stringer questions

[dug]

Due to miscalculations on tread thickness and finished floor height my stair stringers turned out perfect- except for the top and bottom steps, the top one being about 1 and 1/2 inches short on the rise. This was too much for me to live with so I re-cut them (while up) lopping about 1 inch off the rise and now they are all the same.

The problem now is that there is not a lot of wood left under the bottom of the cuts, only about 4 inches. I recently read that there should be at least 6 inches of wood left between the bottom of the rise/run cut and edge of the board, which sounds reasonable to me. When they were first cut using 2 by 12's and wasting none of the lumber I had not quite 5 inches of solid material left, and one I made from a rough sawn (full 2 by 12) piece still didn't give me the required 6 inches. My stringers are 7 1/2 inch rise by 9 1/2 inch run, which seems pretty normal. Is it possible to adhere to code using 2 by 12 lumber?

My question is how best to strengthen them. There are three, about 16 inch OC. One is attached to a wall and the outside one is doubled up with the wider rough sawn piece so it is the center one I am most concerned with. Would it help to sister a 2 by 6 (maybe two 2 by 6's, one on either side) on the center stringer? I could build a support on the center of the run but would rather not because I'd like the space to be as open as possible for the small closet that will be located there.

[Don_P]

Stair structure defies code and the NDS to begin with 
Yes there should be no less than 5 inches of wood remaining. The risers and treads are spanning from support to support on the side walls. However I typically sister a 2x4 along the bottom on each side of the center stringer. I also run a 2x4 on the wall side of each side stringer to leave a gap to drop wall finishes into.

when cutting the notches in a stringer most folks carry the notch to a point... you can slide up, not coming to a point and leave more meat along the bottom of the stringer.

When I draw a set of stairs I begin by drawing from all finish surfaces, finish floor above, below and surfaces of treads and risers. Then deduct the thickness of finishes, subtreads, etc. A straight run is not difficult but when you begin to add landings or winders with 3/4" thick "treads" and normal treads at 1" thick it takes a little head scratching for me.