Understanding Stair Terminology
Rise: The vertical height of each step. Run: The horizontal depth of each step (tread depth minus nosing). Stringer: The diagonal boards that support the treads and risers.
Building Code Requirements (IRC)
| Measurement | Residential | Commercial |
|---|---|---|
| Max Rise | 7-3/4" (7.75") | 7" |
| Min Run | 10" | 11" |
| Min Tread Depth | 10" (w/ 1" nosing) | 11" |
| Min Width | 36" | 44" |
| Max Variance | 3/8" between steps | 3/8" |
| Min Headroom | 6'-8" | 6'-8" |
The 17-18 Rule
For comfortable stairs, the sum of one rise plus one run should equal 17-18 inches. Example: 7" rise + 11" run = 18" (comfortable). This ratio ensures a natural walking stride up the stairs.
Ideal Stair Dimensions
| Rise | Run | Sum | Comfort |
|---|---|---|---|
| 7" | 11" | 18" | Ideal |
| 7.5" | 10" | 17.5" | Good |
| 7.75" | 10" | 17.75" | Code Max |
| 6.5" | 12" | 18.5" | Gentle |
Stringer Sizing
- 2x12 Stringers: Required for cut stringers to maintain 3.5" minimum throat after cuts
- Spacing: Stringers should be no more than 16" apart for 2x treads, 12" for decking
- Support: Top of stringer needs solid blocking attachment to deck/floor
- Bottom: Stringers should rest on concrete pad or secured to landing
Stair Building Tips
- Measure total rise at multiple points - floors may not be level
- Calculate carefully - all rises must be equal within 3/8"
- Account for finish materials when calculating (floor covering, tread thickness)
- Cut one stringer first and test-fit before cutting others
- Use a framing square with stair gauges for accurate marking
- Handrails required for 4+ risers (30-38" height for residential)