It is a real shame to see this over and over and over again. There are a lot of American historic roofers who have only ever known flat lock. I will continue to shame them.
Flat lock, was a mistake in the past and it is still a mistake today. Roof tinners in early America developed flat lock, out of necessity and ignorance. They had complex shapes to cover, and they had no guild training so they weren’t aware of free seaming without solder. It was a “good enough” approach.
Why is this bad?
- It ignores better practices, and older methods with a longer track record.
- The man-hours and cost to sweat panels together on a pitched surface are exorbitant.
- The same amount of copper and less man hours could have been applied to a free-seamed assembly with proper joinery. better value.
- Repairability: That whole thing is one big monolith now. If a repair is needed in 10, 50, 100 years, it will have to be all re-done. If it were a free-seamed god-honoring assembly: you only need to remove the damaged panel, and can stitch a new one in. We have precedents for copper roofing done in the guild style (solder less) from aachen that are 800 years old.
- Expansion and contraction: This detail will fight against itself, slowly working on the joints until one of them fails.. and again when it does the only recourse is remove it.
- Field repairs to aged copper with more solder are notoriously feeble.