Well let’s start with the facts first shall we, yes - in fact mastic asphalt is one of the very few waterproofing materials that can with the right preparation and application can be rectified easily.
Impact damage and movement cracks are readily seen in this totally seamless waterproofing. A hot poultice of mastic asphalt can be applied around the defective area heating the old existing enabling to cut out a patch with a trowel down to roof structure. Equally, this can be achieved with a small hand-held battery grinder.
Either option takes away the health and safety hazard of a gas bottle and torch and hot work permit. Once patch is removed, a break joint can be cut out using mastic asphalt as to lay 2 coats. Sheathing felt will be cut in to patch as a separating membrane.
A first coat will be made good to first break joint, then when cold, top coat will be installed, warming and cleaning the old existing lifting and making good with a sand rubbed finish leaving flat and completely waterproofed.
This is how you can repair a mastic asphalt flat roof.