Abstract: To fasten a seam of sheet material, a seam roller is positioned over a seam of roofing material having overlapping portions of sheets of roofing material which are forced together as rollers pass over. Each of a plurality of rollers is mounted to rotate around a horizontal roller axis in circumferentially spaced apart relationship with each other at an angle to the radii of their circles of rotation, which circle is in a plane at an angle to the horizontal. With this arrangement, the rollers apply a forward pulling action on the seam roller, and apply a pressure to the sheet material in a sweeping motion that removes air.