whether they are awake and alert, whether they can leave on their own, whether they know the building, and how much combustible material and hazardous activity there is. Almost every later requirement depends on the classification, including the permitted building size, the construction type, fire separations, sprinklers, exits, plumbing fixtures and accessibility. A wrong classification can make the whole code analysis wrong.

This lesson explains the occupancy groups in your country's code, how to classify a building with more than one use, how to deal with accessory spaces and how a change of occupancy triggers new requirements. Construction managers need this knowledge to understand design decisions, to spot problems when an owner changes the intended use during construction, and to manage tenant improvements in existing buildings.

Why occupancy drives the code

Consider three buildings of the same size: a warehouse, an office and a nightclub. The warehouse may have few people but a large fire load from stored goods. The office has more people, but they are awake, familiar with the building and spread out. The nightclub has many people crowded together in low light, possibly impaired, unfamiliar with the exits and focused on a stage. The life safety risk in the nightclub is far higher, so the code requires more exits, wider egress, more fire protection and tighter limits on size and construction. Hospitals and prisons add another dimension: occupants who cannot evacuate on their own, so the building must protect them in place.

Codes therefore classify by use rather than by building type. A single building may contain several occupancies, such as shops on the ground floor, offices above and parking below. Each part is classified separately and the code then decides how the parts must be separated or how the whole building must be treated.

The NBC major occupancy groups

The National Building Code classifies each building or part of a building by its major occupancy, in six groups, several of which are divided into divisions. Provincial codes based on the NBC use the same system with minor changes.

Classifying a building step by step

Classification is a structured process. Experienced designers follow roughly the same steps for every project.

• List every use: sales floor, storage room, offices, staff room, kitchen, seating area, parking, mechanical rooms.
• Assign each use to an occupancy group using the code definitions and the examples in the code's appendix or commentary. Where a use is not listed, classify it by the group it most closely resembles in hazard.
• Decide which uses are separate occupancies and which are accessory or subsidiary to a main use.
• Identify the major or main occupancy of the building and any others that need separation or special treatment.
• Record the classification in the code analysis on the drawings, with floor areas for each occupancy.

The NBC defines a major occupancy as the principal occupancy for which a building or part of it is used, including subsidiary occupancies that are an integral part of it. A storage room in a store is part of the mercantile major occupancy, but a large warehouse attached to the store may be a separate F-2 or F-3 major occupancy. Appendix notes in the code give many examples of classification.

Buildings with more than one major occupancy

When a building contains more than one major occupancy, the NBC generally requires each major occupancy to be separated from adjoining major occupancies by fire separations with a fire-resistance rating set by a table in Subsection 3.1.3, which depends on the two occupancies involved. For example, a residential occupancy must be separated from a mercantile occupancy, and the rating is higher when one of the occupancies is high-hazard industrial. The building's size and construction requirements are then determined by applying the requirements for each major occupancy, and where requirements differ, the code usually requires the more restrictive to apply to the whole building, unless the occupancy is located entirely above another, in which case each storey can be treated according to its own occupancy in some cases. Some combinations are prohibited: for example, a high-hazard industrial occupancy is not permitted in the same building as an assembly, care, treatment, detention or residential occupancy.

The choice affects cost. Separating occupancies adds rated walls, floors, doors and dampers but may allow a cheaper construction type or avoid a requirement that would otherwise apply to the whole building. Not separating them saves rated construction but imposes the most restrictive rules everywhere. Designers usually test both options early in design.

Special uses and conditions

Some uses attract special requirements beyond their basic occupancy group. Codes have specific provisions for high buildings, buildings with interconnected floor spaces such as atriums, underground buildings, parking garages, covered malls, health care facilities, buildings containing hazardous materials, and assembly spaces with stages and platforms. Construction managers should flag these early, because they often bring smoke control, emergency power, special fire alarm features and commissioning that affect schedule.

In the NBC, Subsection 3.2.6 sets additional requirements for high buildings, generally buildings whose floors are more than 18 m above grade or that contain some care, treatment or detention occupancies above a lower height. Subsection 3.2.8 covers interconnected floor spaces, Section 3.3 covers safety within floor areas for specific occupancies, and the code contains special provisions for storage garages, laboratories and other uses.

Occupant load

Occupancy classification answers what the space is used for; occupant load answers how many people the code assumes are in it. The occupant load drives exit capacity, the number of exits, plumbing fixture counts and some fire protection triggers. It is calculated by dividing the floor area by an occupant load factor, in area per person, for the use, unless the design provides fixed seats or the owner proposes a higher number. Lesson 7 uses occupant loads in detail for egress.

Change of occupancy and tenant improvements

Buildings outlive their first use. Warehouses become offices, offices become schools, stores become restaurants and old industrial buildings become apartments. A change of occupancy can bring a building under requirements it was never designed for: more exits, sprinklers, fire separations, accessibility upgrades, more plumbing fixtures, higher floor live loads and energy upgrades. Construction managers on renovation projects should identify proposed changes of occupancy early, because the cost of bringing an old building up to the new use can exceed the cost of the interior fit-out.

Under the NBC and provincial codes, a change of major occupancy generally requires the part of the building affected to be made to comply with the code for the new occupancy, with some flexibility for existing buildings. Ontario's code contains Part 11 for renovation of existing buildings, which uses a performance level approach: a renovation may not reduce the performance level of the building, and a change of major occupancy that increases the hazard may require upgrades. Some other provinces publish guides for existing buildings, and the NRC has been developing provisions for alterations to existing buildings in the national codes.