Stair That Does Not Fit the Framed Opening
A remodel requires a new straight-run stair from the first floor to the second. The finished floor-to-floor height is 9 ft 4 in. The available stairwell rough opening is 11 ft 0 in. long and 3 ft 6 in. wide. The owner wants a 7½ in. riser and a 10½ in. unit run with a 1 in. nosing. The framer plans 2x10 stringers at 16 in. o.c., with ¾ in. oak treads and ⅝ in. drywall on the ceiling below. The floor joists are 2x10 at 16 in. o.c. running perpendicular to the stair, and the opening was framed before the stair was laid out. The framer says the stair will 'just fit' if the top riser is made 5 in. and the bottom riser is 8½ in. The inspector has not been called yet.
1. Calculate the number of risers and the uniform riser height for a 9 ft 4 in. total rise. State whether the proposed 7½ in. riser works and why the framer’s unequal top and bottom risers are unacceptable.
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9 ft 4 in. = 112 in. Divide by the desired 7½ in.: 112 ÷ 7.5 = 14.93. You cannot have a fraction of a riser, so round up to 15 risers. Uniform riser = 112 ÷ 15 = 7.4667 in., about 7 15/32 in. (7.47 in.). That is under the common 7¾ in. maximum, so a 15-riser stair works. The proposed 7½ in. riser is close but does not fit exactly; if used with 15 risers the total rise would be 112.5 in., so the top or bottom would have to absorb ½ in., which is already a layout error. The framer’s 5 in. top riser and 8½ in. bottom riser are not acceptable: 8½ in. exceeds the maximum riser, and the 3½ in. difference between largest and smallest riser is far beyond the typical 3/8 in. tolerance. Unequal risers are a tripping hazard because the body learns one step height and then meets a different one. All risers in a flight should be equal within 3/8 in.
2. Using 14 treads at 10½ in. unit run, calculate the total run. Determine whether the 11 ft 0 in. rough opening is long enough, including the top landing or nosing, and propose two compliant remedies if it is not.
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Number of treads = risers − 1 = 15 − 1 = 14. Total run = 14 × 10.5 in. = 147 in. = 12 ft 3 in. The stated rough opening is 11 ft 0 in. = 132 in., so the run is 15 in. longer than the opening even before allowing for the top nosing or landing edge. A 10 in. unit run (the usual minimum) would give 14 × 10 = 140 in. = 11 ft 8 in., still 8 in. too long. Therefore the stair does not 'just fit,' and cutting the top riser to 5 in. only makes it noncompliant. Remedies: (1) enlarge the floor opening by moving the header back at least 15 in., preferably more for finish and headroom, and frame a proper landing at the top; (2) redesign the stair with winders or a landing to turn the flight, or use a different stair configuration, so the required run fits the available opening. Reducing tread depth below the minimum or using unequal risers is not an acceptable third option.
3. Check headroom and stringer support. With 2x10 stringers spanning 12 ft 3 in. and no mid-span support, what would you verify, and what is the minimum headroom typically required?
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Headroom is measured vertically from the nosing line or tread line to the ceiling above, and the typical minimum is 6 ft 8 in. (80 in.). With a 15-riser stair and a framed opening only 11 ft long, the critical point is where the stair passes under the floor header; check the actual clearance at every tread, not just at the opening center, and remember that a thicker finish ceiling or a dropped beam can reduce it. For the stringers, a 2x10 spanning 12 ft 3 in. at 16 in. o.c. is a structural question, not a rule-of-thumb call: verify the stringer size, grade, species, and support spacing against a span table or have the designer or engineer confirm. Stairs carry concentrated loads and impact, and a 2x10 at that span may be marginal depending on grade and loading. If it is over-spanned, add a mid-span support wall or beam, or increase to a deeper stringer. Also check that the stringers bear properly at top and bottom, are not notched excessively for treads, and are braced laterally.