A roof panel seamer which is self-propelled and bidirectional for forming a standing seam joining two adjacent metal roof panels. Two seamers are disclosed, one for forming a 90° seam and the other for forming a 180° seam. Friction wheels, rather than gears, are used to transfer power from the main support body to the auxiliary support body of the seamer. The seamer includes five roll forming stations, the leading three of which are effective for each direction of travel. One of the roll forming stations for each direction of travel in the 90° seamer imparts a curve to the leg being bent.