LESLIE: Well, calculating the rise and run of a staircase can be maddening but the actual construction is rather straightforward.
TOM: Well, that’s right. But if you get it wrong, it can be a real tripping hazard. Here with some tips to get it right is Tom Silva, the general contractor for This Old House.
TOM SILVA: Well, thank you. It’s nice to be here, as always.
TOM: You know, invariably, when I walk up and down a step, if I occasionally catch my foot or something feels a little bit off, I’ll look at it. It will be because some part of the stair was not built correctly. Maybe the last step was shorter than the second step. And I think what folks don’t understand is that you sort of get programmed to a step height. And once you take that first step, you expect everything else to be exactly the same.
TOM SILVA: And legally, it’s supposed to be, according to building code. The building code says that a stairway cannot vary over its entire run more than 3/8-inch. That means that risers can’t change their height over the entire run of the stair. So it’s very important that they all match.
LESLIE: Now, is the equation based on the distance you’re trying to cover or is it always an 8-inch rise and a 10-inch run?
TOM SILVA: It’s based on the distance that you’re trying to cover. And the most comfortable riser height is about 7½ inches.
TOM SILVA: To get to that number equally over the run, you take the number that is from the finished at the bottom to the finished at the top and you divide that by eight. The reason that you divide that by eight is because that’s the maximum height that I would want the riser to be. I divide that number by eight, I have a fraction. Let’s say the fraction is 13½. That means that I have 13½ risers. I don’t want a half. I want to round it off to the next. I want 14 risers. I take that 14 and I divide it into that same number that I divided the 8 into and that will end up in a fraction of 7 and something.
TOM: So it has to be eight or less?
TOM SILVA: Eight or less.
TOM: Let’s say we’re building a deck stair and we go from the surface of the deck down to the sidewalk and it’s exactly 48 inches. That’s easy.
TOM SILVA: Mm-hmm. That’s easy.
TOM: We’ve got six risers.
TOM SILVA: You’ve got 6 risers at 8 inches.
TOM: Right. But if it turns out that it’s 50 inches, well, then it’s not so easy because you can’t have an 8¼-inch step or something like that.
TOM SILVA: No. Legally, you can but it’s too high.
TOM SILVA: You take that 8 that you divided into 50 and now you’re going to have a fraction of 6½. So you take the 50 and divide that by 7 and you have a riser height that is 7-1/16.
TOM: Seven-something, right.
LESLIE: Now, if you’re dealing with common core, you have to put the 10s in one column and the 1s in another and then that’s a whole other problem.
TOM SILVA: It sounds complicated but if you remember eight and round it off to the next fraction up and then divide that in.
TOM: Now, we also see stair stringers sold in home centers. And those distances can almost never work out right.
TOM SILVA: Yeah, yeah, it’s true. But if you’re going to use them, I would always say that make the step at the bottom – the riser height at the bottom – be the one that you have to alter. And you can alter that by changing the grade around it. Maybe bring some gravel in, make a step that – out of masonry that will bring it flush to that bottom step.
TOM: Yeah, that’s a great point. Easy way to adjust it at the bottom. Dirt’s a lot easier to handle than calculating the stair stringer.
TOM SILVA: Calculations aren’t that bad.
TOM: Tom Silva, the general contractor from TV’s This Old House, thank you so much for stopping by The Money Pit.
TOM SILVA: Always a pleasure. Nice to be here.