Kevin from Thunder Bay, Ontario has a sag in his roof that he wants to shore up.
First Kevin you need to inspect inside to be sure it is not sagging because of rot in the roof joists. If they are sound, but just not properly supported from below you can guild a knee wall half way up the roof, just like as if you wanted to make an attic room. Be careful because you can push the roof right off of the wall below, as you see where I drew an arrow in the lower right hand corner. You might even need to put metal straps from the ceiling joists to the roof joists at this point to prevent un-nailing the roof. These are actually called Hurricane Ties and are used in windy areas to hold the roof down better than toe-nailing.
If you need to move the sag a long way, you will probably want to build the wall in such a way as to be able to move it into place over several months time, slowly pushing the roof back to its straight line.
Keep an eye on the ceiling below, you don't want to push it down at the same time. Ideally there would be a wall in the room below right under the knee wall in the attic. That is rarely true but there are often walls not too far away. It is also possible to build your support "wall" from the point where the support wall below is located, then up diagonally to the centre of the roof face, preventing any pressure on the ceiling below.