I'm rebuilding our staircase from the basement to the first level. The old treads and risers are out. The existing stringers are 2x 12's that were routed out and will be reused. I have access to underneath the staircase. My question is about the riser. Does the riser need to be drilled/nailed to the back of tread? Or can the riser rest on the tread below it and be drilled/nailed from below? I looked high and low for any reason or logic for or against putting the riser in between the treads.
Something needs to hold the risers in place. If you plan to set the riser boards on the treads, you can nail or screw upward through the bottom of the tread and into the bottom of the riser. Placing the riser over the back edge of the tread also allows you to hide the nails/screws anchoring the back of the tread to the stringers below. The top of the riser will be lapped over by a nosing approximately 1", so you cannot hide nails here unless you plan to paint it all. So just place a couple of nails through the top of the riser into the stringer behind, placing them closely to the tread nosing so the nails are obscured by the overhang's shadow. The tread nosing will need to be nailed straight down through the top into the stringers, I would just use nails with smaller heads if it bothers you. If you look at old wooden stairs you can see the nails through the treads though.
Closed stringer staircases I've worked on have shims under the treads and behind the risers to hold them in place. The shims are glued in and a finish nail is shot through each shim. The only time I see nails connecting the tread to the riser is when some attempt has been made to stop squeaks in the stair case.