On Mon, 6 May 2013 14:26:55 -0700 (PDT), Nona
wrote in Re
Reinforce Roof Against Falling Trees?:

We have a two-story gable roof with plywood sheathing and architectural shingles. Would it help to reinforce the trusses with horizontal or cross metal braces? Could we put metal sheathing beneath the plywood? Our goal is not to make the roof impervious to trees but rather to give us at least a small amount of time to escape the house safely should a heavy tree fall on the roof.

I'm assuming that you have a wood frame house under that roof; in
which case there is not much you can do if one of those trees fall.
The weight of the tree is likely to collapse the frame exterior walls
as well as destroy the roof.