In any case, those are all great suggestions, but, for a roof, if it's collapsing, it's too high.

No, going from a 45 degree roof to a 26 degree roof will not fix it, but reducing the height of the peak will, if you can, which, means reducing the span.

If it's a wood roof on a wood wall, supporting the wall better will help. If it's a stone roof on a stone wall, supporting the roof with cross beams will help. Support beams from the floor will always help.

A wood roof on a stone wall can get support by adding cross beams with vertical support beams from the cross beams, but that's tricky, since the cross beams are likely going to be pretty weak, since the span is large (except for reinforced beams, which is usually the answer if you can make them).

Without knowing how it's built, it's hard to suggest fixes.