A clamp for attaching solar panel grids to standing seam roof structures. The clamp includes opposing clamp halves having inter-engageable projections and recesses. One or more fasteners engage the halves to draw them together. A first set of recesses on one clamp half receives a first set of projections on the other half. A second set of recesses on the other clamp half receives a second set of projections. When loose, the first set of projections engage the first set of recesses like a detent, while the second set of projections are spaced apart from the second set of recesses to enable the clamp to be slid down onto the vertical seam of a standing seam roof. Tightening the fasteners causes the second set of projections to deform the seam into the second set of recesses, fixing the clamp to the seam.