A spiral stairway or staircase is another type of stair system that the code allows in-lieu of conventional stairs. When a spiral stairway is installed within an individual dwelling unit, it must comply with the International Residential Code (IRC).
Unlike buildings subject to the International Building Code (IBC) where there are limitations to the use of spiral stairs in these buildings, dwelling units subject to the IRC have no limitations to the use of this type of stair as long as it meets the size and clearance requirements within Section R311.7.10.1 of the IRC.
Fun Tip: To learn the difference between the International Residential Code (IRC) and the International Building Code (IBC), be sure to check out this POST.
Although this type of stair system might be difficult to use or even move furniture around, it is allowed under the International Residential Code.
Therefore to understand if a spiral staircase is compliant, let us take a look at code section R311.7.10.1 within the 2018 International Residential Code (IRC).
Spiral Staircase Width
What is the required width of a spiral staircase?
The clear width at and below the handrails at spiral stairways shall be not less than 26 inches and the walkline radius shall be not greater than 24-1/2 inches.
Since this is a type of stair that is supported around a center pole, it has several dimensions it must meet. To best illustrate this, see the image below.
Although you might not understand the reason for the walkline dimension here, you will when we get into the minimum tread depth.
Tread Depth for Spiral Staircase
What is the minimum depth of a spiral staircase tread?
Each tread shall have a depth of not less than 6-3/4 inches at the walkline. Treads shall be identical.
Unlike conventional stair treads that can have a tolerance of no more than 3/8 inch, spiral staircase treads must be identical meaning no variation in size.
Since the minimum depth is measure from the walkline, it is best to understand where the ‘walkline’ is located along the tread. This term is not defined in the definition but it is mentioned in the conventional stair section found in R311.7.4.
It says that the walkline shall be located 12 inches from the inside of the turn, at the widest point of the clear stair width at the walking surface. Basically from the inside radius of the stair.
To best illustrate this, see the image above.
Riser Height for Spiral Staircase
What is the maximum height of a spiral staircase riser?
The rise shall be not more than 9-1/2 inches.
The riser height can be of any dimension from riser to riser, just not more than 9-1/2 inches.
To best illustrate this, see the image below.
Headroom Clearance for Spiral Staircase
What is the minimum headroom clearance for a spiral staircase?
Headroom shall be not less than 6 feet 6 inches.
The spiral stairs must have at least 6′-6″ of headroom clearance measured vertically from the leading edge of the tread.
To best illustrate this, see the image above.
So this basically sums up the basic code requirements for spiral staircases within residential dwelling units that are subject to the International Residential Code (IRC).
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* Reference Source – 2018 International Residential Code – [Buy on Amazon]
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