Calculation Results
About This Calculator
Calculate deck stair stringer cuts, total rise and run, individual unit rise (IRC max 7-3/4") and run (min 10"), bottom riser thickness deduction, and 2x12 lumber length.
Frequently Asked Questions
What are the International Residential Code (IRC) stair dimensions?
The IRC mandates that the maximum riser height cannot exceed 7-3/4 inches (7.75"), the minimum tread depth cannot be less than 10 inches, and the difference between the largest and smallest riser in a flight cannot exceed 3/8 inch.
Why must you cut the bottom of the stair stringer by the tread thickness?
Because you add a tread on top of every step, failing to trim the tread thickness (e.g. 1.0 inch) off the bottom foot of the stringer makes the bottom step one inch too high and the top step one inch too low, violating building codes and creating a dangerous trip hazard.
What size lumber must be used for cut deck stringers?
Building codes require solid 2x12 structural lumber for cut stair stringers. After cutting out the tread and riser triangles, at least 5 inches of solid wood throat must remain between the cut notch and the bottom edge of the board.
How many stringers are needed for a 36-inch wide deck staircase?
For standard solid wood treads, stringers can be spaced up to 16 inches on center (requiring 3 stringers for a 36" wide stair). For composite decking (such as Trex or TimberTech), stringers must be spaced no more than 12 inches on center (requiring 4 stringers).