We are soundproofing a garage, which currently has a "cathedral" ceiling (2x6 rafters supporting a composite shingle roof over plywood). We'd like to duplicate in the garage, the ceiling currently in our living room, which consists of tongue and groove pine laying over 4x8" joists, 48" on center. It should also be noted that we will put some soundproofing materials on top of the tongue-and-groove ceiling: drywall boards and insulation on top. In the garage, the 4x8" joists supporting the new ceiling will sit on, and run between, the top plates(sill)of two opposite walls. These two walls are being soundproofed (e.g. stucco + OSB Board on the outside,and inside: R13 insulation, shear-plywood, soundboard (Celotex), 5/8" Wonderboard/Durorock, resiliant channel, and one layer of 5/8" drywall.
The mud sill (lower plate) of the wooden frame of these walls sits on, and is attached to, approx. 2 1/2 feet of concrete (the foundation wall surrounding the garage floor (the house is on a raised foundation).
Question: one friend in the construction business has said we need a post and beam support system, where the beam would run perpendicular to, and sit under and support, the 4x8" joists (the supporting posts and beam would be located down the middle of the 4x8" joists)?
Another friend, also familiar with construction, has said that the 4x8's (again, at 48" on center, the ends of which will sit on top of, and be attached to the top plates of two opposite, sound-reinforced walls) themselves are strong enough, and don't need a supporting beam. We understand your advise here is informational only, and we just want to get an idea of whether the proposed ceiling plan is even feasible.