Exit Stair Design Requirements: Geometry, Enclosure, Ventilation and Pressurization

IBC Section 1011 sets stair geometry. Minimum width is 44 inches for stairs serving an occupant load of 50 or more, and 36 inches for those serving fewer than 50, both measured between the handrails (handrails may project 4-1/2 inches each side). Riser height ranges from 4 inches to 7 inches in most occupancies; tread depth must be at least 11 inches measured horizontally between vertical planes of the nosings. Uniformity is critical — within any flight, the largest and smallest risers can differ by no more than 3/8 inch, and treads by no more than 3/16 inch. Headroom is at least 80 inches measured vertically from a line connecting the edge of the nosings. Stair geometry that varies even slightly between flights creates a tripping hazard that has been implicated in many evacuation-related injuries. NFPA 101 Section 7.2.2 contains nearly identical geometry rules with very minor variations between code editions.

IBC Section 1014 requires handrails on both sides of every stair flight, regardless of occupant load. Handrail height is 34 to 38 inches measured vertically above the stair nosings. The handrail must be continuous along the flight without interruption and must extend at least 12 inches horizontally beyond the top riser and at least one tread depth beyond the bottom riser, returning to a wall or a guard. The handrail cross-section must be graspable — a Type I handrail (1-1/4 to 2 inches diameter) is the typical solution. Guards along open sides of stairs must be at least 42 inches high and must not have openings that allow passage of a 4-inch sphere. Landings at the top and bottom of each flight must be at least as wide as the stair and at least 48 inches deep in the direction of travel (or as deep as the stair width, whichever is less). Doors swinging into the landing cannot reduce the landing depth by more than half when fully open.

IBC Section 1023 requires interior exit stairways to be enclosed in fire-resistance-rated construction: 1-hour for buildings less than four stories above grade plane and 2-hour for buildings four stories or more. The enclosure must extend from the lowest level of exit discharge to the underside of the roof or top floor. Openings into the enclosure must be limited to those for normal egress and must be protected by fire door assemblies — 60-minute doors in 1-hour enclosures and 90-minute doors in 2-hour enclosures, all equipped with self-closing or automatic-closing devices and positive-latching hardware. Penetrations of the enclosure by ducts, pipes and electrical conduit must be sealed with through-penetration firestop systems tested to ASTM E814 / UL 1479 to maintain the rating. The enclosure interior may not be used for storage, mechanical equipment unrelated to the stair, or any function other than egress and required signage.

IBC Section 1023.11 requires smokeproof enclosures for interior exit stairways serving stories more than 75 feet above the lowest level of fire department vehicle access (high-rise) and for stairways more than 30 feet below the level of exit discharge. Smokeproof enclosures can be created by ventilated vestibules (a separately ventilated space between the corridor and the stair) or by stair pressurization (a fan-driven positive pressure differential that keeps smoke out of the stair). Pressurization design follows ASHRAE Handbook and IBC Section 909, requiring 0.10 to 0.35 inches water gauge differential across the stair-enclosure doors, with the system activated automatically by the fire alarm. The pressurization fan must be on emergency power and must be sized to maintain pressurization with one or more enclosure doors open. Testing and commissioning are required prior to occupancy and at least annually thereafter, with the differential measured at multiple stair levels under different door-open scenarios.

IBC Section 1023.9 and NFPA 101 Section 7.2.2.5.4 require stair identification signs inside each interior exit stairway at every floor. The sign must indicate the stair designation (commonly Stair A, Stair B), the floor level, the upper and lower terminus of the stair (top floor and lowest level of exit discharge), the floor numbering convention, and whether roof access is available. The sign must include tactile characters per ICC A117.1. Letters and numbers must be a minimum of 1-1/2 inches for floor level and 1 inch for stair designation. The sign must be located approximately 60 inches above the floor measured to the center of the sign, mounted adjacent to the door on the latch side. Floor-level signs in stairs serving more than three stories must include high-contrast retroreflective marking. NFPA 101 also requires signs at each exit door identifying the stair (so an occupant in the corridor knows which stair they are entering). EvacPlan Generator (www.evacplangenerator.com) supports adding stair-identification labels to posted plans so the same stair designations appear consistently on the diagram and on the in-stair signage.

Most interior exit stairs terminate at the level of exit discharge with a door directly to the exterior or with an exit passageway leading to an exterior door. Where the stair extends to the roof, IBC Section 1011.12 requires roof access to be marked with a sign at each landing. In high-rise buildings, IBC Section 1011.12.1 requires roof access from at least one interior exit stairway when the roof slope is less than 4:12 and the building has an occupied roof or has equipment that requires service access. At the discharge level, IBC Section 1028.1 generally requires stairs to discharge directly to the exterior or to an exit passageway. Where stairs discharge through a vestibule or lobby (exception in Section 1028.1), the discharge path must be separated from the rest of the floor by 1-hour construction and must be limited to 50% of the building's required exits with the remainder discharging directly to the exterior. Posted plans must clearly show whether the stair discharge is direct-to-exterior or through-lobby so occupants know what to expect when they reach the discharge level.

Maintenance is governed by IBC Section 1031 (the maintenance section), NFPA 101 Section 4.6, and the building's fire prevention code. Doors must be inspected annually under NFPA 80 to verify positive-latching, self-closing operation, full perimeter sealing and undamaged frame and hardware. Pressurization systems must be tested at least annually by qualified technicians per NFPA 92. Stair-identification signs and floor-level signs must be inspected during routine fire-marshal inspections and replaced when damaged or illegible. Storage in stair enclosures must be prohibited and removed when discovered — this is the single most-cited stair-related violation. Tripping hazards from worn nosings, damaged treads or accumulated debris must be repaired. Emergency lighting in the stair must be tested monthly with a 30-second function test and annually with a 90-minute discharge test. EvacPlan Generator's two-tier data model — PDF edits versus annotations — makes it straightforward to update a stair label, a route arrow or a stair-identification note on every page of a multi-page plan when the in-stair signage is updated, keeping the posted plan and the building condition in agreement.