I seem so proud of how long it took me to fix a hole in my garage roof that I need to tell about it. Maybe El Nino is happening to you, too?
The basic problem was that we had a flat roof extending over about the last 5 feet of our garage. (Rule 1: NEVER let your architect or builder convince you that some new miracle fiber can keep water from getting through a flat roof!)
I needed to create some slope, so I attached one end of some fiberglass sheets to the front wall of my house, above the garabe. Then, because the sheets are not very strong, and so that I could still go out on the little "deck," I built a boardwalk over the sheets. This picture shows the sheets attached to the house, and the boardwalk over them.
As you can see here, I didn't worry about sealing the edges of the sheets. This provides a bit of ventilation, and I'm just hoping not that much water can go vertical.
This whole mess is fortunately hidden from the front, by the deck's railing which also provides the other anchor for the boardwalk. In the corner you can see a pair of "deck slippers" I made to let me walk on the plastic sheets.
The hero of this story turns out to be the fiberglass sheets. They are very strong and compared to a lot of other alternatives, not too expensive. The hardest part about them was cutting them. I wound up with a pretty effective jig, involving some of the rippled moulding and a few clamps. When braced like this, a hack saw cuts simply.