How to fix sagging ceiling?

Question

I have an older building on my property that I am in the process of fixing up. The person that built it used 2x4s for the ceiling joists and it is beginning to sag a bit in the middle of the ceiling.

I would like to fix this and also hopefully prevent future sagging. My current plan is to jack up the ceiling until it is level and then sister 2x6s to the existing 2x4s.

I have a few questions about this approach:

1. is it a reasonable approach to resolve the current/future sagging issue?
2. The ceilings are already kind of low - could I potentially use a 4x4 instead of a 2x6 to get the same support and save myself the 2" of head room?
3. The current 2x4 joists continue through the wall to the outside - am I ok stopping at the wall with the new 2x6 joists? They would just rest on the wall frame.
4. Am I ok to sister the 2x4 and 2x6 joists using screws every 18 inches or so?

Update:

• There are two spans of about 12 feet that meet in the center of the ceiling.
• The joists are spaced somewhat irregularly - between 18 and 24 inches.
• As @LeeSam mentioned the ceiling joists are the roof joists as well.
• There is no material attached to the bottom of the 2x4s.
• There is plywood and tar paper on top of the 2x4s but nothing else.
• The angle of the roof is fairly shallow, it raises about a foot over the 12' span.
• Water doesn't currently pool on the roof

Crude drawing:

Answer 1

Is it a reasonable approach to resolve the current/future sagging issue?
Yes, adding 2x6 lumber should resolve your issue. 12 feet is still a long span for unsupported 2x6 boards, but it'll be a definite improvement. Be sure to crown each rafter to maximize benefit.

Could I potentially use a 4x4 instead of a 2x6 to get the same support and save myself the 2" of head room?
This is inadvisable. The additional weight and lower rigidity of 4x4 lumber make it a poor rafter choice. They'd also be quite expensive.

Am I ok stopping at the wall with the new 2x6 joists?
Yes. 2x4 rafter tails should be adequate. It's not critical that the new rafters bear either on the wall or the ridge beam. Fit them as close as is convenient. The short spans of 2x4 beyond the 2x6 boards have the strength needed to carry the load. You might level off the bottom ends of the 2x6 boards to create a taper into the 2x4, like so (but obviously with less pitch):

                /      /
               /      /
              /      /
             /      /
            /      /
           /______/
          /    /
         /____/

Am I ok to sister the 2x4 and 2x6 joists using screws every 18 inches or so?
Yes, if your screws are 2-1/2" gold construction screws (or better--no black oxide), you use two per 18", spaced widely, and you be sure to clamp the rafters tightly together as you progress.

Answer 2

1 To correct the situation the way you propose is feasable. 2. Take a 2x6 and not 4x4. The bending moment and not the strengh is the problem. 3. Since the bending moment is the problem there is no harm in leaving the sister joists free from the beam and wall. Only go as close as you can. 4 I would prefer a screw distance of 12"

Overlooking the situation. You could do next. Sister the joists with a 2x6 (6 in the vertical plane) and screw them together every 12 inches. You can leave the ends just free from the wall and the beam. That does not form a problem. The bending moment of the joist in the middle is the problem. At the ends the joists are strong enough.

If possible you could try to correct the sagging. For that you have to fix the outer ends of the 2x6 first and then try to bring the original joist in line. If to difficult just leave it the way it is.