RE: jig for installing stair treads between existing stair skirts
Tread to Riser formula is relitively simple...the total tread/riser relationship should equal 17" to 18". That is to say, if your treads are 10" wide, the risers should be about 7-8" high. This will produce a 'stair angle' that is both comfortable and safe to use. Normally, you don't want risers less than 7" (too shallow) or much higher then 8 inches (too steep).
To determine the number of treads require, take the total rise (from lower floor level to the top of the next level) and divide by '7'. Lets say you just built a deck that's 6' off the ground. Divide the total rise (72") by 7 (the majic number) and you find you need 10 treads. Now divide the 10 treads into the 72" total rise and you find each riser is 7.2" (close enough to 7 1/4). Use a framing square to mark off the successive '10' and '7 1/4' layout lines.
After you cut the stringers, remove the thichness of the tread (say 1 1/2") from the bottom of the stringer. You will be adding this amount back when you attach the treads. For example, if your risers are 7 3/4" and the treads are 2X10's (which are only 1 1/2" thich) remove 1 1/2" from the bottom of the stringer. Now the first tread is only 6 1/4 tall, the rest are 7 3/4, and the stringer comes up 1 1/2" short of the decking. By adding the treads (1 1/2" thick) everything comes out the same 7 3/4" and the top tread is the same level as the decking.
Too confusing??? Just give me a call. I'd be happy to come over and give you a hand building the stairs (my pet project).
For those of us DIYs who are measuring tape challenged, there is a Jig for cutting treads and risers. Itís called the Step Doctor at www.stepdoctor.com. With it you can make tight fitting treads even when they are not square.