Curved Stairs Riser and Tread Dimensions

Stair geometry is tricky even on a straight run. Here, woodworkers wrap their heads around a curved staircase layout question. December 7, 2007

Question
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.

Forum Responses
(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.

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From contributor H:
You only have three choices for a rise:
6.94, aka 7"
7.33, aka 7 3/8"
7.76, aka 7 3/4"

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.

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From contributor J:
Assuming the total run length can be whatever you want indicates that you haven't laid this out, and do not know where it will actually start. For this reason, you have not yet determined your total rise. Dropping a tape straight down from the landing, or going off of the plans, are recipes for disaster. If you want this to go smoothly, get out your water level.

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From contributor A:
Assuming this is a residential structure, a good rule of thumb to use is (2 Rise + 1 Tread) = 24 to 25 inches. This measurement is exclusive of the nosing. This geometry is very comfortable regardless of your tread size.

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.

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From contributor P:
"Line of travel" or "datum line" is that area 12" in from the inner radius on the inside of the tread you have to comply with code. At that point you have to have a minimum nominal measurement on tread depth to meet code. This is where your stair should be laid out from, as the rise and run is different from inside stringer to outside stringer. I go to the site when possible and transfer all points down with a plumb-bob and swing an arc with a trammel, lay out full scale, copy all measurements to notebook, and then redo another full scale layout including tenons, rabbets, dadoes, etc. based on all the given variables at my shop.

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.