Find out what the covering is
Older felt, modern felt systems, EPDM rubber, GRP fibreglass and asphalt behave differently. A repair material that bonds well to one surface may be poor on another. Identify the covering before buying tubs of general-purpose roof coating.
Ponding has a cause
Shallow water that disappears after rain is not the same as a deep pond that sits for days. Persistent ponding can point to poor falls, deflection or a restricted outlet. Look at where the water should leave the roof before blaming the membrane.
Edges often fail first
Upstands against walls, edge trims, pipe penetrations and corners move and weather differently from the main surface. A small split at one edge can wet a large area below because water spreads across decking or insulation.
A patch needs a sound surrounding area
Local repairs make sense where the rest of the covering is reasonably sound. A roof covered in blisters, cracks, lifted laps and old patches may keep producing new failures. Ask to see the defect and enough surrounding roof to judge that context.