Disclosed is a fire retardant roof system with a roof deck, a sealant material applied to the roof deck to prevent fluid migration, a polystyrene insulation board upwardly adjacent the sealant material and a sealing material upwardly adjacent the polystyrene material.Further disclosed is a method for making a fire retardant roof structure. The method includes sealing all fluid passageways in a roof deck with a sealing material, applying a polystyrene material upwardly adjacent the roof deck and the sealing material and applying a further sealing material or cover board upwardly adjacent the polystyrene material.