Because they are implemented to ensure safety for all kinds of people, especially children and the elderly, building codes are very specific and must be followed. Stairs have their own set of rules within the code and it is important to learn them when planning your stairs. Every part of a staircase needs to be designed accordingly, including even the smallest parts to the overall structure.
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Rise is the overall height of the entire staircase, from the lowest point to the highest point. Staircases vary in shape, but their rise is an important measurement for determining where landings are placed. Typically, a landing must be provided for every 12 feet (365.8 cm) of rise, as per fire safety codes. Other than the code for landing placement, there is no rise length requirement since every staircase is different.
Run refers to the length of the staircase, regardless of height. Run is used in conjunction with the rise and helps determine how many stairs there will be. Each individual step also has its own unit rise and unit run -- the rise and run of each individual step. Standardized codes with unit rise and run requirements make it easier to design a staircase, regardless of run of the entire staircase. A staircase's run, however, is important if the staircase has one or more landings because there will be multiple run lengths. These will vary depending on the particular staircase that you are building.
Width refers to both the individual step as well as the entire staircase. The width of one stair, after all, will need to be consistent with the rest of the stairs. The stair's width usually varies depending on the type of building the staircase is in, but for a normal residence, the standard tends to be 3 feet, 6 inches (106.7 cm). The minimum, in most places, is 2 feet 8 inches (81.3 cm). If a staircase exceeds 44 inches (111.8 cm), handrails are required for both sides.
Unit Rise and Run
Unit rise is the vertical measurement of the height between two individual stairs. Unit run is the horizontal measurement of from the front to the back of a single stair, which is known as the "tread." Standard code requires that each stair tread fall between 9 inches and 11 inches (22.9 cm and 27.9 cm) to allow people to safely ascend and descend stairs. Unit run is carefully considered based on both safety and cost. Increasing the length of the run by the smallest unit of measurement increases cost and the length of a staircase. Unit rise is normally about 7.5 inches (19.1 cm).