Hello dmakin, I'm presently working on a project with the same type of construction as yours, built in tin gutters, replacing w/ copper. Rule of thumb, 1st. Clean a good 2" on underside of one sheet, and top side on next sheet with fine sandpaper, flux surfaces well, and overlap about 1" . #2 Rivet the sheets together with COPPER rivets. #3 You are going to solder the sheets just as you would a copper pipe. ( heat the overlapping sheet with a mild torch and gently feed in your solder until the entire length of sheet has been flooded w/ solder.) You should be able to see the solder between the sheets from both the top and bottom of sheet. Continue on likewise. I hope this was of help, and not too late in the coming. Be sure to solder the rivets as well, covering the heads completely.