Today we're learning how to figure and cut stair stringers for a basic set of stairs if this is your first time building your own set of stairs I imagine you're pretty overwhelmed right now and that's okay because by the end of this two-part series you're going to be able to figure the math out and cut your own stringers and then learn how to install them using three different methods to get it started you're going to need to know some basic terminology the first term is stringer stringers are normally made up of dimensional lumber and they make up the main structure of the staircase a typical stairway will have three to four stringers the second term is stairwell header the stairwell header is important because it's the structural member that supports the floor above and it's also where the stringers are attached the next term is tread treads are the finished part of the stairs that you actually step on and the last basic term is riser risers are the finished material that run vertically up the rise of the stairs if we dig a little bit deeper there are four other important terms that we need to know and they are total run total rise unit run and unit rise first is the total run this is the stairs total horizontal length next is the total rise total rise is the total vertical distance from one finished floor to another sometimes that finished floor can be concrete like in a basement or a garage it can be in a home where it's hard wood or tile and it can even be dirt or patio blocks for an outside project like a set of deck stairs to better understand unit run and unit rise i've made this very simple visual aid unit run which is represented by this side of the triangle is the horizontal distance from the face of one riser to the next adjacent one and for our stairs that number is going to be 10 inches or 254 mil and that's a pretty standard dimension for a rough tread the unit rise which is this side of the triangle is the vertical distance from the top of one tread to the top of another and for our stairs here that number worked out to be 7 and 5 16 or 185 mil and i'll show you how we worked out that number here in a minute the last side here is the stair slope which is determined by the unit run and unit rise it's important to know that the sum of one rise and one tread should add up to between 17 and 18 inches or 431 to 457 mil for example if we had a tread that was 12 inches and a rise that was 7 inches that would add up to 19 inches which would not be ideal and in some cases would not pass local building codes and speaking of building codes be sure to check your local codes before you build any set of stairs because this video is really demonstrating the math and the basic techniques to stair building okay with that out of the way let me show you how i solved for our unit rise the first step to determine unit rise is to determine the total rise again the total rise is the total vertical distance from one finished floor to another finished floor for example here this plywood represents the subfloor on the inside of a house which is not the finished floor therefore we need to take a piece of sample finish floor and hold it into its position before measuring the total rise the other thing to think through before getting your total rise is the floor below for our example the concrete floor here is the finished floor but let's say that it wasn't your finished floor you would have to do the same thing as we did up top which is to take a piece of sample finished flooring and lay it down to measure off of but since our concrete floor is the finished floor the total rise from the top of the finished concrete to the top of the finished pine floor measures 36.5 inches or 927 mil and before we move on i forgot to mention something pretty important and that is you need to check how level the floor is further out away from the header to check this extend the level beyond the header and measure down to see if the total rise is the same in that area as it is in the area underneath the header if the measurement further away from the header is different you either must make them the same which can be very difficult or go with the measurement that's out further where the actual stringer will be setting so for our example here the measurement is quite the same so i'm going to go ahead and go with the measurement that's right under the header with the total rise now identified let's go ahead and move on to step two which is the math behind solving for unit rise the ideal rise for a stair is seven inches or 177 mil knowing that take the total rise which is 36.5 inches and divide that by seven which again is the ideal rise to get 5.21 this number represents the total number of risers but of course you can't have a partial riser so you have to round to the nearest whole number and in this case that would be 5. now divide the total rise of 36.5 inches by 5 this time to get 7 and 5 16 or 185 mil which is indeed the unit rise for our stairs so how many total treads and risers will we need for our stairs well it all depends on how we decide to attach the stringer to the header there are two main ways to do that the first is to have the tread at the top of the stairs even with the finished floor doing it this way makes the number of risers and treads the same the second way to attach the stringer and the way we're going to do it today is to drop the stringer down and use the top of the finished floor as one of the treads doing it this way makes the number of treads one less than the number of risers therefore for our stairs today we need to cut four risers and four treads knowing that the stair header is acting as one of the risers and this will make more sense as soon as we start to lay out our first stringer at this point if you didn't already have your total run of your stairs you could take the total number of treads and multiply that by the unit run so for the first method that would be 5 treads times 10 inches which is our unit run for a total of 50 inches and for the second method four treads times 10 or 40 inches for your total run now that the math is all done we know that our stairs are going to have a total of five risers and four treads and we know that we need to cut four treads and four risers it's time to lay out for our first stringer the first step is to find a really nice straight piece of lumber in this case i'm using a 2 by 12. next grab a framing square a straight piece of wood and a few clamps using the blade of the square line up the 10 which is our unit run with the edge of the wood and clamp it in place then using the tongue of the square line up the 7 and 5 16 which is our rise with the edge of the wood and clamp that in place i like to do a test mark to make sure i'm set up in the correct position so i use the back of the lumber and draw both lines once that's done i double check my marks with a tape measure to ensure that i have exactly what i'm intending for some
