International Residential Code (IRC) Stair Guidelines
Building a safe, code-compliant staircase requires precise adherence to the International Residential Code (IRC R311.7). Uniformity is the single most critical safety requirement: the greatest riser height within any flight of stairs cannot exceed the smallest by more than 3/8 inch (9.5 mm).
Core Residential Stair Specifications
| Stair Component |
IRC Code Requirement |
Ergonomic Best Practice |
| Riser Height |
Maximum 7-3/4 inches (196 mm) |
7.0 to 7.5 inches |
| Tread Run |
Minimum 10.0 inches (254 mm) |
10.5 to 11.0 inches |
| Stair Width |
Minimum 36 inches clear width |
38 to 42 inches |
| Headroom Clearance |
Minimum 6 ft 8 in (80 inches) |
7 ft 0 in (84 inches) |
| Handrail Height |
34 to 38 inches vertically from tread |
36 inches |
| Stringer Lumber |
2x12 dimensional lumber with 3.5" throat |
Solid No. 2 Douglas Fir or SPF |
The Ergonomic Comfort Rule (2R + T)
For centuries, carpenters and architects have relied on Blondel's Formula to verify that stair proportions provide a natural, effortless walking stride:
2 × Rise(in) + 1 × Tread(in) = 24 to 25.5 inches
If your calculated rise is 7.25 inches, the ideal tread depth is: 25 - (2 × 7.25) = 10.5 inches.
Stringer Board Selection & Cutting
Stair stringers bear the entire structural load of the staircase. Always use 2x12 No. 2 dimensional lumber free of large knots. After cutting your riser and tread notches, ensure the uncut throat of the stringer retains at least 3.5 inches of continuous solid lumber. For exterior decks or stairs wider than 36 inches, install stringers every 16 inches on center (minimum 3 stringers).
Frequently Asked Questions About Stair Framing
What is the maximum stair riser height allowed by building code?
Under Section R311.7.5.1 of the International Residential Code (IRC), the maximum riser height for residential stairs is 7-3/4 inches (196 mm). Commercial staircases governed by the IBC allow a maximum rise of 7 inches.
What is the minimum tread depth for stairs?
The IRC mandates a minimum tread depth of 10 inches (254 mm) measured horizontally from the nosing of one step to the nosing of the next. When treads are less than 11 inches, a 3/4-inch to 1-1/4-inch nosing projection is required.
What is the standard rule for comfortable stair proportions?
The traditional rule of thumb (Blondel formula) states that 2 × Rise + 1 × Run should equal between 24 and 25.5 inches. For example, a 7.25-inch rise with a 10.5-inch run equals 25 inches, representing the ergonomic gold standard.
What lumber size is required for cutting stair stringers?
Stair stringers must be cut from 2x12 dimensional lumber. After cutting out the notches for the treads and risers, building codes require at least 3.5 inches of solid uncut wood (the stringer throat) along the back edge to maintain structural integrity.
How much headroom is required above a staircase?
The IRC requires a minimum headroom clearance of 6 feet 8 inches (80 inches / 2,032 mm) measured vertically from the leading edge of the stair tread nosing to the ceiling plane directly above.
Why is the bottom riser cut shorter than the other risers?
When installing finished tread material (such as 1-inch oak or 5/4-inch deck boards), you must deduct the thickness of the tread from the bottom of the stringer. If you do not trim the bottom of the stringer, your first step will be too tall and your top step will be too short, creating a dangerous code violation.
When is a stair landing required by building code?
Under the IRC, a floor or landing is required at the top and bottom of each stairway. Additionally, a flight of stairs cannot have a vertical rise greater than 147 inches (12 feet 3 inches) between floor levels or landings without an intermediate landing.
What is the difference between open and closed stringers?
An open (cut) stringer has triangular saw cuts where the treads and risers rest on top of the board. A closed (housed) stringer has routed grooves or cleat supports on the inside face where the treads and risers slide into the wood, concealing the step edges.
