OK, so we are finishing up house plans (just waiting on construction drawings from architect), and will hopefully start construction on our dream home before the fall rain sets in. This design will leave me a sizable area underneath the roof on 3rd floor that can be dedicated to theater, lobby, equipment etc.
I have a couple of early construction questions that might affect some of those final construction details, and once all that is set and I can nail down final dimensions of the space to work with we can start talking about actual room design in more detail.
1. Roof rafters. The unsupported span from one knee wall to peak is around 18 ft. Looking at span tables for doug fir and southern pine, I'm thinking that would probably require 2 x 12. We will have i-joist for flooring system 2nd and 3rd floors. Does anyone have any experience using i-joist for roof framing? Looking at their span tables, it seems like I could get away with the 9.5" deep joist, maybe even spaced wider at 19" or 24". In a room under the slope, a couple of inches depth translates to several inches wider room and we all know every inch can be used. Any thoughts here?
2. Knee wall and sub-floor, early isolation considerations. With living spaces below the theater I will make a good effort on sound isolation. At a minimum I'll do the sub-floor, rubber mat, and then two layers of sub-floor floating on the mat. There will be a knee wall to support some of the rafter span (roughly where dotted verticals are above), for now I'm assuming at roughly 6' ceiling height. Would you guys recommend the support wall be outside the floating sub-floor? I'm guessing so... and if so, would you then construct another knee wall that is decoupled and sits on top of the floating sub-floor assembly as a form of room within room, or just clips/channel/dd/gg on the single knee wall? The answer to that probably affects where I have them place the structural knee wall. Doing a double wall means I'd move the support wall out a little to keep finished ceiling height on target.
Trying to think of anything that might affect actual house framing. After that is more general sound isolation strategies within that framed space, HVAC, electrical, etc. Then I can worry about the layout of the room itself!