Basement stair under the main beam
What's wrong here?
- The handrail must be on the left side, the side people use going down
- Headroom only has to be 6 ft 4 in. in a basement, so this is fine
- Headroom under the beam is about 6 ft 2-1/2 in., less than the 6 ft 8 in. minimum
- Headroom is measured from the basement floor, and the beam is 7 ft 6 in. above the floor, so it is fine
Show the answer and the code reference
The answer
The steel beam crossing the stair leaves only about 74-1/2 in. (6 ft 2-1/2 in.) of headroom above the tread nosings; R311.7.2 requires at least 6 ft 8 in.
Correct choice: 3. Headroom under the beam is about 6 ft 2-1/2 in., less than the 6 ft 8 in. minimum
R311.7.2 requires at least 6 ft 8 in. of clear headroom in all parts of a stairway, measured vertically from the sloped plane adjoining the tread nosings (or from a landing floor). A beam, duct or soffit that drops across a stair has to clear that line everywhere along the flight. Here the beam bottom is about 74-1/2 in. above the second-tread nosing, so anyone over about 6 ft 2 in. tall walks into the steel coming down the last steps, and everyone else ducks. The beam was set where the structure needed it without checking the stair; the stair and the beam have to be coordinated at framing.
How to fix it
Get 6 ft 8 in. above the nosing line: move the stair so the beam falls over the basement floor beyond the bottom riser, redesign the beam as a flush beam (for example, an engineered flush beam with joist hangers) so it sits within the floor depth, or lower the stair's bottom with a landing designed to keep the nosings far enough below the beam. Any change to the beam needs the designer's or engineer's sign-off.
Code references
IRC 2018 R311.7.2 — Headroom
The minimum headroom in all parts of a stairway is 6 ft 8 in., measured vertically from the sloped line adjoining the tread nosings or from the floor surface of a landing or platform. Exceptions cover floor-opening edges that project no more than 4-3/4 in. into the headroom at the side of a flight, and spiral stairs.
Drills citing R311.7.2IRC 2018 R305.1.1 — Basements
Portions of basements that do not contain habitable space or hallways need a ceiling height of at least 6 ft 8 in., except that at beams, girders, ducts or other obstructions the ceiling height may be as low as 6 ft 4 in. from the finished floor.
Drills citing R305.1.1
2024 IRC:renumbered to R318.7.2. The 2024 IRC moved stairways from R311.7 to R318.7; secondary sources place headroom at R318.7.2 with the 6 ft 8 in. minimum unchanged. Not checked line by line against the printed 2024 IRC. Confidence: low
Minnesota
Minn. Rules 1309.0311 amends parts of IRC R311, and search summaries of the Minnesota stairway text show the same 6 ft 8 in. headroom, but the rule text could not be read to confirm whether R311.7.2 is adopted as published. Some local handouts describe a reduced headroom allowance for existing basement stairs in remodels; that was not confirmed either, and it would not apply to a new stair. DLI is adopting the 2024 IRC as the next edition; see the 2024 IRC note.
About the Minnesota Residential CodeWhy the other choices are wrong
- The handrail must be on the left side, the side people use going down. R311.7.8 requires a handrail on at least one side; either side is acceptable.
- Headroom only has to be 6 ft 4 in. in a basement, so this is fine. 6 ft 4 in. is the basement ceiling-height allowance under beams, girders and ducts (R305.1.1), not a stair headroom rule. New stairs need 6 ft 8 in. along the whole flight (R311.7.2), and 74-1/2 in. would not meet 6 ft 4 in. anyway.
- Headroom is measured from the basement floor, and the beam is 7 ft 6 in. above the floor, so it is fine. Over a stair, headroom is measured vertically from the sloped plane through the tread nosings (R311.7.2), not from the floor at the bottom.