A continuously moving foam spray nozzle which supplies liquid plastic such as polyurethane moves in a path to define and create a solid structure which solidifies immediately as the nozzles moves away and the result can be a curved solid building structure similar to a conventional "Quonset hut" made from solid plastic or can be wall structures of different shapes. The foaming spray nozzle is carried on a guide frame which moves along a predetermined track and in the case of curved structures like a "Quonset hut" the foam nozzle moves in an arch from one side to the other and at the same time is moving lineally on the track to create a continuous curved shell of whatever length is desired. (Nozzles may be mounted facing each other in opposition to coat a shell formed there between by the foam which is applied from other means.) In another form, simultaneously with the creation of plastic structure a covering of sheet plastic or other similar material is supplied from rolls carried with the nozzle, whereby the exterior surfaces both outside and inside may be simultaneously covered with sheet plastic or other similar material which adheres to the foam. With the nozzle operating from side to side and simultaneously moving lineally, wall panels may be created continuously and simultaneous covered with a skin of plastic or other similar material. According to the method of the present invention, the material used must be self-supporting in atmosphere (e.g. plastic foam) so as to remain in place and resist dripping by gravity until it has solidified sufficiently to form a solid structure. The method is that of continuously supplying a foam material and continuously moving said supply in a pre-determined pattern to create a three-dimensional structure by moving said supply both lineally and in a direction either straight or curved transverse thereto.