Whether a partial or full tear-off of a flat roof is needed is straight-forward, or at least it should be. After all, the most common reason a flat roof tear-off is needed is to remove any part of the roofing system that is wet below the roof surface. There are other reasons, including building code and the building owner’s preference, that may require the tear-off of part or all of the existing roof, but these typically come into play much less frequently.
Most every roofing system manufacturer specifically calls out that their system should not be installed over existing roofing substrate that is wet. In addition to it being poor construction practice in general to leave wet building components in place when part of the building envelope is repaired or replaced, there are important structural aspects of a new roof that would be compromised over time by leaving wet substrate in place.
There are a number of ways to determine whether there are areas of a flat roof that are wet. While some, like thermal imaging, are somewhat high-tech and expensive, taking core samples in areas that are likely to be wet is a quick, easy, and very low-cost way to know whether a roof is wet, merely damp, or dry below the surface.