When building a staircase, the width and depth of the treads and the height of the risers are essential to the safety and functionality of the finished stairs.
The dimensions of the treads and risers are regulated by building codes. You must check with the municipality in whose jurisdiction the property is located, for its specific code sizes for treads and risers. Failure to comply with building codes can result in fines and building projects being closed until code compliance is reached.
According to the International Code Council the minimum width permitted in residences is about two feet eight inches, although three feet is preferred, and three feet six inches is the standard for normal occupancy. The unit run (depth of tread) should be between nine and eleven inches.
When stairs turn, such as in the case of spiral stairs, it is important to ensure that the stairs comply with code by measuring them at a specific point on each step. According to Don Vandervort's Home Tips website many codes demand tread depth to be measured and within code standards at a point twelve to fourteen inches from the narrow side.