Try to use a some trigonometry before deciding you need to measure the roof. If you know how wide and long the footprint of the house or a part of it is, can estimate the distance the roof rises from the eaves to it's peak, and you can estimate the overhang on the eaves, with the use of trigonometry you can calculate roof area.

For a rectangular structure. The width of the roof can be deterined by knowing the width of the structure and the rise of the roof. Squaring both these numbers, adding them and then finding the square root of the sum would give you that width of the roof. The length of the roof should not change much from the length of the structure, and it should not change over the width of the roof. Multiply by the length by the square root of the sum mentioned, and it should give you the area of the roof. The math is much safer than getting out the ladder.

Not sure if I would buy tarps to cover all of the roof, but how much you want to cover is all up to you.

As far as tie down straps, I might forego them. If I needed to put a tarp in place to cover a hole in the roof, and I was in a hurry, I would probably nail it down.

Good luck with the clean up, etc.