Roofing over is when metal roofing is installed over existing shingles to save money. The problem with this logic is that most homes have some kind of wood rot under the shingles. It rare to find a roof that doesn't. Every year home owners call us with leaking metal roofs and more often then not, they were installed over existing shingles. When its time to replace your roof please ensure its installed correctly and just say NO to roofing over. It will save you piece of mind and money in the long run.