Can a coating be applied over a leaking roof?
Existing leaks and the materials they have affected need assessment and repair. A coating system is not a reason to leave wet insulation or damaged support in place.
Is coating always cheaper than roof replacement?
Not necessarily for the work the building actually needs. Preparation, repair and the extent of deterioration affect whether restoration remains sensible. Compare complete scopes, not just surface material.
How long will a coating last?
The selected system, surface preparation, application thickness, drainage and ongoing maintenance all affect service life. Compare the manufacturer’s requirements and the actual coverage offered for the proposed system. A warranty term by itself is not a prediction of how long this roof will remain serviceable.