Commercial metal, hot asphalt “tar & gravel” and many other roof materials are prone to leaks, especially when flat or low slope or have multiple angle/valley seams.  Leaking most often shows up around flashing, HVAC units, vents, piping, sky lights and other structures penetrating the surface. Removing an old roof to install a new one is very expensive.  Many inadequate repairs simply involve pouring on more asphalt but this seldom fixes the problem long term.  And, asphalt is climbing in cost.  Instead, consider these ways of repairing with polyurethane or polyurea coatings and sprayed polyurethane foam (SPF).