i'm trying to layout my bsmt for a theater room and have a spot that'll accomodate one measuring roughly 17ft x 21ft. i've read about building "a room w/i a room" and wanted to know how to support the theater's ceiling beams. if i were to use 2x4's for the ceiling, the 17ft length seems pretty long. add the weight of 2 layers of drywall plus dampening material, i'm worried that the 2x4's will sag. and since the theater's ceiling will not be attached to the bsmt's ceiling, i'm not sure if the 17ft distance is too great. would like to hear some ideas as to what others have done. thanks.