My home is antique and has 9" x 9" stairs (extremely steep). I'm having them rebuilt adding only one stair since I am limited by floor space and headroom. They will then have an 8.1" rise and I would like them to have a 10" run because it feels roomy but the formula (2X rise) + run is supposed to be within an inch of 25. The only way to get it below 26 would be to reduce my run which I really didn't want to do.. What should I do?
Code dictates a tread depth of no less than 10". The rise should be no more than 7 3/4", but since your house had an original stair system that predated anything like that they require now, "grandfathered" is the term that comes to mind. If your structure, as in walls, floors etc., keep your steps from being brought up to code, there is an exception to the code that you can go to.
With that in mind, do as you plan, add the extra tread and increase the depth of tread to 10", that is a close as you can get to code without moving walls etc.
You will appreciate the difference...
The Code requires a tread depth of no less than 9”. (See ICC R3188.8.131.52.). However, it also states that the maximum difference between “any tread” can’t be more than 3/8”.
Btw, risers can’t be more than 8”. (R3184.108.40.206.) Ditto on the 3/8” diff.
Interesting fact. There is a recommended “comfort range” of riser height to tread depth. (That’s why those 18” treads going up to the Lincoln Memorial has 3” risers.) I remember 7:11, but you can probably google it if you need it.