Framing for drywall under Spiral staircase
Hi folks. As part of the never-ending basement renovation, I'm working on a framing plan for the basement staircase so I can get on with drywall. The main staircase (ground floor to upper level) is a curved, 90 degree turn construction, which from the underside presents a gentle S-shaped curve. The basement staircase, however, is a basic square, 90 degree consisting of two short flights and a landing which extends about 24" beyond the spiral stair stringer at the outside corner. Widest span I have to deal with is 68" from the inside curved wall to the outside farthest corner of the square landing.
I need to come up with a framing plan that will maximize headroom on the basement stairs and, of course, be strong enough to support the weight of drywall. My initial impression is that simply running 2x3s on edge across the bottom of the curved staircase stringers and extending to tie into the stud walls around the square basement stair landing would do the trick, paying close attention to framing everything level and no more than 24" on centre. (I'm envisioning some rather pie-shaped sections of framing around the bend, with straight sections (more or less) at either end). Scribed lines on both stud walls (inside curved and outside square) would act as guides to keep the smooth flow of framing.
My question here, in addition to whatever framing expertise is out there to help me plan this, is: Is it permissible to attach framing for drywall directly to the upper staircase to support the drywall in the middle of the span? I have had several drywall contractors in to quote the drywall job, and received conflicting answers to this. I am located in Ontario, btw.
I'm leaning on the 'permissible" side of this question myself as the original drywall framing had 2x3's nailed directly to the back of at least a few stair treads, but there's always the chance that the original builder "bent" the code on this one......
Can anyone offer their experience with curved staircases and/or their understanding of the Ontario Building Code?