I need some help configuring a set of curved stairs. The total rise is 11 feet. The inside radius would be about 7’ and the treads will be about 4 feet wide. Assuming the total run length can be what I want, what is the ideal rise and run for such an animal? AWI gives minimum allowable sizes, but I want to know what feels best.
(Architectural Woodworking Forum)
From contributor E:
The general rule of thumb (or is it toe?), for stairs is to try to get close to a 7/10 layout; 7 inches of rise, and 10 inches of run. In the case of a curved stair, I would try to figure the 10 inch run in the "walk line" of the treads, and find a number near 7" that divides evenly into your total rise. I think that it is better to go above the 7" figure, rather than below it, because smaller rises start to feel funny. There are some building codes concerning the width of the stair at the minor radius, but this may not apply to your layout.
What feels best is up to you. Your run at the line of travel is what's important. Line of travel is 12" in from the inside radius.
AWI isn’t a good source of stair-building specifications, but rather a great resource for acceptable tolerances. Please review your local codes and/or review a copy of the International Code Councils – IBC or IRC (residential version) - Building Code. You can also find a great visual interpretation of the SMA website, stairways.org.
But contributor H is right - the run can be as wide as allowed, but the rise based on an eleven foot floor to floor will be only one of three. 7" is a little shallow for my taste and 8" is just plain too steep. I always shoot for between 7.25" and 7.75" if possible.
Laying out the treads is easy once you familiarize yourself with all this. Forming the stringers with two different rises and runs can be a little more trying, as the hypotenuse of a right triangle does in fact change when wrapped around a cylinder. Mathematically, no, but this is the real world and stringer thickness, length of tread tenons, etc. will dramatically change the dynamics.