BridgeMan

Actually, 2 x 6s could be sistered onto each 2 x 4 ceiling joist by taper-cutting the ends where the roof slope interferes. But a stiffer ceiling will result if each 2 x 4 joist would have a 2 x 4 flat member glued/screwed to its bottom. Crunching the (approximate) numbers for the section modulus of each configuration, it works out to 13.62 cu. in. vs. 10.63 cu. in., or a difference of approximately 28% stiffer with the flat 2 x 4s on the bottom.

Dionysia

Thanks BridgeMan - I will pass this info onto the Mr. As always, I appreciate your number-crunching!

joecaption

In your case I'd just use at least 2 X 6's. That's the way it's always done with a new home with a low slope roof.
Those 2X 4's are not even close to being wide enough to support the load or plaster. I'd remove the plaster and use light weight 5/8 sheetrock.

papakevin

This will likely be my last post on this thread unless something else comes up.

Ended up getting a few more 2x4's to finish up the floor under the living room. It might have been a mental thing, but it seemed like I could tell where I stopped putting the 2x4's in the middle, so I went ahead and finished off the room. Execpt for the insulation hanging down, I think it turned out alright.

Thanks again Bridgeman for the idea. Love it when a plan comes together.