A number of responses that I received to my earlier question alluded to multiple layers of shingles. While I've heard of this, I always wondered whether it was a good idea. Any problems associated with simply putting down one layer on top of another? Does your existing roof have to be leak-free in order to make this work? Any situations where it should be avoided? I wouldn't mind saving some bucks if this is the way to go. I hope to be living in Boca Raton in a retirement community by the time I need to replace the next roof. :>) TIA,