Targeted repair
When the roof is mostly sound and leaks come from one or two spots: a canale, a parapet corner, a vent. Fix the source, seal it and protect it.
Recoat or re-foam
When the surface is worn but the material underneath is dry. Recoat sound foam, or add new foam over an eroded but dry roof.
Replace
When moisture is widespread, the deck is soft, there are too many layers, or the roof leaks in many places.
The deciding factor: moisture
Putting anything over wet material traps water. An honest contractor checks before pricing. See flat roof inspection.
Common questions
Can I just keep patching?
For a while, if the roof is mostly dry. When patches multiply, the money is better spent on a recoat or replacement.
Is recoating a waste on an old roof?
Not if the foam is dry and sound. It's often the best value.