Disclosed is a seam clamp for securing equipment, such as solar PV panels, to standing roofs. The seam clamp includes an outer clamp member, an outer support member, and an inner clamp member. The inner clamp member and outer clamp member both include a clamping jaw projecting inward toward each other. The inner clamping member movably attached to the outer support member allowing the clamping jaws to engage and secure a standing seam. For ease of assembly, the outer clamp member and outer support member are slideably engaged and can be optionally captively slideably engaged. The seam clamp can be configured to accommodate different standing seam shapes by substituting alternative outer clamp members.