My shingles were a little worse for wear and I had a "friend-of-a-friend-who-used-to-be-a-roofer" come take a look at it and he said I could save some money on the demolition by just laying another course of shingles over the first.
He claimed it was a common technique and they did it all the time at his job, but it sounds a little hinky to me.
Is this really commonly done? Is it a good idea? What Are there downsides?