Smoke in the Stairwells: The Right Steps to Smoke Management within Building Stair Enclosures

The New York City Building Code contains several different sections regarding smoke management features and building stair enclosures. There is often some confusion over which aspects of these code sections apply to specific projects. The three primary means of smoke management within stair enclosures are listed below:

  1. Stairwell Smoke Venting
  2. Post-Fire Smoke Purge
  3. Stairway Pressurization

While these three requirements may have some similarities, understanding their differences is critical to the design of buildings. This article will detail each item and clarify when they are required under the 2022 NYC Building Code.

Stairwell Smoke Venting

Smoke vents at the top of stair enclosures are intended to release smoke to atmosphere in an active fire situation, preventing the egress stairs from becoming filled with smoke. The requirements for stair smoke venting are detailed in Section 713.12.1, which states “…vertical exit enclosures…shall be provided with a smoke vent.” These smoke vents are a requirement within all buildings unless the building has been provided with smokeproof enclosures in accordance with Section 1023.11. Per Section 713.12.1.2, the smoke vent must provide at minimum 3½ percent of the maximum shaft area at any floor. For a typical scissor stair in an R-2 building, this equates to a vent with a free area of ±5.25 square feet.  Consideration should be given to floors where stairs transfer or connect to an egress passageway. If permitted by code, providing a door between the stair enclosure and an egress passageway can drastically reduce the size of the required vent.

Smoke vents for stair enclosures are permitted to terminate through the stair bulkhead roof in accordance with Section 713., or through the exterior wall at the highest level in accordance with Section 713. As such, it is important that building egress stairs have a means of communicate directly with the atmosphere. Consider a stair that terminates at the highest level within the building but has a mechanical room or other similar space on the roof directly above the stair enclosure. Such a stair would need to be vented through a duct providing minimum 3½ percent of the stair shaft area at its largest level. As mentioned above, the typical scissor stair in an R-2 building requires a vent with a free area of ±5.25 square feet. This equates to a 48×16 duct or equivalent. This duct in turn would need to be routed through the mechanical room within a shaft enclosure providing an equivalent rating to the stair enclosure. Accounting for the construction of the shaft enclosure, this vent would take up a footprint ±6.0 square feet. This type of floor space is often difficult to allocate.

Stair smoke vents can take many forms. Per Section 713.12.1.1, windows, louvers, skylights, vent ducts, or other similar devices are permitted means of smoke venting. The two most common methods for venting stairs are smoke vent hatches, similar to Bilco Type SV, or louvers. The simplest solution is typically to provide a smoke vent hatch on top of the stair enclosure. This generally results in the smallest vent opening while also avoiding obstructions within egress passageways. Additionally, it is typically easier to meet the termination requirements when venting through the roof. Lastly, the smoke vent hatch can also double as access to stair bulkheads. The second most common means for venting stair enclosures is to provide a louver with a motorized damper interlocked with a smoke detector at the top of the stair enclosure. There are several complications that arise from this strategy, however. Going back to the typical R-2 scissor stair example, a ±5.25 square feet vent would require a louver that is a minimum of 48” wide by 36” high, assuming 45% free area. This louver would be required to terminate minimum 5’-0” horizontally on either side from operable openings into the building. This minimum distance requirement is difficult to achieve at rooftops and often not possible depending on the rooftop programming (i.e. stair doors, mechanical room doors, elevator machine room doors, etc. nearby). If a suitable location does exist to terminate a stair smoke vent horizontally, the next consideration that must be given is avoiding conflict with both the turning radius and sprinkler/standpipe riser within the stairwell. While the turning radius must only e maintained to a height of 7’-6” above the stair landing, there typically is not enough space above this height to accommodate the required louver size. As such, the louver must be located outside of the turning radius. Note that the louver would require a small duct collar to house the damper assembly and actuator. Depending on how tight the enclosure is at the roof, this may not be possible. Additionally, at least one side of the stair outside of the turning radius is typically occupied by the building’s sprinkler/standpipe riser and roof manifold. There is also likely to be an electric heater provided at the top of the stair enclosure for freeze protection, yet another item that can interfere with a horizontal vent.

Post-Fire Smoke Purge

As its name suggests, post-fire smoke purge is intended to restore the timely restoration of operations and overhaul activities after a fire has been extinguished. It is important to note that smoke purge smoke venting. Smoke venting is a life safeties measure to evacuate smoke from a building during an active fire condition. Post-fire smoke purge is meant to exhaust cold smoke after a fire has been extinguished and is not considered as a life safeties measure. These are different requirements with distinct functions, and they must each be provided with dedicated infrastructure. Per Section 917.1, post-fire smoke purge systems are required in the following scenarios:

  1. High-rise buildings subject to Section 403.
  2. Buildings with any story exceeding 50,000 square feet in floor area.
  3. Buildings with spaces exceeding 100 ft from natural ventilation openings. Natural ventilation openings shall consist of operable windows and doors of at least 5 percent of the floor area or roof vents per Section 910.
  4. High-piled or rack storage in accordance with the New York City Fire Code.

For this section, we will focus on post-fire smoke purge systems in occupancy Group R-2, which is the only occupancy permitted to use stair enclosures as part of the post-fire smoke purge system. Per Section 917.3.1., “the top of all enclosed exit stairs within an R-2 high rise building shall be provided with a reversible fan system capable of introducing fresh air or exhausting air at a rate of 6 air changes per hour or 1 CFM/square foot, whichever is greater, based on the area of the largest floor.” Simplifying this requirement further, floors with an underside of slab height of 10’-0” or less must be designed to the rate of 1 CFM/square foot of floor area and floors with an underside of slab height of 10’-0” or more must be designed to the rate of 6 air changes per hour. For R-2 buildings, it is typically mechanical transfer floors or podium floors with higher floor-to-floor heights that govern the design requirements of post-fire smoke purge systems.

Let’s go back to the example of a scissor stair within an R-2 building. Note that within R-2 buildings, a scissor stair is considered as two separate stair enclosures per Section 1007.1.1 Exception 3. This requirement was also clarified further in Buildings Bulletin 2023-005. That means that the top of each scissor stair section must be provided with a reversible fan system. Putting it all together, the top of each scissor stair section within an R-2 building would require both a smoke vent as detailed in the “Stair Smoke Venting” section above, and a separate post-fire smoke purge fan. These two differing components competing for the same space are often easy to overlook during the design phase. It is critical that this is coordinated properly to avoid complications and additional cost during the construction phase.

One final note on post-fire smoke purge systems – The 2022 Building Code introduced a new code section prohibiting occupancies other than Group R-2 from using the stair enclosure as part of a post-fire smoke purge system. Per Section 917.2.4 “Interior exist stairways or ramps or exit passageways shall not be used as a portion of the post-fire smoke purge system in occupancies other than Group R-2. Doors in interior exit stairways or ramps or exit passageways shall not be permitted to be used as a portion of the post-fire smoke purge system. Air transfer and duct openings associated with the post-fire smoke purge system shall not be permitted in the interior exit stairway of ramp or exit passageway.” This means that non-R-2 occupancies within a mixed-use residential building, such as a community facility or a retail tenant on the ground floor, must be provided with their own dedicated post-fire smoke purge system that is completely separate from the system serving the R-2 portion of the building. Note that such a system need only be sized for the largest fire area per Section 917.2.2 and that different non-R-2 tenants are permitted to share a single post-fire smoke purge system provided the system is designed with the appropriate zone dampers and sequencing.

Stairway Pressurization

Stairway pressurization systems deliver fresh air to stair enclosures, pressurizing them with respect to adjacent spaces. This ensures that smoke is prevented from migrating into the stair enclosure, allowing occupants to safely evacuate during an active fire. Section 1023.11 states that smokeproof enclosures must be provided for each of the interior exit stairways and ramps for buildings required to comply with Section 403 or Section 405.  Section 909.20 details the compliant means of providing smokeproof enclosures. Given the complications and spatial requirements for achieving smokeproof enclosures architecturally as well as the cost and complexity associated with the mechanical ventilation alternative, stairway pressurization systems are the most common pathway to providing smokeproof enclosures when required by the building code.

Per Section 909.20.5, stairway pressurization systems shall be capable of pressurizing a stair enclosure to a minimum of 0.10 inches of water and not more than 0.35 inches of water in the shaft relative to the building measured with all interior exit stairway and ramp doors closed under the maximum anticipated conditions of stack effect and wind effect. For buildings taller than 8 stories, multiple injection points at different floor levels will be necessary to ensure the required pressure differentials are maintained. This means a fresh air shaft would be required adjacent to the stair enclosure with injection points ducted in every three to five floors depending on the building height.   

Sections 909.20.6 & 909.20.6.1 detail specific requirements for stair pressurization ventilating equipment. The fan(s) for stairway pressurization systems must be activated by smoke detectors installed at each floor level at an approved location at the entrance to the smokeproof enclosure, or upon power failure.  Importantly, and unlike post-fire smoke purge systems, the ventilation systems for smokeproof enclosures must be independent systems and cannot be combined with or otherwise shared with other building systems. Section 909.20.6.1 further details connection and separation requirements for stairwell pressurization systems. Most commonly, the equipment, control wiring, power wiring, and ductwork is located exterior to the building with the ductwork fed into the smokeproof enclosure. However, the system is also permitted to be located within the smokeproof enclosure or within the building if the appropriate separation measures are taken.

While Section 403 governs high rise buildings, it is important to note that there is an exception, as there often is, for Group R-2 buildings. Per Section 403.5.4 “smokeproof enclosures are not required in occupancy Group R-2 unless provided pursuant to Exception 2 of 917.1. However, if an R-2 building contained floor levels more than 30 feet below its level of exit discharge, every required exit stairway serving such a floor would require smokeproof enclosures in accordance with Section 405.7.2. As stair pressurization is considered a life safeties system, they are required to be provided with a source of standby power in accordance with Section 403.4.8.3, Section 405.8, and Section 2702.2.20.2. This means that buildings that require stair pressurization will also require a diesel generator and a fuel oil system in accordance with Section 2702.1.1. Lastly, as stairway pressurization systems are considered a smoke control system, they are subject to the special inspection and testing requirements of Section 1705.18.