Roof diverters
 

I want to have a roof diverter installed on a vault roof shape
which would require the diverter to be bent or shaped in an arc.

What do you want the diverter to be made of - metal, like aluminum or copper?

Do it with overlapping pieces, kind of like step flashing is done.

Good idea - I was going there . You can buy precut pieces of flashing (squares of aluminum) at a big box home improvement or lumber store.

What kind of roof covering do you have - shingles?

Step flashing consist's of small pieces of flashing overlapped to form a continuous flashing, like you see where a chimney meets a sloped roof. It looks kind of like steps. ;)

Roof deflecter
 

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I have asphalt shingles. What is the size of the pieces of flashing?

They come in precut squares or in a roll, but you could easily cut them down to a smaller size. You would bend them into an "L" shape and tuck one side under the shingle. The other side would face up and divert water. You may need to experiment with size to conform to the curve of the roof.

Look at Home Depot in the roofing area. Also see this link for ideas:

http://www.guttersupply.com/p-coil.gstml?s=gaw&kw=roofing+Flashing