In Europe, where the history and heritage of metal roofing comes from: flat lock was never used. It never even existed because they knew how to seam, and design their roofs properly.
They train for years to build actual permanent roofs, using methods they were adopted around the year 800 and are still in use today. Many of these european roofs have benchmarks for performing multiple centuries with no human intervention after the initial install.
Why is this important? It’s wasteful for one. Copper, and metal, and budgets on large public projects are not cheap in the US. The same amount of materials and labor could be applied with proper training and methods to produce much better results: results that will last centuries.
There is another issue: repairability. When you spend labor to slice up copper into “plates” fit them on the roof, and then solder them back together: you’e charged a whole bunch of money to put down a monolith, that will never be anything other than that. With free-seamed panels however if a repair of a single pan is ever required (bullets falling from the air, other trades mounting stuff and penetrating the roof at some point in it’s life… ) Then a single part of the roof can be repaired or re-worked without disturbing the entire assembly.
Trad Building 