Can new shingles be installed over an existing shingle roof?

In many cases, building codes allow one layer of new shingles to be installed over an existing layer, a practice known as roofing over. However, Douglas Construction generally doesn't recommend it. Roofing over hides the roof deck and any underlying damage, such as rot or soft spots, adds extra weight to the structure, and can shorten the lifespan of the new shingles since they aren't laying on a flat, solid surface. It can also void some manufacturer warranties. A full tear-off gives our team a chance to inspect the deck, fix any hidden problems, and install the new shingles properly for the longest possible lifespan.