Protected escape route

A protected escape route is the part of an escape route that is separated from the rest of the building by fire- or smoke-resisting construction, or by open air. Occupants in it can reach a final exit without being exposed to the fire. In New Zealand, C/VM2 (second edition, 2025) calls these protected parts exitways and safe paths. In Australia they are known as fire-isolated exits, such as fire-isolated stairs and passageways. The protected part follows the open path through the room or floor where the fire may be, and it ends at a final exit to a safe place outside.

Protected escape routes are a core element of the means of escape in multi-storey buildings. They rely on fire compartmentation, on fire doors that close behind the occupants and, in many tall buildings, on stair pressurisation or other smoke control systems. The aim is that people who have left the open path need not walk through smoke, because smoke is what most often prevents escape. Design rules therefore set requirements for the fire resistance of the enclosure, the doors, the linings, the separation between alternative routes and the travel distance to the protected route.

Definition and terminology

C/VM2 (second edition, 2025) defines an escape route as a continuous, unobstructed route from any occupied space to a final exit, so that occupants can reach a safe place. An escape route consists of open paths, safe paths or both. An open path is the part within a firecell where occupants may be exposed to fire or smoke while they escape. The exitway is the part of the escape route protected by fire or smoke separations, or by distance where it is open to the outside air, and it ends at a final exit. A safe path is the part of an exitway that is protected from the effects of fire by fire separations, external walls or distance in the open air.

The parts of an escape route: the open path inside the firecell and the protected exitway that leads to the final exit. Based on Ministry of Business, Innovation and Employment (2025b), pp. 63, 67 and 69.

The final exit is the point where the escape route gives direct access to a safe place, such as a street or open space where people can disperse. C/VM2 also allows a place of safety inside a building. Such a place must be enclosed by fire separations that withstand burnout, lie in a sprinklered building, hold the intended number of people and have its own means of escape to the outside.

The Australian data sheets for the Fire Safety Verification Method use the term "protected exit", with a fire-isolated stair or passageway as examples. The Building Code of Australia (BCA) states the matching performance requirement. As quoted in the International Fire Engineering Guidelines (2005), exits must be fire-isolated "to the degree necessary", depending on the storeys connected, the fire safety systems, the use of the building and fire brigade intervention. In the Netherlands, maintaining the escape routes for building users is one of the sub-goals of the fire safety rules in the Bbl, the Dutch building regulations.

Why escape routes are protected

In a fire, escape is usually prevented because hot smoke makes visibility poor, although temperature and toxicity can also play a part. Visibility inside a smoke layer can fall very quickly. In an example in the Introduction to Fire Safety Engineering, a lit exit sign is visible from only 0.57 m two minutes after the fire starts. Escaping through such a layer is not a realistic option. Several forces drive smoke into other spaces. The expanding gases push smoke through seams, gaps and holes, and stack effect, wind and building services spread it further.

Smoke often travels far from the fire. Klote notes that stairs and lift shafts frequently become smoke-logged, which blocks or hinders evacuation. Proulx describes how stairs in high-rise buildings become contaminated when occupants of the fire floor and of upper floors open the stair doors during a simultaneous evacuation. This happens even if the stair is pressurised. Proulx concludes that the stairs may be the most dangerous place in a high-rise fire after the room of origin and the fire floor. In the 1993 bombing of the World Trade Center, 94 percent of the occupants of the first tower moved through smoke.

A protected route is designed to remain usable for as long as occupants need it. In New Zealand, C/VM2 (second edition, 2025) requires tenability to be assessed in the escape routes over the period in which occupants are escaping. The tenability criteria are the fractional effective dose of carbon monoxide, the fractional effective dose of heat and the visibility, all measured 2.0 m above the floor. The explanatory notes to the Dutch Bbl give comparable criteria. For a non-enclosed space through which an escape route runs, they include a smoke-free height of 2.5 m above the route and at most 2 kW/m² of radiation below the smoke layer.

Enclosure and construction

The enclosure of a protected route is formed by fire separations and smoke separations. Klote explains that the effectiveness of such barriers depends on their leakage paths, such as cracks around doors and holes for pipes, and on the pressure difference across them. For smoke modelling, C/VM2 assumes that fire and smoke separations have no leakage apart from their doors. A smoke control door is assumed to leak through a 10 mm gap under the door. A fire door without smoke seals is modelled with a 10 mm gap over its full height.

The required fire resistance depends on the building and on the scenario. In the challenging fire scenario of C/VM2, the floors, stairs and walkways that form escape routes must keep their structural adequacy, integrity and insulation for the whole required safe egress time. The fire and smoke separations that protect these routes must do the same. When this construction is assessed, the effect of sprinklers in controlling the fire must be ignored. For firefighting access, buildings with an escape height above 10 m need stairs designed as exitways, fire separated from the rest of the building and able to resist fire spread until burnout.

The materials inside the route also matter. C/VM2 sets its strictest wall and ceiling lining class, Group 1-S, for exitways in unsprinklered buildings, the same as for sleeping areas where care is provided. In most buildings, floor coverings in exitways must have a critical radiant flux of at least 2.2 kW/m². The commentary explains that linings in exitways may be the largest fire load in those spaces, while the spaces themselves are critical paths for escape.

Openings in the enclosure are a weak point. The International Fire Engineering Guidelines give the example of a window in the wall between an apartment and an internal stair in a multi-storey residential building in Australia. Such a window does not comply with the deemed-to-satisfy provisions, because the wall no longer has the required fire rating. The structural strength of the enclosure can matter as well.

In the World Trade Center towers, the stairs were enclosed by cast gypsum panels covered with gypsum board. These enclosures were fire rated but not required to withstand accidental loads. On 11 September 2001, all stairs in the impact zone of the first tower were destroyed, and debris from damaged enclosures obstructed stairs in both towers.

Doors

Doors are the most frequent openings in a protected route and must close again after use. C/VM2 assumes that fire doors and smoke control doors with self-closers are closed unless occupants are using them. When few people use a door, it is taken to be open for 3 s per occupant. When queuing is expected, the door is assumed open for the whole time of queuing. If 20 people pass a door one by one, it is therefore open for 60 s in total. Open doors count as half their width in smoke flow calculations.

In practice, doors do not always work as intended. In a New Zealand survey quoted in the Australian data sheets, 4.8 percent of fire doors and 10.3 percent of smoke control doors had missing smoke seals. About 2 percent of the doors were wedged or blocked, and 2.9 percent did not close fully. Estimated reliabilities in the same data sheet range from about 65 to 93 percent, with a mean of about 80 percent. The British draft DD240 gives 90 percent for a self-closing door to a protected stair.

Estimated reliability of fire and smoke doors from surveys and guidance documents. Based on Australian Building Codes Board (2020a), Data Sheet C7, p. 215.

C/VM2 therefore treats fire doors and smoke control doors as key fire safety systems, whose failure must be checked in a separate design scenario. Doors on hold-open devices released by the fire alarm in sprinklered buildings are an exception. In that scenario, each key system is assumed to fail in turn, and the design must still show that the available safe egress time exceeds the required time.

Pressurisation and smoke control

Smoke control combines barriers such as walls, floors and doors with airflows and pressure differences generated by fans. The high-pressure side of a barrier can be a refuge area or an escape route. Air flowing through the cracks around a closed door then prevents smoke from entering. When the door is opened, smoke can flow against a slow airflow into the escape route. A high enough velocity through the doorway prevents this backflow.

Pressurisation of an escape route: air leaking through a closed door, and smoke backflow through an open door at low and at high air velocity. Based on Klote (2002), p. 4-278.

Pressurised stairs are designed to provide a smoke-free escape route and, as a secondary aim, a smoke-free staging area for firefighters. Klote estimates that stopping smoke backflow through an open door needs about 4 m/s for an intense fire of 2.4 MW, and about 1.5 m/s for a fire of 125 kW. In a sprinklered building, where the smoke is cooled, 0.25 to 1.25 m/s may be used. Australian deemed-to-satisfy provisions refer to AS 1668.1. It requires at least 1 m/s through each open doorway to the fire-affected compartment when those doors and the main discharge doors of the stairs are open.

Pressure creates its own problem: occupants must still be able to open the doors. For a door of 2.13 m by 0.91 m, a pressure difference of 75 Pa and a closer force of 53 N give a total opening force of 133 N. Pressurisation systems also need relief openings, so that excessive pressures do not make doors hard to open or stop them from closing automatically. The Australian data sheets give a typical reliability of only 50 percent for pressurisation of fire-isolated exits, with a range of 30 to 60 percent.

Layout, distance and alternative routes

The travel to safety is often considered in two stages: the travel to a protected exit, and the travel through it. The length of the first stage, on the open path, is the part most exposed to the fire. In an example in the Australian handbook for the Fire Safety Verification Method, the deemed-to-satisfy travel distance from an apartment door to a fire-isolated stair is 6 m. A proposed design extends it to 12 m.

Where two escape routes are required, they must not both be blocked by one fire. C/VM2 (second edition, 2025) requires alternative escape routes to remain separated until they reach a final exit. They must diverge at an angle of at least 90° until they are 8 m apart, or 20 m apart when more than 250 people use only two routes. Smoke separations and smoke control doors are an alternative to these distances. A route with a single direction of travel may be at most 50 m long for occupants familiar with the building and 40 m for others.

Alternative escape routes in C/VM2: they diverge at 90° or more until 8 m apart, or 20 m apart when more than 250 people use only two routes. Based on Ministry of Business, Innovation and Employment (2025b), p. 37.

The World Trade Center showed the importance of separation between protected stairs. On floors 83 and higher, two stairs were only about 21 m apart, while on floors 77 to 82 they were 54 to 63 m apart. In the first tower, where the stairs in the impact zone were clustered in the core, all were destroyed. In the second tower, where they were furthest apart, one stair remained marginally passable. NIST therefore recommended that egress systems maximise the remoteness of stairs and keep their functional integrity in foreseeable emergencies.

Use, maintenance and limits

A protected route only helps if people use it. The Introduction to Fire Safety Engineering notes that emergency doors and routes reserved for incidents are hardly used in practice. A practical approach is to integrate escape routes with the routes used every day. The International Fire Engineering Guidelines place the duty to keep egress paths clear, doors working and signs in good condition with building management.

Protected routes can also become the place of a fire. The Australian handbook notes that fire-isolated stairs are sometimes used to store rubbish or furniture, and that combustibles may be brought in and ignited deliberately. It treats a fire at the base of a fire-isolated stair as a reference scenario for this reason. For sprinklered sleeping occupancies with a single vertical escape route, C/VM2 (second edition, 2025) requires visibility in that route of at least 5.0 m for the whole required safe egress time.

See also

• Evacuation strategy
• Phased evacuation
• Exit capacity
• Emergency lighting
• Escape signage

References

• Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
• Australian Building Codes Board. (2020b). Handbook: Fire Safety Verification Method (Print version 1.1).
• Averill, J. D., Mileti, D. S., Peacock, R. D., Kuligowski, E. D., Groner, N., Proulx, G., Reneke, P. A., & Nelson, H. E. (2005). Occupant behavior, egress, and emergency communications (NIST NCSTAR 1-7). National Institute of Standards and Technology.
• Klote, J. H. (2002). Smoke control. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-274–4-291). National Fire Protection Association; Society of Fire Protection Engineers.
• Ministry of Business, Innovation and Employment. (2025b). Verification Method C/VM2: Framework for fire safety design (2nd ed.).
• National Institute of Standards and Technology. (2005). Final report on the collapse of the World Trade Center towers (NIST NCSTAR 1).
• National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
• Proulx, G. (2002). Movement of people: The evacuation timing. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-342–3-366). National Fire Protection Association; Society of Fire Protection Engineers.
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.