Stairs should be installed at an angle between 30 and 50 degrees from the horizontal. The most common and comfortable stair angles tend to range from about 35 to 45 degrees, balancing safety, convenience, and space efficiency. For example, angles around 37 degrees are considered optimal for comfort and safety.
Industrial or steeper stairs may approach the higher end of this range, close to 50 degrees, but beyond that, stairs become more ladder-like and less safe for general use. Shallower stairs with angles closer to 30 degrees are more comfortable but require more space.
In summary, the recommended installation angle for stairs is generally between 30° and 50° from horizontal, with about 35° to 45° being optimal for typical use.