N Q L,
I agree with ML that I do not think the roof material and gravel will have any effect on the Muskrats. We have resorted to 6 foot tall chain link fence lining the inside of both dams to try to slow down the damage to the dams. They still manage to find, or dig under the fence to burrow into the dam.
Unless you get rid of all the Muskrats you will have the same problems in the near future. And, they will come back, year after year.
Check State and Local regulations so you don't get into trouble. Use the search feature and put in Muskrats and you should come up with several pasts posts on how others have dealt with 'Rat removal.
Good Luck,
Dan
