Basement stair under the main beam
What's wrong here?
- Headroom under the beam is about 6 ft 2-1/2 in., less than the 6 ft 8 in. minimum
- Headroom only has to be 6 ft 4 in. in a basement, so this is fine
- Headroom is measured from the basement floor, and the beam is 7 ft 6 in. above the floor, so it is fine
- The handrail must be on the left side, the side people use going down
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: 1. 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
Minnesota adopts the 2018 IRC with Minnesota amendments as the Minnesota Residential Code (Minn. Rules ch. 1309). 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
- 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.
- 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.