According to stair design rules, what should be the sum of two risers plus one tread?

The correct range for the sum of two risers plus one tread being 24 to 25 inches is based on standard stair design principles aimed at creating safe and comfortable stairs. This guideline is rooted in human ergonomics, where the dimensions of risers (the vertical portions) and treads (the horizontal portions) need to accommodate average step lengths and heights for most people.

When designing stairs, it is vital to ensure that the combined height of two risers (which represents two consecutive steps) along with the depth of one tread allows for a natural and smooth motion while ascending or descending. The ideal range of 24 to 25 inches helps maintain a rhythm and flow that reduces the risk of tripping or misstepping.

Options outside this range either make the stairs too steep or too shallow, complicating safe navigation and potentially causing discomfort or accidents. Thus, maintaining the sum within this specified range of 24 to 25 inches is essential in stair construction for ensuring usability and safety.