As I mentioned previously, building a staircase is not terribly difficult, but you need basic carpentry skills, basic math, and good common sense and planning skills. The materials and tools needed are pretty basic. I built the majority of this staircase with only a circular saw, a hand saw, and a square equipped with brass stops (these are 5$ for a set). A table saw is also useful for ripping down the plywood and treads.
Calculating stairs is also not too difficult. The main measurements you need are the full height (finished floor, to finished floor), and the full length (the spot where the top ends, and where the foot of the stairs begin). This can be tricky to measure when the stairs are still in place, but if you need to, you can drill a hole in a step for a tape measure, or use a combination of levels and a plumb bob to measure a large zig-zag across the wall to find these measurements.
In my case, the full height (floor to floor) was 110.5", and the total length was 103.5".
The normal "standard" or "code" sizes for residential stairs is a step height of 7.5", and stair treads of 10" width. If you've ever been in an old house, however, measurements like these are completely useless, and you can chuck them right out the window. They serve merely as a guideline for the height of a comfortable staircase. Obviously you don't want steps 10" high, or only 4" wide. To give you an idea of why the standard measurements won't work in my house, my staircase would need to be at least 114-120" long, which would leave me with an upstairs hallway less than a foot wide. Alternatively I'd have to install a "platform step" at the foot of the stairs, but then I wouldn't be able to open my front door properly, and I'd have a step to walk over into the living room across the door frame. A lot of older homes simply had steep and shallow steps, and there isn't a lot you can do to change this depending on the location of your staircase. In my case, the stairs end at a door frame (living room), and into a hallway upstairs, so the length is pretty "fixed".
The existing stairs (rough sizes) worked relatively well, so I used the same divisions. 12 steps, 13 "rises" (basically the thirteenth step is the second floor). This gave me 8 5/8" steps, and 8 1/2" rises. Basically the steps will be one inch taller than "standard", and the final steps (over the 8 5/8" tread) will be around 9" deep (because they overhang a bit), so 1" narrower than standard. These are the exact measurements you use to cut the stringers (the notched boards). It took me a while to figure out how all the different elevation problems/calculations depending on how many "layers" make up the steps and riser boards would work out, but all of that is simply adjusted by shortening the bottom step, and adjusting the length with the depth of the top step.
Hint: Try to get a nice measurement for your step depths. Originally I was aiming for 103" length, but divided by 12 gave me 8.58333333333333" which is a hair over 9/16", while if I just added half an inch (103.5) it gave me exactly 8.625, which is 8 5/8". You can't do much about the height, though. It must be divided exactly by what you have.
Knowing my proper rise/tread sizes, and the amount of steps to cut, I set up my square, and traced these onto one of my stringer boards (a 2" x 10" x 14'). Once you have the first stringer all marked out, transfer the locations of all the points onto your other stringer(s) to make sure they will be exactly the same, then trace out the other stringer(s) with the square.
When everything is traced, you can figure out your adjustments at the top and bottom steps, and you're good to go. In my case, the rough stairs will have 5/8" plywood risers, later capped with 1/2" finished wood, then 1 1/2" (2" x 10") rough steps, later capped with 3/4" thick hardwood steps. This means that the bottom step must be 2 1/4" shorter (1.5 + 0.75), and the top step must be cut back 1 1/8". In my case, the top step was not going to be wide enough to reach the beam properly (it would have been just a skinny 3/4" sliver) so I just cut the top step longer across the top. In most cases, this might land against a wall or another framing member. You can see the top tread here in this photo. Instead of being 8 5/8, it's a little over 10".
Nothing fancy was used to cut the stringers. There are several techniques, including drilling a 1/2" hole in the corners, or using a saw guide, but I just cut all of these freehand (cut up to the corner, then the rest with the hand saw). I have years of experience, so my precision is pretty good. Use whatever setup you feel comfortable with.
After cutting the stringers, several of the "triangles" on one board were severely weakened by a split running through the board. These were knocked free, or spread open, and glued/clamped.
If I had hand-picked these boards, I wouldn't have had this problem, but these materials were ordered and delivered, so I didn't choose them myself. Not a big deal, just an extra few hours to let the glue dry. Once everything is screwed together, this won't be any weaker than if it didn't have the split.
You will end up with a whole bunch of these scrap corner pieces. Luckily for me, this will be great scrap wood for campfire use at my friend's place.
All cut and ready! Note, in this photo, I had the original bottom steps (the three at the front) cut to 7" high, because I was going to lay them down against the sub floor. This took under consideration 3/4" hardwood as the finished floor, so 7" + 1.5" rough step, plus 3/4" hardwood step, minus 3/4" hardwood, gives 7". Later, (for reasons I'll explain again) I removed another 1.5" from the bottom steps, and installed a 2" x 8" board under the first step (which still gives me the 7"). Hopefully this wasn't confusing.
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