Patch repairs rarely last on a failed flat roof. The durable fix is to strip back, confirm the deck and falls are sound, correct the insulation and vapour control arrangement, and re-cover with a single-ply, liquid or high-performance felt system.
Most flat roof failures come from ponding water caused by inadequate falls, from movement at upstands and junctions, or from condensation within a cold roof build-up that was never adequately ventilated.
A warm roof, with the insulation above the deck and a vapour control layer beneath it, avoids most condensation risk and is the usual approach on a re-cover. It raises the roof level, so check upstand heights at doors and walls.