Inclined stairways are the most common method of traversing floors in private homes because they are sturdy, require no external energy for use and have long histories with builders for their construction. However, builders may find traditional stairs take up too much living space, especially in small areas. Spiral stairs provide the same multi-floor access in nearly half the square footage that is required for standard stairs.
Contractors and manufacturers rely on two types of standards when determining the feet needed for spiral stairs. First is the International Residential Code, which specifies regulations for single- or dual-family residences. Second is the International Building Code, which applies to multi-family residences or commercial structures. Both codes come from the International Code Council, a member organization founded in 1994. Local, state and some federal building codes follow these building codes either directly or with small modifications.
The 2012 International Residential Code mandates that the minimum tread widths of spiral staircases be 26 inches or 2.16 feet below the handrail. Each step must have a minimum depth of 7.5 inches or 0.625 feet, as measured 1 foot from the narrower edge of the tread. Treads must have identical measurements, with a rise (vertical separation) of under 9.5 inches or 0.79 feet. The stairs need minimum headroom of 6.5 feet. The 2012 International Building Code specifies the same measurements as the residential code. It further stipulates that spiral staircases may be used as the only way of leaving a space inside residential units, or in spaces under 250 square feet with five or fewer occupants.
Because the spiral stair section of the International Residential Code says nothing about handrails, builders must rely on the codes that apply to standard stairs when installing these required fixtures. Spiral staircases require at least one handrail on one side for each continuous run of four or more steps. The rail must be from 34 inches (2.83 feet) to 38 inches (3.16 feet), as measured vertically from either the sloped plane of the tread nosings or the ramp slope. Rails must be continuous from the lowest riser to the highest riser of the staircase, and be at least 1.5 inches from any adjacent walls.
Spiral staircases are fairly complex, requiring the experience of contractors and precise calculations when built from scratch. Fortunately, many manufacturers offer modular versions that the home handyman can install. Because many building codes require spiral staircases to be at least 5 feet in diameter, modular versions start with the same dimensions. The space available for the stair must be the same upstairs and downstairs, and must be a minimum of two to three inches larger than the structure. For example, a 5-foot diameter spiral stair needs at least an area that measures 5 feet 2 inches square.
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