In 2015 I left Rhode Island, and Casa Buena Builders to bring seamed roofing to the english-speaking world. I had been studying at night, bringing my skills up with folding and joinery; while working full-time in historic preservation, remodeling, and slate roofing. My favorite project of the tour that year was this seminar I was invited to give at the HPTC (Historic Preservation Training Center) headquarters in Frederick, MD.
Almost all of these techniques were completely foreign to the seasoned pros from the National Parks Service. I hope to one-day have all of these rules codified in English, and accepted at least internally within organizations like NPS. We may never be able to regulate the entire market like the better countries but within institutions and even preservation districts it is possible. I know all properties benefit from having roofs that are designed to last as long as the building.
This handout (PDF link) below demonstrates how to layout and cut the valley seam, from the ground as long as you know the two pitches of the intersection roof faces. This is very important with metal roof seaming. Much like in timber framing: the piece must be fully fabricated to exact specifications before they are assembled. In the same way: we do most of our design and layout on the drawing board, and on the cutting bench. There is very little “in place” fabrication, only folding assemblies.