A roof replacement tear off is when a roofing contractor removes all roofing material to bare roof sheathing. The contractor installs all new shingles, underlayment, flashings and vents. With a reroof the roofing contractor cannot inspect the decking because they just install a new layer of shingles on top of the existing. It is important to note that with a full replacement the contractor should offer a full warranty which is not available when installing a second layer. Our recommendation is to always do a full tear off to ensure the roof decking is solid and all proper accessories and flashings are installed.