.....a farmer once told me, " cut an eighteen-inch square hole in the roof. Then stand back."
Within ten years the roof will be gone, and when the roof is gone..... it don't take too long before the rest of the barn follows suit. This is in The World Without Us, by Alan Weisman.
I wouldn't want you to think that I'm demonstrating the truth of this with a controlled experiment on my shed roof, but it is beginning to look that way.
I really, really need to get that new roof on!