Roofing : Pros and cons of re-roofing or re-layering
Do not relayer. Here are some of the reasons.


The added layer adds needless extra weight to the roof.
Second and third layer shingles do not last as long as a new base layer.
The added layer will never lay as flat as a base layer and creates more chances for pockets to form where water can backup under the shingles.
Full rip off and replacement allows for inspection of roof boards / sheathing and replacement if necessary.
Full replacement allows for much better re-do of the flashing. Relayering generally makes a total mess of the flashing trying to add new on top of old and inviting new leaks.