Although code does allow for an overlay of one layer of existing asphalt roof shingles, in many cases, we do not recommend it.  With a tear off of old materials, we can look for any structural damage that may have occurred due to storm damage, or age or both. If there are any defects in the roof deck like rotting, this would be revealed during tear off.  Also, starting with a clean deck, with fresh underlayments, results in a smoother finished roof system.

That being said, there is a stone coated steel shingle that does work well with an overlay because of the design of the panel. (Click here for more info)