Sagging roof question

Hello all,

I'm new to these forums and I'm a new home owner. I've been in this home for ~6 months. It has a shop/shed in the backyard that is at least 10 years old (couldn't find the records) that is in need of some repairs; mainly, replacing the siding, the door (bottom is rotted), and the roof. I have some ideas on how to fix the first two but am seeking advice on the roof.



As you can tell, the roof sags in the middle and the two long walls (the structure is ~20 ft x 40 ft) are being pushed out slightly so they are out of plumb. The reason is obvious in that the roof has rafters but no horizontal joists, as seen from these images:



Here are some additional pictures of the rafters:





I figure the roof structure needs horizontal joists to keep the roof level and to keep the roof from pushing the walls out.

My question is is there some way to "jack" up the roof and install horizontal joists to repair the above problem? If so, is this something an individual can do with limited experience or do I need to hire a roof contractor?

Thanks for any help,
Chris

Yes you may be able to do this.
Open the bottom of the walls and see if they are bolted down and if not, bolt them down with ancher bolts and tie the rafters in place to the walls with huricane hangers. Add eye nuts to the top of the bolts and use a cable come-a-long from one walls hold down bolts to the top of the other wall.
Drill a hole thru the top of the sheeting on the other wall so you can use a sling to wrap the top plate of the wall to pull on.
Once you have that section of wall plumb, brace it with a diangled 2x4 to hold it in place and move the come-a-long set up and repeat untill you have that wall straight and plumb.
Once you have one wall all braced up, move the set up to the other side. The second side will not want to move so easily, you may have to remove a few coller ties after you are holding it with the come-a-long. Once you have a section of that wall plumb add a 2x6 ceiling joist from wall to wall and move your set up a long and do the next section.
Once you have both walls straight and plumb, you will have some rafter that will need work but that for another day.