A “roof over” is considered to be O.K. in certain areas of the US. In this form of re-roofing a crew simply nails a new layer of shingles over an existing layer or layers of shingles. However, in a “roof over” situation there is no way of telling whether the decking has damage or needs to be replaced, plus the additional weight it adds to potentially compromised decking. You also have to consider the probability that the roofing material already has aged and broken down. The felt material breaks down over time and becomes inefficient. Overlaying shingles can also cause a ripple effect on your roof that would be very unappealing to look at. Make sure you know all of the facts about overlayment before considering paying a roofer to do a job that may cost you more in the long run.