Quote of the Day
History is a tool used by politicians to justify their intentions.
— Ted Koppel
I was reading this month's Journal of Light Construction (JLC) when I saw an article on building safe stairways per the International Residential Code (IRC). I have written about stairway rise angles before, but from the standpoint of three commonly used stairway design rules of thumb. The IRC provides a hard upper limit that is lower than the maximum permitted by the most liberal of these rules of thumb.
The JLC article (which I cannot find online) provides an interesting graphic (Figure 2) that shows a maximum stair angle of 37.78° (red line).
These angles are important because people are quite sensitive to even minor changes in stair rise angles.
The IRC sets the maximum allowed stairway rise angle by setting the maximum rise and minimum run. Figure 3 shows how to use these values to calculate the maximum stairway rise angle (green highlight) allowed by the IRC. I have also included the stairway rise angles for some other common rise/run combinations. The vast majority of stairways have rise angles between 30° and 35°.
I have to admit that I love designing stairs and roofs. There is something magical about using a few formulas to create something that is both beautiful and functional.
"Tread" appears to represent the total depth of the flat piece of wood you step on - typically called the tread!
But slope ratios are properly based on the run - which is the horizontal increment from one riser to the next, or from one nosing to the next.
Run = tread - nosing overhang
Good catch! I have updated the post and included a reference with the rise and run mandates.