Occupancy classification

Kostyshyn starts the third week with a game. She reads out the names of buildings and the students shout the occupancy. A hockey arena. A bank branch. A hospital. A brewery with a taproom. A church hall where a seniors' club plays cards. A yoga studio above a shoe store. The answers come quickly at first, then more slowly, and by the end the students are arguing. 'Good,' she says. 'Classification is where every code analysis starts, and it is where many of them go wrong. Get the occupancy wrong and every requirement that follows is wrong with it: the area, the height, the construction, the sprinklers, the exits, the fire separations. Get it right and the rest of the code falls into order.'

Why buildings are classified by use

The risks in a building depend more on how it is used than on how it looks. A hundred people in a nightclub, standing in the dark with music playing, behave very differently in a fire from a hundred people in an office who know the building, are awake and sober, and hold regular fire drills. Patients in a hospital may not be able to move at all. People asleep in a hotel or an apartment will be slow to wake. A warehouse full of plastic goods has a far heavier fire load than a gallery of paintings. Codes respond to these differences by sorting buildings into occupancy classifications, each with its own requirements, so that a daycare is not built to the same rules as a garage.

The classification asks several questions about the people and the contents of a space. How many people will there be, and how densely will they gather? Will they be awake and alert, or asleep? Will they know the building, or be strangers to it? Can they leave by themselves, or will they need help, or are they locked in? What will be stored or made there, and how readily will it burn or explode? Every occupancy group is a particular answer to these questions.

The occupancy groups

The National Building Code divides buildings into six groups, most of them subdivided into divisions. Group A is assembly: Division 1 for the production and viewing of the performing arts, such as theatres and concert halls; Division 2 for assembly occupancies not elsewhere classified, such as libraries, schools, community halls, restaurants, places of worship, museums and daycare facilities; Division 3 for arena-type occupancies, such as ice rinks and indoor swimming pools; and Division 4 for open-air assembly, such as grandstands and stadiums. Group B covers people who cannot readily leave: Division 1 detention, such as jails; Division 2 treatment, such as hospitals; and Division 3 care, such as nursing homes and some residences for seniors. Group C is residential: apartments, houses, hotels, residences and dormitories. Group D is business and personal services: offices, banks, clinics, hair salons and police stations without cells. Group E is mercantile: stores, shops and markets. Group F is industrial: Division 1 high hazard, Division 2 medium hazard and Division 3 low hazard.

The examples in the code's tables and explanatory material are a help but not a complete list. Many buildings do not match any example, and the designer must classify them by asking the questions above. Kostyshyn gives the yoga studio as an example. A small class of people exercising in a room is an assembly of people, not an office, even if the studio's reception desk looks like an office. It is Group A, Division 2, not Group D. The brewery with a taproom is two occupancies at once: an industrial occupancy where the beer is made and an assembly occupancy where it is drunk.

Classifying the community hub

The students' community hub from Design Studio III makes a good first case. It contains a glass commons, a library branch, a hall and gymnasium with a small stage, a community kitchen, a daycare for 48 children, meeting rooms, offices for staff and the usual washrooms, storage and mechanical rooms. Kostyshyn asks them to classify each part.

Under the National Building Code almost all of the hub is Group A, Division 2. The commons, the library, the meeting rooms and the hall are places where people gather for a common purpose. The hall has a small stage and will host the occasional concert, but it is primarily a multipurpose room for meetings, dinners and volleyball, so it is Division 2, not Division 1; a purpose-built theatre with a fly tower and fixed seats would be Division 1. The daycare is also Group A, Division 2: the code classifies daycare facilities as assembly, not as care, because the children are there by day and are supervised by staff. The staff offices are Group D, business and personal services, but they are small, and the code allows a minor occupancy within a larger one to be treated as part of it for some purposes, as the next section explains.

Kostyshyn points out what the students have just done. They have not only classified the hub; they have seen how a design decision, made in Design Studio III for reasons of light and play, changed its classification. Classification is not something that happens to a building after it is designed. It is a consequence of design, and an architect who understands it can use it.

Mixed occupancies

Most buildings contain more than one use. A school has offices and a cafeteria; an apartment building has a lobby, a bicycle room and perhaps a store at street level; a hospital has kitchens, laundries and parking. Codes deal with mixed occupancies in three ways. The first is to treat a minor use as part of the main one, if it is small enough. The second is to apply the most restrictive requirements of all the occupancies to the whole building, without separating them. The third is to separate the occupancies from one another with fire-rated construction and apply each one's requirements to its own part.

The National Building Code calls each principal use a major occupancy and requires that major occupancies be separated from one another by fire separations whose ratings are set in a table: a separation between a residential occupancy and a mercantile one, for example, must have a fire-resistance rating that depends on the pair. For the requirements on building size and construction, a building with more than one major occupancy is generally designed to the most restrictive requirements for any of them, although the code allows a major occupancy that takes up only a small share of a storey, about a tenth of its floor area, to be ignored for this purpose in some cases. Within a major occupancy there may also be rooms or spaces that serve it, such as an office in a library, which do not change its classification at all.

The script below sorts the hub's spaces into occupancies floor by floor and tests which of them are small enough to be treated as minor. The results show why the choice of method matters. A small use can ride along with the main one; a large one cannot, and then the designer must either separate it or design the whole building to its stricter rules.

# Classify the hub's spaces and test the minor-occupancy share per level
spaces = [  # (space, level, net area m2, group)
    ("commons", 1, 140, "A2"), ("hall and gym", 1, 520, "A2"), ("hall storage", 1, 40, "A2"),
    ("community kitchen", 1, 70, "A2"), ("daycare", 1, 290, "A2"), ("washrooms", 1, 45, "A2"),
    ("mechanical", 1, 90, "A2"), ("custodial and loading", 1, 40, "A2"),
    ("library", 2, 450, "A2"), ("meeting rooms", 2, 90, "A2"), ("staff offices", 2, 60, "D"),
    ("washrooms", 2, 25, "A2"),
]
for level in (1, 2):
    rows = [s for s in spaces if s[1] == level]
    total = sum(r[2] for r in rows)
    by_group = {}
    for name, lv, a, g in rows:
        by_group[g] = by_group.get(g, 0) + a
    for g, a in sorted(by_group.items()):
        share = a / total
        note = "minor share" if share <= 0.10 else "major occupancy"
        print(f"level {level}: group {g:<3} {a:>5} m2 of {total} m2 = {share:5.1%}  {note}")

level 1: group A2   1235 m2 of 1235 m2 = 100.0%  major occupancy
level 2: group A2    565 m2 of 625 m2 = 90.4%  major occupancy
level 2: group D      60 m2 of 625 m2 =  9.6%  minor share

On the ground floor everything is Group A, Division 2, so there is nothing to test. Upstairs the offices are about 9% of the floor and can be treated as part of the assembly occupancy for the size and construction requirements. Even if they were larger, the consequence would be small, because the requirements for Group D are generally less restrictive than those for Group A. The hub is, for most purposes, a single assembly building.

Classifying Greenway Homes

Greenway Homes is simpler. The apartments and townhouses are Group C, residential. The common room on the ground floor, where residents hold meetings and children's birthday parties, is used for assembly, but it is small and serves the residents, so it does not make the building a mixed occupancy. The bicycle storage, the garbage room and the laundry serve the residential occupancy. If Greenway had put a corner store or a café on Aldous Street, the building would have had a second major occupancy, Group E or Group A, Division 2, separated from the homes by a fire separation.

Kostyshyn adds a warning about residential care. As people age, the residents of an ordinary apartment building may come to need help. When a building is designed or operated to provide care, such as help with bathing, medication or moving about, it may become a care occupancy with much stricter requirements. Under the National Building Code, a building in which care is provided to residents who may need help to evacuate may be Group B, Division 3, and some provinces have added requirements for retirement homes and seniors' residences. Greenway's plan to rent some homes to seniors does not change the classification, but a plan to provide care services in the building would.

Exercise 3: occupancy classification

• Classify every space in your Design Studio IV housing project, and state whether the building has more than one principal occupancy.
• Take three buildings near the school, classify them, and give your reasons.
• Find a building that changed use in your city, such as a church turned into homes or a warehouse into offices, and describe how its classification changed.
• For the hub daycare, write a short note explaining how the classification would change if the playrooms were moved to the second floor.

Self-check

• Have I classified spaces by how they are used, not by what they are called?
• Have I identified every principal occupancy and decided how mixed occupancies will be treated?
• Have I checked whether any design decision changes the classification?