I am remodeling a loft above a garage in a 50 year old detached building. The constuction is 2x4 on (approx.!) 24" centers except for the floor joists which are 2x6, and span 16'. The floor is quite springy. I'm considering sistering 2x10s to the existing joists, but I'm not sure if that's the best way to go. One consideration: there is no top plate above the 1st floor studs. The existing 2x6s are nailed directly to the studs! What are my options?