How high up off the ground is this roof? If you're working with wood and core wood, you're going to have trouble going beyond about three poles (12m) high with your roof.

But if there is anything grounded nearby, like a tree or a tall rock or a cliff, it counts as a ground level support for anything it touches. Jutting a beam horizontally of it to as high a point as possible can be one way to get a lot of extra stability.