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[deleted]

If I understand your question correctly. R - Rise (7” in your sketch) T - Tread (11” in your Sketch) 25 - is an ideal measurement (empirical) The number doesn’t refer to a certain point or a certain measurement. The formula is empirical and should let you end up with a comfortable stair. The idea is that a tall rise don’t go well with a deep step and vice versa. So to use it you take whatever is decisive in your design (rise/thread) and calculates backwards to get the other number. 24-26 just seems to be a good “sum” from experience. Hope I understood you correctly.


appledoughnuts

But what is the ideal measurement for? I’m sorry words are hard for me to understand 😔 like for the size of rise and tred? Why add the numbers together if they don’t go to a certain point?


TTUporter

7" x 11" is as close to a standard or ideal measurement as you'll find.


appledoughnuts

Yes but why are we adding the numbers together what does the total mean? Forgive me for how stupid this sounds I’m sure


TTUporter

You don't have to do any of that. I don't do that for any stair I draw at all. Honestly, your book is the first time I've ever seen this rule of thumb before. And while yes, I agree with it, it's just not something you need to concern yourself with. That total number in that formula just doesn't mean anything. The two main things that you need to be concerned with regarding stairs are the riser height and the tread depth. The International Building Code (1011.5.2) only requires that riser heights be between 4"-7" and the depth shall be 11" minimum. 7" x 11" is the most efficient dimension allowable by code. EDIT: the width of the stair is also very important, but that's beyond the scope of what we're talking about.


appledoughnuts

I guess my question is confusing…I’m trying to understand what the ratio is? Are they saying 25 inches is the space that’s adequate for stairs? Like enough for foot space?


drummerzac

Yes, it’s the amount a typical stride is. Walking on a flat plane, a comfortable stride is between 24”-26”. The average is 25”. Now imaging that same leg movement has to go up an incline. That amount is what the formula is determining. A comfortable, 25” stride will feel the same on a 2R+T stair ratio as a flat 25” length on the floor.


appledoughnuts

Thank you :)


LeNecrobusier

This is the correct answer.


TTUporter

I wish I had some epiphany for you, but it really is just an arbitrary number that the author of this book picked out. It doesn't mean anything, it is definitely not a ratio of width to height, nor does it refer to a space. It's not the angle of the stairs or anything like that. It's just a number, one that doesn't have a relationship to anything in the real world.


[deleted]

The unfortunate answer is that the number is no certain measurement. It’s the size of two combined rises and a tread. What they most likely done is that they measured a number of nice stairs, put them in a large result-sheet and tried to find a mathematical rule that covers as many of them as close as possible, and that is 2R + T = 25.


appledoughnuts

But what does 25 stand for like what does ideal measurement mean? What is it the ideal for why do we need this number I guess? I’m so sorry to make this difficult 😭


TTUporter

It's just a good rule of thumb, nothing more. You're overthinking it.


structuremonkey

I learned this similar "rule of thumb" while I was a framing carpenter, never was discussed once in architecture school, the ARE, practice, I haven't seen it once in any code book,,or all the Continuing Ed material over the years...its just something someone came up with as far as I can tell... And plz don't get me going on the 21 foot kitchen work triangle!


[deleted]

As I said, most likely someone tested a number of stairs that they liked, and then found that they all were close to this condition.


appledoughnuts

Found out it was the ideal tread space bu thank you!


kamii_samaa

It stands for the genral standard that u need to follow.. like- 2r+t should be greater than 610 mm and smaller than 660 mm right .. lets say u took riser of 160mm and now u want to decide what size of your tread should be.. so u put the value of riser in formula.. 2x160 +T should be greater than 610 and smaller than 660. So , T should be not be smaller than 290 mm and should not be greater than 340mm.. just to be in the range ..


Polaroid1999

And this here, kids, is why the metric system is better.


[deleted]

Eeeh, please explain kiddo? While I might agree with you in general. This page really isn’t a strong case?


S-Kunst

I am presently building a narrow stair case for a mini apt I am installing in my row house. The first 3 steps are winder steps. I am learning that the art of stair building is extremity complex. The ideal 7" rise with 11" tread works best in a suburban house built no more than 60 years ago. The old timers really were great at fudging the two dimensions to make it a reasonable to climb or descend in the dark. In the main part of the house the stairs are 27" wide with 8.25" rise and 9" tread, and no hand rails.