Sagging Ceiling (Photos) - Advice?

Hi all. I have an old house that, at some point, was added on to by a guy who didn't do the best of jobs. He used 2x4s as raftors, which I hear is a no-no; that they should be at least 2x6. Anyway, I tore out the drywall on the ceiling at the attached photos show what I found.

The raftor beams have bowed in the center of the room. Also, you can see that some of the sheathing planks have outright cracked apart. So here's my plan; please tell me if it's wise.

Based on what I see, I think I should probably tear out the old plank sheathing, replace it with plywood and, while I'm up there, replace the raftors with 2x6 beams. Sound good? PHOTOS ATTACHED.

 

I'd replace it all. The one rafter is split by the window, on top (the compression side. It has failed.
1. What is the ceiling joist span, how long is it?

2. A ceiling joist 2x4 will span 7'10" at 24"o.c. UBC, 20# live load

3. A 2x4 rafter at 24" o.c. will span IRC- 7'10" with no snow load, UBC- not
permitted

4. 2x6- 24"o.c.- 11'9" IRC no snow load Rafter

5. 2x6 ceiling 24"o.c.= 12'3" UBC

The main problem is lack of insulation, rather room for, plus venting.
All your raters need 1-1/2" of bearing, and none have that. THe rafters look to be a #3 (unsuitible for rafters) because of the size and number of knots in most. The builder did not know how to install or select rafter stock.

Is the roof deck 1x6 or 8"? or is it 2x6" or 8"? Bet it fun to walk/bounce on?
Be safe, G

 

Hi GBR, thanks for the response. The joist span is 11'8". When you say, you'd replace it all, I take it you also mean the sheathing, not just "all" the rafters? Also, sorry for the ignorance, but what do you mean by "bearing?"