Measuring the flight, you find riser heights ranging from 7 inches to 7 and five-eighths inches, and tread depths ranging from 10 inches to 10 and one-quarter inches. No riser exceeds the maximum and no tread is below the minimum. What is the correct call?
- AThe stairway passes, because every riser is under the 7 3/4-inch maximum and every tread meets the 10-inch minimum, which is all the code checks
Maximum riser and minimum tread are only two of the three checks. The code adds a uniformity limit, and a flight can satisfy both dimensional limits while still varying enough between risers to trip a user.
- The stairway fails, because the difference between the tallest and the shortest riser is greater than the three-eighths inch the code permits
- CThe stairway fails, because the tread depths vary at all, and treads within a single flight are required to be identical with no tolerance
The code allows a 3/8-inch tolerance precisely because field-built stairs never come out identical. Reading the rule as requiring zero variation would fail nearly every stair built, and it fails this one for the wrong reason.
- DThe stairway passes, because the riser uniformity limit is written for stairways serving more than one story and not for a single flight
Uniformity applies to every flight regardless of how many stories the stairway serves. The story count changes nothing in the stair geometry section; the misreading comes from mixing it in with the handrail continuity provisions.
Why B is correct
The code limits variation within a flight to three-eighths of an inch between the largest and smallest riser, and separately between the largest and smallest tread. Riser variation here is five-eighths of an inch, which exceeds the limit and fails. Tread variation is a quarter inch and passes. Compliance with the 7.75-inch maximum and the 10-inch minimum does not substitute for uniformity, because it is the change in rhythm that makes people fall.
What this question is testing
The item tests whether the candidate applies the uniformity limit independently of the maximum and minimum dimensions, and whether the candidate can identify which of two measured ranges actually exceeds the allowable variation.
On the job
Remodel stairs are where this lives. A contractor rebuilds a rotted bottom step or pours a new slab at the base, and the bottom riser ends up short or tall while the rest of the flight is untouched. The resulting trip happens on the way down, when the occupant has already established a stride. Measure every riser, not the first and last, and measure from nosing to nosing rather than from the framing, because carpet and treads change the finished dimension.
Memory technique
Three-eighths is about the thickness of a pencil. If you can feel the difference underfoot, it is too much.
Exam tip
Check three things on every flight: maximum riser, minimum tread and variation across the flight. Two of them can pass while the third fails.
Where to look it up
IRC Chapter 3, Section R311.7.5. The uniformity language sits with the riser and tread provisions themselves.