A device adapted to dispense or spray a coating such as a foam in a predetermined pattern or manner on a substrate, preferably a roof. The spray coating apparatus comprises a spray assembly having a carriage which is operatively mounted on a track that preferably provides a linear travel path. The carriage is controlled by a drive mechanism which causes reciprocating movement of the carriage. A spray gun is mounted on a holder of the carriage and controlled by an actuator and is used to uniformly apply coatings at a predetermined thickness controlled in part by a spray rate on the intended substrate. In one embodiment, the apparatus includes a cart which is either motorized or manual. The apparatus is lightweight and easily disassembled into sections in order to transport the device from a ground surface to a roof.