Lap joining copper seams for this small roof. I will have 3 sheets soldered, and all pre-tinned. Always open to suggestions if anyone has done copper, slate or carpentry work. 

Here is the roof with just the drip edge:

Getting ready for soldering

The top of the valley being installed. Screw the thing in place then replace with rivets. Works great. Next time I'll just rivet then take it down so it's easier to solder the joint. You can lay it flat instead of soldering on a slope. Then install with nails on the edge.