Measuring spiral stair tread minimum width?

[RBerryman]

We are trying to fit spiral stairs in a 62" square opening. The code says "Spiral stairways are permitted, provided the minimum width shall be 26 inches with each tread having a 7 1/2-inches minimum tread depth at 12 inches from the narrower edge".

When the treads overlap, where is the 12 inches measured from: the pole or the edge of the stair tread as seen from above? Thanks in advance.

[John Kogel]

For safety, go with the widest tread possible, exceeding the min, in other words. You wouldn't want to be under by mistake.

[Jerry Peck]

Not being in my office or having the codes with me, and using the code wording you posted, if you used a square, placed the square tight to the leading edge of the thread, then slid the square toward the pole until the square touches the pole, then measured from the edge of the square touching the pole out 12 inches, marking the 12 inch point on the leading edge of the tread.

Now slide the square from the pole out to the mark, making sure the square is still tight to the leading edge of the tread, measure back from the leading edge along the square to 7-1/2 inches, draw with line along the square and mark the 7-1/2 inch point.

Now place the bottom edge of the square on the tread and on that line with the other leg of the square vertically up - the square should be able to stand vertically without the tread above hitting the vertical leg of the square.

If the tread above is in the way of the square then the tread is not 7-1/2 inches deep at 12 inches out from the narrow end.

[RBerryman]

Very clear and lucid explanation. It matches what I have gleaned from other codes. Thanks.

[Jerry Peck]

Very clear and lucid explanation. It matches what I have gleaned from other codes. Thanks.

Your drawing shows different measurements, not as I described. Just clarifying that you should not use the drawing you posted.

[Robert Koen]

Heres something that might help. Its from "Carpentry" fourth edition, Floyd Vogt.

[Jerry Peck]

R311.7.5.2 Treads.
- The minimum tread depth shall be 10 inches (254 mm). The tread depth shall be measured horizontally between the vertical planes of the foremost projection of adjacent treads and at a right angle to the tread’s leading edge. The greatest tread depth within any flight of stairs shall not exceed the smallest by more than 3/8 inch (9.5 mm).


R311.7.5.2.1 Winder treads.
- Winder treads shall have a minimum tread depth of 10 inches (254 mm) measured between the vertical planes of the foremost projection of adjacent treads at the intersections with the walkline. Winder treads shall have a minimum tread depth of 6 inches (152 mm) at any point within the clear width of the stair. Within any flight of stairs, the largest winder tread depth at the walkline shall not exceed the smallest winder tread by more than 3/8 inch (9.5 mm). Consistently shaped winders at the walkline shall be allowed within the same flight of stairs as rectangular treads and do not have to be within 3/8 inch (9.5 mm) of the rectangular tread depth.

R311.7.5.2 Treads. says the depth is measured at a right angle to the tread's leading edge.

R311.7.5.2.1 Winder treads. says the depth is measured at the intersections with the walkline.
- To me this says that the point of measurement for each winder tread depth of a wider is measured at the point of the walkline, but the depth is measured at right angles to the leading edge in accordance with R311.7.5.2.1 for the depth, however ... it could also be saying that winder tread depths are measured from a point on a tread at the walkline from the leading edge to the adjacent leading edge at a point on the walkline - except that the drawings in the code commentary shows winder tread depth is measured at right angles to the leading edge.

That drawing is based on the erroneous reading that the tread depth is measured from the points of the walkline. My drawing is from an older edition of the IRC, I have not updated the drawing, but the intent and drawing is currently the same.

[RBerryman]

If I understand this correctly, "Measuring winder tread depths from a point on a tread at the walkline from the leading edge to the adjacent leading edge at a point on the walkline" would look like this. Measuring at 90 degrees at the start of the walkline makes almost no difference in this case. So my tread is going to have to be made a little wider at the ends to meet that 7 1/2" requirement. Thanks for all of your help.
30094

[Jerry Peck]

If I understand this correctly, ...

Which you are not.

Refer to the drawing I posted or to the two drawings from the IRC Commentary I posted - both show the same thing ... which is not what you show in your drawing.