Building a new set of stairs is a project that sounds harder than it actually is. If your building simple cellar stairs it’s a job that can be done in just a few hours. If your doing finish stairs that will take a bit longer but could be completed in the same weekend.
To begin, a couple of measurements must be made.
The first measurement is called rise. Rise is the measurement from the top landing to the floor at the bottom.
The second is run. Run is the length of the stairs. To measure run you need to plumb a line down from the face of your upper landing to the floor beneath. Put a mark on the lower floor where the plumb bob hits. Now measure from this line to where you want the stairs to end. This dimension is your run.
For the purposes of this article I’m going to use 96″ as a rise and 120″ as a run
I usually start at about 14 risers. So if you take our rise of 96″ and divide by 14 you get 6.85″. Changing to a fraction that is 6-13/16″. That would actually work but I would like the rise to be at 7″ or a bit above 7″. So rather than dividing by 14 I will divide by 13. So if you take our rise of 96″ and divide by 13 we get 7.384. That in a fraction is about 7-3/8″ which I like better. So we will have 13 risers at 7-3/8″ each.
Now for the run. The only fast and hard rule about run is it is always one less than rise. So, being that we determined we were using 13 rise we will automatically know were going to have 12 run. Using our run measurement of 120″ we will divide that by 12 which comes out to 10″. That is the width of each stair tread.
Why is run always one less than rise? Because when we nail our stringers up they will be mounted one step down. The space above the top step to the landing above is counted when we divide up our risers.
If the stairs are going to include risers you need to adjust the cut in the stringer. The most common riser width would be .75″ so in the example above we need to add the .75 to our tread width of 10 inches now making it 10.75″.
Now on to the stringers. Usually with standard stairs you will need to use a 2×10 or a 2×12 for the stringer. Being that we are going to be cutting triangles out of this stringer determined by our dimensions above we need enough wood left to be safe. In most areas here in the USA you need 4″ of width left after the triangles are cut out to abide by building codes. As always it’s best to check with your local building department for the rules in your area.Put a mark on the square at the rise dimension and the run dimension. You can purchase stair chairs if your going to be doing a lot of stairs in the future. Stair chairs are small clamps that connect to your building square. You can set them at any dimension on your square for repeatability. Mark out the stringer using the marks you made on the square. Start with the rise dimension on the square closest to the top of the stringer. Mark the triangle on the stringer. Slide down and mark again. You will end up with a zigzag pattern all the way down. You will have to square back the line at the very top and bottom. At this point you will have the stringer marked but you need to make one final adjustment before cutting them out. You must reduce the bottom step by the width of your stair treads. If your using 2x material you need to cut 1.5″ off the bottom step.
Now to cut out the stringers. When cutting out the triangles in the stringer be very careful to cut straight and on the waste side of the line. Neatness at this step will pay off in the finished stairs. Cut to the intersection of each triangle then finish the cuts with a handsaw or sawzall if you have one. Once you have one stringer cut out hold it up on edge at about a 30 degree angle and voila you will see exactly what your stairs will look like. If all is well with the first one you can trace the second one for an exact match.
When installing the stringers quite often you will need a ledger to nail your stringers to. This is because stairs are installed one step down and sometimes there is not enough to nail to at that point. A ledger board is usually a piece of plywood the width of your opening and high enough to reach to the bottom of the stringer. Once the ledger is nailed up we need to determine at what height to nail the stringers. Measure from top of the ledger down the amount of your rise plus the thickness of the tread. That will be the top of the stringer. From here it’s only a matter of nailing or screwing your parts together to form your finished stairs.
Railings are required on most all stairs today. When building railings it is very important to check your local building codes.