i need to replace a 2x4 in an old ceiling. the room is 17' long though. the 2x4s in there now are that long. i am wondering if i can sister two shorter ones together without having the sister board go all the way to one side. can i just use like a 2 foot section in the middle to sister them together. there is no weight on these. just to hold the drywall.
Ideally, you'd use a full-length 2x4 tight to the old member when sistering to this ceiling joist. That would gain you the most strength. But, any material you add would add strength to the ceiling. I'd try to install the longest piece you can unbroken in the middle of the span, and have the shorter pieces and spices near the outside of the span. Nail the new and the old together very securely.