Abstract: A hand-operated batten seamer tool (30) for folding a pair of opposed standing seams (96) over to a pair of double seams. A pair of arms (38) pivotally mounted to a common pivot point (50) to rotate a pair of arms (38) pivotally mounted to a batten cap plate (36) that rests on the batten cap (90). Force applied to the handles (32) is transmitted through a force multiplying mechanism comprising upper link arms (78), side plates (76) and lower link arms (80). A pair of cleats (70) engages the seams (96) during the folding operation to hold the batten seamer tool (30) in engagement with the batten cap (90) and support block (88) to form a pair of tight and leakproof batten seams (86).