IRC residential vs IBC commercial
| Requirement | IRC (residential) | IBC (commercial) |
|---|---|---|
| Maximum riser heightcommon | 7-3/4" | 7" |
| Minimum riser height | 4" | 4" |
| Minimum tread depth | 10" | 11" |
| Maximum variation in a flight | 3/8" | 3/8" |
| Minimum stair width | 36" | 44" (typical) |
| Minimum headroom | 6' 8" | 6' 8" |
| Handrail height above nosing | 34"–38" | 34"–38" |
| Guardrail height | 36" | 42" |
| Maximum sphere through guard | 4" | 4" |
| Nosing projection (tread under 11") | 3/4"–1-1/4" | 3/4"–1-1/4" |
| Maximum rise between landings | 12' 7" | 12' |
IRC 2021 R311.7 and IBC equivalents. Local jurisdictions amend these — always confirm with your building department before cutting stringers.
The ⅜ inch rule that fails inspections
Within any single flight, the tallest riser may not exceed the shortest by more than ⅜ inch. The same tolerance applies to tread depths.
Almost every failure traces to the same cause: measuring total rise to the subfloor and then adding finish flooring at only one end. If the upper floor gets ¾ inch of hardwood and the lower stays bare concrete, every riser shifts and the bottom one ends up ¾ inch off.
The second common cause is the bottom stringer cut. When you notch a stringer, the bottom riser finishes taller than the rest by exactly the tread thickness unless you trim the stringer foot by that same amount. This catches experienced framers.
Measure finished floor to finished floor before you cut anything, and account for the tread material in the stringer foot.
Comfort beyond code
A stair can pass every code check and still feel wrong. The Blondel rule predicts it: 2 × riser + tread should fall between 24 and 25 inches.
A 7 inch riser with a 10.5 inch tread gives 24.5 — comfortable. A 7¾ inch riser with a 10 inch tread gives 25.5, which is legal but reads as steep and tiring. A 6 inch riser with a 14 inch tread gives 26, legal but stretched, forcing an unnatural stride.
The comfortable angle range is 30 to 35 degrees. Below 30 the stair consumes floor space without feeling easier; above 37 it starts to feel like a ladder, particularly on the way down.
Before you cut
- Measure total rise from finished floor to finished floor, including flooring that has not been installed yet.
- Divide by 7 and round to the nearest whole number to get your riser count, then divide total rise by that count for the actual riser height.
- Confirm the resulting riser is at or under 7¾ inches. If not, add a riser and recalculate.
- Check that total run fits the available floor space before committing — a code-compliant stair that lands in a doorway is still a rebuild.
- Cut one stringer and test-fit it in place before cutting the rest. A ⅛ inch error repeated across three stringers is three wasted 2×12s.
- Use 2×12 stock minimum. After notching you need at least 5 inches of remaining throat, and a 2×10 does not leave enough.
Frequently asked questions
What is the maximum stair riser height?
7¾ inches for residential stairs under the IRC, and 7 inches for commercial under the IBC. Risers within one flight cannot vary by more than ⅜ inch.
What is the minimum tread depth for stairs?
Ten inches residential, 11 inches commercial, measured nosing to nosing. If the tread is under 11 inches, a nosing of ¾ to 1¼ inches is required.
How wide do stairs have to be?
Thirty-six inches minimum for residential, measured above the handrail height. Commercial stairs are typically 44 inches, though occupancy and egress load can require more.
When does a stair need a handrail?
Any flight with four or more risers needs at least one handrail, mounted 34 to 38 inches above the nosing line.
How much can stair risers vary?
Three-eighths of an inch between the tallest and shortest in a single flight. This is the most commonly failed stair inspection item.