Stairway Rise Angles

Quote of the Day

History is a tool used by politicians to justify their intentions.

— Ted Koppel

Figure 1: A Well-Designed Stairway is
Mathematics Carved Wood (Source).

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.

Figure 2: JLC Graphic Showing IRC Maximum Angle.

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°.

Figure 3: Angles for Common Stairway Risers and Tread Lengths.

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.