Basics of Egress Stair Design
For many of us, walking up and down stairs is a routine part of our day. We may use stairs at work, at entertainment venues, and in our home without thinking twice about how their design and function contribute greatly to life safety in both emergency and non-emergency situations. Recently, I wrote about the details and the importance of handrail design for safe and efficient stair use. Here I will focus on other details of stair design including riser height, tread depth, stair width, stair landings, and construction uniformity that are mandated in order to create a safe path of travel when using the stairs to move throughout the building. These standard stair design details are mandated for egress stairs in the exit access, exits and exit discharge. (Where you have other than standard stairs such as curved stairs, spiral stairs or winders within a means of egress, consult NFPA 101, Life Safety Code, Chapter 7 for further details on their design.)
All stairs serving as required means of egress must be of permanent fixed construction (unless they are stairs serving seating that is designed to be repositioned, such as those in theaters, for example, where seating sections are added, removed, or relocated and it is impractical for stairs associated with that seating to be of fixed, permanent construction). In buildings required by NFPA 101, Life Safety Code, to be of Type I or Type II construction, each stair, platform, and landing, not including handrails and existing stairs, are required to be of noncombustible material throughout. Stairs can be of combustible construction if the building is not required by that occupancy to be of Type I or Type II construction. For example, an occupancy might not have any requirements related to minimum building construction type, or the occupancy chapter might permit Type III, Type IV, or Type V construction. If the building is required to be of Type I or Type II construction, the materials used for new stair construction (stairs, platforms, and landings) must be noncombustible.
Dimensional Criteria and Uniformity
Providing adequate width is one of the most important features of egress stair design as the width ensures that the stairs can accommodate enough people safely and efficiently during an evacuation. Providing appropriate stair riser height and tread depth ensures that stairs are safe, usable, and presents tripping and discomfort when traveling up or down the stairs. The minimum required width as well as other dimensional criteria for both new and existing stairs is summarized in the tables below (reference: Chapter 7 of NFPA 101). It should be noted that in some cases, the egress capacity will require a stair to have a greater width than the minimum specified here.
The minimum width of new stairs is 36 in. (915 mm) where the total occupant load of all stories served by the stair is fewer than 50. Where new stairs serve a total cumulative occupant load (assigned to that stair) of 50 or more people but less than 2000 people the minimum width is 44 in. (1120 mm) and where the total cumulative occupant load assigned to the stair is greater than or equal to 2000 people the minimum width is 56 in. (1420 mm).
Riser height is measured as the vertical distance between tread nosings. Tread depth is measured horizontally, between the vertical planes of the leading projection of adjacent treads and at a right angle to the tread’s leading edge. Measuring both riser height and tread depth needs to represent the actual space available to those using the stairs. It cannot include any part of the tread that is not available for someone to place their foot. Installing floor coverings to existing stairs might also reduce the available space for use on the stairs.
Irregularities in stair geometry, either from one step to the next or over an entire run of stairs, can cause accidents, tripping and falling when using the stairs. When many people are using the stair at once, just one accident can cause delays and disruptions in movement and use of the stairs, and increase the overall time of evacuation. There should be no design irregularities. Very small variations due to construction are permitted between adjacent treads and risers and the overall different over the entire flight of stairs. The variation between the sizes of the largest and smallest riser or between the largest and smallest tread depths shall not exceed 3∕ 8 in. (9.5 mm) in any flight.
As a general rule, stairs must have landings at door openings because it is unsafe to move through a door opening and immediately begin vertical travel on a stair. In existing buildings, a door assembly at the top of a stair is permitted to open directly to the stair, without first providing a level landing, provided that the door leaf does not swing over the stair (rather, it swings away from the stair) and the door opening serves an area with an occupant load of fewer than 50 people.
Stairs and intermediate landings must continue with no decrease in width along the direction of egress travel. A reduction in width of a stair landing could reduce the overall capacity of the stair. In new buildings, every landing will have a dimension, measured in the direction of travel, that is not less than the width of the stair. Landings are not required to exceed 48 in. (1220 mm) in the direction of travel, provided that the stair has a straight run. Intermediate stair landings serve as effective breaks in runs of stairs, which allow persons who slip or trip to halt their fall.
Stair Tread and Stair Landing Surfaces
Stair treads and landings must be solid, without perforations, except for noncombustible grated stair treads and landings as otherwise provided in the following occupancies: assembly, detention and correctional, industrial and storage. Solid treads and solid landing floors provide a visual barrier that shields the user’s view of the vertical drop beneath the stair. People with a fear of high places are more comfortable using these stairs. Grated and expanded metal treads and landings could catch the heel of a shoe and present a tripping hazard. Noncombustible, grated stair treads are permitted in areas not accessed by the general public, such as catwalks and gridirons in theaters, resident housing areas in prisons, factories and other industrial occupancies, and storage occupancies.
Stair treads and landings must also be free of projections or lips that could trip stair users. The tripping hazard occurs especially when someone is traveling down the stairs, where the tread walking surface has projections. The installation of a surface-mounted stair nosing or a strip of material onto an existing stair tread might produce a projection that creates a tripping hazard. Tread nosings that project over adjacent treads can also be a tripping hazard. (Additional considerations for minimizing tripping hazards for accessibility is also addressed in ICC A117.1, Accessible and Usable Buildings and Facilities.)
Stair treads and landings within the same stairway must have consistent surface traction. This means that slip resistance is reasonably uniform and sufficient to minimize risk of slipping across the treads. Consistency is important because misleading a person’s expectation of the surface they will be walking on is a major factor in missteps and falls involving slipping. Materials used for floors that are acceptable as slip resistant generally provide adequate slip resistance where used for stair treads. If stair treads are wet, there is also increased danger of slipping, just as there is an increased danger of slipping on wet floors of similar materials.
The many details of stair design may seem minute and unimportant in the overall picture of fire and life safety, but stairs can be dangerous and an impediment to egress if not designed correctly. Tripping, falling, and a lack of confidence by those using egress stairs can interrupt efficient egress travel and building evacuation. Paying careful attention to stair design will greatly contribute to occupant safety during both day to day and emergency conditions