spiral stair settings

[steve]

What riser and run settings and location line run settings do I use to get a spiral stair that has these code requirments:

Spiral stairways are permitted, provided the minimum width shall be 26 inches (660 mm) with each tread having a 7 1⁄2 inch (190 mm) minimum tread depth at 12 inches from the narrower edge. All treads shall be identical, and the rise shall be no more than 9 1⁄2 inches

I keep getting a 36" Radius stair with a +/- 1'-5" outside tread.  

[ToanDN]

Sketch a code compliance diagram using drafting lines and see what you will need for the stair.

[ToanDN · Accepted solution]

Examples:

[steve]

I see, so it can only spec the distance between risers at the CL of the run.  Thanks

[ToanDN · Accepted solution]

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@steve wrote:

I see, so it can only spec the distance between risers at the CL of the run.  Thanks

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Correct.  For example:

[ToanDN · Accepted solution]

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@steve wrote:

I see, so it can only spec the distance between risers at the CL of the run.  Thanks

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If you know the inside radius (R1) and outside radius (R2) of the spiral stair, and you are a math guy not a visual guy, you can use this formula to calculate the Tread Width for your spiral stair:

Tread Width = ((R1 + R2) * 3.75") / (R1 + 12")