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Spiral Stair Code Question

globaldiver | Posted in General Discussion on April 3, 2010 12:54pm

I am finally getting around to building and installing the spiral staircase for my long-suffering shop project.  This staircase accesses a 300 sq ft office space.  It must be code compliant, and I just can’t get my head around one aspect of the various parameters required for code compliance.

Code says:

1.  minimum tread width at 12″ from narrow end of tread must be 7.5 inches;

2.  tread width minimum of 26 inches (no problem for me as I’m going to be about 35 inches wide)

3.  maximum 9.5 inch rise

4.  all treads must be identical.

There are some other requirements not relevant to my issue, all of which my design meets.  These treads will wind around an existing 4.5 inch steel column that supports the corner of the office space overhead.  The treads are all a basic pie-shape, with the pointed end removed to fit around the steel post.

I have a layout/tread design which works fine meeting the code IF the small width of the tread (the “inside” width) is about 2 13/16 wide.  At that width, the stairs overlap about 2 inches at the post — the next tread up overhangs the lower tread by about 2 inches at the post.  This overhang diminishes evenly to the outside edge of the tread.

Measured at 12 inches from the post, the tread depth is in excess of 7.5 inches.  That is, of course, if I ignore the overhang at that point.  The actual overhang at that point is 1 5/16.

In a regular stair, the code seems clear that you measure the minimum tread depth only to the point the nosing overhangs the riser. 

Does that rule also apply to spiral stairs?  If it does, I’m having difficulty coming up with any rise/run calculation that meets the code requirements. 

I suppose it would be possible to alter the shape of the tread to remove the overhang down to the 12″ dimension, but the commercial spiral stairs I look at don’t seem to have that modified shape. 

I’m grateful for any words of wisdom.  I have a sketchup file showing this, but I don’t think the forum allows that format to be uploaded.

–Ken

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Replies

  1. Snort | Apr 03, 2010 08:34am | #1

    What building code are you using?

    If you are getting a generic code interpretation off the internet, it may not be the same as Idaho's, which may not be the same as your county's.

    NC code calls for 9" depth at 12" out, and a minimum depth of 6", so unless you have a problem with it, I'd call inspections and see how they interpret.

    I think you can post sketchup files.

    1. globaldiver | Apr 03, 2010 12:19pm | #5

      I think it is the '03 code; it is the code applicable to my permit, not just a generic web find.

      NC is 9 inches!  Wow.  That's for spiral and not just for a winder or curved stair?

  2. DanH | Apr 03, 2010 09:05am | #2

    Increase the diameter of the post.  Or just start the tread several inches out (attach some sort of blocking to the tread to render the inner portion unusable).

    1. globaldiver | Apr 03, 2010 12:11pm | #3

      tried that....

      I do have a design that starts the narrow end of the tread away from the post. It is about 3 inches.  The post is already in place, and it is structural, so I'm not changing it. 

      The increased width makes the total size very large, so I was interested in the clarification of the code issue as a means of reducing the overall diameter.

  3. globaldiver | Apr 03, 2010 12:17pm | #4

    The sketchup file should be attached.

    File format
  4. Clewless1 | Apr 03, 2010 02:36pm | #6

    I'm a bit rusty on this, but some food for thought. I always thought the tread depth was from riser to nose, so whatever the nose overhang is shouldn't affect your design.

    Also ... this is a code question, so in addition to our feedback, it's important to connect w/ the local BO and have a frank discussion with him about your initial approach. Ask his opinion/interp of some of the specific req.

    What specific jurisdiction are you in? I've done work in that neck of the woods; might be able to give you some additional insight.

    Also ... there may be another requirement that may affect you that I haven't resolved myself ... max 4" sphere can't pass through between the treads, so if you have e.g. 1" treads and a 7" rise, you have a 6" gap ... may affect the design of the treads to limit the space between.

    1. globaldiver | Apr 03, 2010 04:22pm | #7

      That's probably the best approach

      I will just give the inspector another call.  I can't seem to find anything that clarifies it.  This is in Boise, Idaho.

      I did ask the inspector last week about the between-tread spacing.  The code applicable to this project didn't require blocking between the treads (you can have "open" treads).  The balluster spacing must not allow a 4" sphere pass through, though.  Apparently a more recent revision also added that requirement to the treads, too.

      --Ken

      1. Clewless1 | Apr 06, 2010 09:42pm | #8

        Good Plan. While our comments lend food for thought, ultimately the specific inspector will make the determination. With the stair codes always seeming to change, it can be hard to keep up. Pretty soon, you won't be able to even have stairs. I've seen some pretty nice stairs ... many of them violate current stair codes w/ sometimes no railing at all on the open side or big spaces between ballusters. I started a house about 5 years ago and learned about the open tread requirement having to be the same as the ballusters ... max. 4" sphere through any part of the balluster/tread openings.

        Keep us posted on the details of what you find out.

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