Means of egress: occupant load, exits and travel distance · CON-140 Lesson 7

Getting people out of a building in an emergency is the oldest life safety purpose of building codes, and the egress provisions are among the most detailed. The code asks how many people may be in each space, how far they may have to walk to reach safety, how many separate ways out they need, how wide those ways must be and how they must be built, lit, signed and protected. The answers come from the occupancy and occupant load, with adjustments for sprinklers and building height. This lesson works through the main egress calculations a construction manager should be able to follow and check: occupant load, number of exits, exit width, travel distance, separation of exits and the protection of exit routes.

Construction managers deal with egress every day. Stair dimensions, door swings, hardware, exit signs, emergency lighting, corridor widths and the absence of obstructions are all inspected at occupancy. Egress must also be maintained during construction and renovation, often while part of the building is occupied, which requires temporary exits and planning.

The egress system

The NBC divides the route out of a building into two parts. Access to exit is the part of the means of egress within a floor area that leads to an exit, such as aisles, corridors and doors from rooms. An exit is the part of the means of egress, including doorways, that leads from the floor area it serves to a separate building, an open public thoroughfare or an exterior open space protected from fire exposure. Exits include doors directly outside, exit stairs and ramps enclosed by fire separations, horizontal exits through firewalls or fire separations into another building or compartment, and exterior passageways. Once a person enters an exit they are protected and should be able to continue to the outside without re-entering a floor area.

Floor plan of an office floor with two enclosed exit stairs at opposite ends. Dashed lines from the most remote point in an office show the travel path along a corridor to each stair. One path is labelled travel distance to the nearest exit. A dimension between the two exit doors shows they are well separated. A legend explains access to exit, exit and the outdoors. — Travel distance is measured along the path of travel from any point in a floor area to the nearest exit; exits must be far enough apart that one fire is unlikely to block both.

Occupant load

Lesson 4 introduced occupant load as floor area divided by an occupant load factor. For egress, the occupant load is calculated for each room, each floor area or storey and the building as a whole, and it is the starting point for every other egress calculation. Where a space has fixed seats, the occupant load is the number of seats. Where a space serves more than one use at different times, such as a hall used for dining and dances, the design uses the more demanding load. The occupant load of a floor also includes people from upper floors who will pass through it on the way out, at the level of exit discharge.

The NBC permits a lower occupant load than the table value if the owner shows that fewer people will use the space, but the posted occupant load then becomes a limit for the life of the building. It also permits higher occupant loads than the table value if the egress is designed for them. Assembly occupancies need the occupant load posted in a conspicuous place.

Number of exits

The NBC generally requires every floor area intended for occupancy to be served by at least two exits, with exceptions for small floor areas with a limited occupant load and travel distance, for example small single-storey buildings and some floor areas served by one exit where the occupant load and travel distance are within limits stated in Article 3.4.2.1. Rooms and suites need two egress doorways when their area or occupant load exceeds limits in Subsection 3.3.1. Higher occupant loads need more exit capacity, which may be provided by wider exits or more of them.

Two exits are required because one fire, or the smoke from it, may block an exit. For the second exit to be a real alternative, the exits must be far enough apart. Codes therefore set a minimum separation between exits.

The NBC requires exits from a floor area to be located so that the distance between them is not less than one half of the maximum overall diagonal dimension of the floor area, but need not be more than 9 m for a floor area with a public corridor, with variations in some editions and occupancies. Exits must also be placed so that the travel distance to the nearest exit is within the limits described below.

Worked problem

Checking exit separation

A rectangular floor area measures 48 m by 20 m. Two exit stairs are proposed with doors 22 m apart, measured in a straight line. Check the separation against the half-diagonal rule.

Show solution

Diagonal: √(48² + 20²) = √(2,304 + 400) = √2,704 = 52 m.
Half the diagonal: 26 m.
Proposed separation: 22 m, which is less than 26 m.
Check whether the 9 m cap applies: if the floor area is served by a public corridor, the required separation need not exceed 9 m in the editions that contain that provision, and 22 m would comply. Without a public corridor, the exits must be moved apart to at least 26 m.

Answer: Diagonal 52 m; half is 26 m; 22 m complies only if the public corridor cap applies, otherwise move the exits to at least 26 m apart.

Travel distance

Travel distance is the distance from any point in a floor area to an exit, measured along the path of travel. The NBC sets maximum travel distances in Article 3.4.2.5 that depend on occupancy and sprinklering. Typical values in recent editions are 30 m in most floor areas, 40 m in business and personal services occupancies and 45 m in floor areas of a sprinklered building, except in high-hazard industrial occupancies, where the limit is lower at 25 m. Where rooms open onto a public corridor, travel distance may be measured from the room door in some cases. The values must be confirmed in the edition and provincial code that applies.

Typical NBC maximum travel distances to an exit (Article 3.4.2.5). Provincial codes and editions vary; confirm the values for the project.
Occupancy (typical)	Unsprinklered (m)	Sprinklered (m)
Most occupancies	30	45
Business and personal services (Group D)	40	45
High-hazard industrial (Group F-1)	25	25

Worked problem

Checking travel distance

In a sprinklered office floor (Group D), the most remote point in a corner office is 9 m from the office door, then 31 m along a corridor to the nearest exit stair door. Is the travel distance acceptable?

Show solution

Travel distance: 9 + 31 = 40 m, measured along the path of travel.
Limit for a sprinklered floor area: typically 45 m.
40 m is within 45 m, so it complies.
If the building were unsprinklered, the Group D limit would typically be 40 m, and the design would just meet it; any increase in the corridor length would require a change.

Answer: 40 m complies with a 45 m sprinklered limit.

Exit width and capacity

The NBC sets the required width of exits from the occupant load they serve. In Article 3.4.3.2, the aggregate width is calculated at 6.1 mm per person for ramps with a slope not more than 1 in 8, doorways, corridors and passageways, and 8 mm per person for stairs with steps having a rise of not more than 180 mm and a run of not less than 280 mm, with higher values for other stairs. Minimum clear widths also apply: typically 1,100 mm for exit stairs and corridors serving more than a few people, with lesser widths for small occupant loads and doors at least 790 to 800 mm clear in many locations. When an exit serves more than one floor, the width is based on the floor with the largest occupant load, not the sum, because occupants of different floors do not arrive at the same time.

Exit capacity must also be balanced. If 60 percent of the occupants are likely to head for the main entrance, codes in both countries limit how much capacity can be lost if one exit is blocked, so that the remaining exits can still serve a reasonable share of the load. Assembly occupancies often require the main entrance to handle a stated fraction of the load.

Worked problem

Calculating required exit width

A storey of an office building has an occupant load of 300 persons and two exit stairs with steps of 180 mm rise and 280 mm run. Calculate the aggregate stair width needed and the width of each stair if they are equal, and check against a 1,100 mm minimum. Then calculate the door width needed into each stair.

Show solution

Stair width: 300 × 8 mm = 2,400 mm aggregate.
With two equal stairs: 2,400 ÷ 2 = 1,200 mm each, which exceeds the 1,100 mm minimum, so 1,200 mm governs.
Door width: 300 × 6.1 mm = 1,830 mm aggregate, or 915 mm per door with two equal doors.
Doors must also meet the minimum clear width; a 915 mm clear opening needs a door leaf of about 1,000 mm.

Answer: Two stairs at 1,200 mm and two doors with at least 915 mm clear width.

Building the exits

The calculations fix how many exits and how wide; detailed rules fix how they are built. These are the requirements inspectors check at occupancy and that construction managers must coordinate across trades.

Stairs: riser height, tread depth and uniformity limits, headroom, landings at each door, handrails on both sides and guards at open edges. Uniformity matters: a single riser that differs from the rest causes falls.
Doors: swing in the direction of travel where the occupant load exceeds a threshold, hardware operable without keys or special knowledge, panic or fire exit hardware in assembly and other occupancies, and limits on the force needed to open.
Exit signs: illuminated signs at each exit door and along routes where the exit is not visible.
Emergency lighting: lighting of exits and principal routes, powered by batteries or a generator when normal power fails, typically for 30 minutes to 2 hours depending on the building.
Protection: exit enclosures built as rated fire separations or fire barriers, with no storage, no unrelated services and limited openings.
Exit discharge: a clear path from the exit to a public thoroughfare or safe open space.

The NBC permits exit signs with the green running man pictogram, required by recent editions for new buildings, and requires emergency lighting to an average of at least 10 lx at floor level in exits and principal routes. Stair dimensions in Part 3 buildings require a rise between 125 and 180 mm and a run of at least 280 mm, with handrails 865 to 965 mm high.

Egress during construction

Existing buildings under renovation, and new buildings partly occupied before completion, must keep their exits working. Blocking an exit with materials, locking a door to secure a work area or removing an exit stair for repair can leave occupants with no way out. Fire codes in both countries require a fire safety plan for construction and demolition sites, and the plan should show how exits are maintained, where temporary exits and signage are provided and how workers and occupants are kept apart.

Part 8 of the NBC and Section 5.6 of the NFC set requirements for safety at construction and demolition sites, including access for fire fighting, protection of adjacent buildings and maintenance of exits. Provincial occupational health and safety regulations also require safe means of exit for workers on site.

Common mistakes

MistakeMeasuring travel distance in a straight line.
InsteadMeasure along the actual path of travel around walls, furniture and fixed equipment.
MistakeAdding the occupant loads of all floors to size a stair.
InsteadSize each exit for the largest load of any single floor it serves, adding only loads that converge at a level.
MistakeIgnoring the separation between exits.
InsteadCheck that exits are at least the required fraction of the diagonal apart so one fire cannot block both.
MistakeAccepting uneven stair risers on site.
InsteadCheck uniformity of risers and treads before finishes; small variations cause falls and fail inspection.
MistakeStoring materials in exit stairs during construction.
InsteadKeep exits clear; the fire safety plan should show protected temporary routes.

Summary and key terms

Means of egress consists of access to exit and exits leading outside.
Occupant load drives the number of exits, their width and other requirements.
Most floor areas need at least two exits separated by at least half the diagonal, subject to the public corridor provision.
Typical travel distance limits are 30 m, 40 m for business occupancies and 45 m when sprinklered.
Exit width is 6.1 mm per person for doors and corridors and 8 mm per person for typical stairs.
Stairs, doors, signs, emergency lighting and protection of exits are inspected at occupancy.
Egress must be maintained during construction and renovation under the fire safety plan.

Means of egress: A continuous path of travel from any point in a building to a safe place outside.
Access to exit: The part of the means of egress within a floor area leading to an exit.
Exit: The protected part of the means of egress leading to the outside or a safe area.
Exit discharge: The path from the end of an exit to a public way.
Travel distance: Distance along the path of travel from a point in a floor area to an exit.
Common path of egress travel: Travel before occupants have a choice of two separate paths.
Horizontal exit: An exit through a fire wall or fire separation into another compartment or building.
Panic hardware: Door hardware that releases the latch when pressure is applied in the direction of travel.
Emergency lighting: Lighting powered when normal power fails, for exits and egress routes.
Exit capacity: The number of people an egress component can serve based on its width.

Practice problems

Work each problem on paper first, then reveal the answer to check.

A floor area is 60 m by 25 m. What is half the diagonal?
Show answer
Diagonal √(3,600 + 625) = 65 m; half is 32.5 m.
An occupant load of 450 uses stairs at 8 mm per person. What aggregate width is needed?
Show answer
3,600 mm.
What door width is needed for 200 persons at 6.1 mm per person?
Show answer
1,220 mm aggregate.
Why is stair width based on the largest floor load and not the sum?
Show answer
Occupants from different floors do not reach the stair at the same time.
What is a typical sprinklered travel distance limit in the NBC?
Show answer
45 m.
How many exits does an IBC story with 700 occupants need?
Show answer
Three.
What are the two purposes of exit separation rules?
Show answer
To make it unlikely that one fire blocks both exits and to give occupants a real choice of routes.
Name four exit features inspected at occupancy.
Show answer
Stair dimensions, door hardware, exit signs and emergency lighting; also clear exit enclosures.
What illumination does the NBC require for emergency lighting in exits?
Show answer
An average of at least 10 lx at floor level.
Why must riser heights be uniform?
Show answer
People adjust their step to the first few risers; a different riser causes trips and falls.