hi.i'm thinking about converting my loft into a single bedroom.at the moment i have 4.5x1.5 joists in it.i have a supporting wall thats goes 3/4's of the way across the loft.i'm thinking about sistering the new joist but not sure what size to fit.my other problem is that the joists after the supporting are roughly about 24 feet from wall plate to wall plate,i think i'll have problems fitting new joists that length or is there a way i can join them even though there is no supporting wall underneath them.
You have 1-1/2" x 4-1/2" floor joists spanning how far? Or are we talking ceiling joists here?
If this is a floor, you're waaaaaaaay undersized and need a serious re-design.
As for breaking/splicing the joists out over the span, no. Can't do it. You can certainly break them over the wall, provided it is a bearing wall. Perhaps a header could be added to support them in the area where the wall stops.
Is it spanning 24' from the inside wall to inside wall?
You don't count the actual wall width, not that it will gain you much
I'd say your are into LVL or I-beams at that distance
I went with I-beams at 19' instead of 2x12's