Generally ruby fluid works fine for new copper soldering but if you are having issues try a flux specifically made for copper.
How to solder copper roofing.
Copper is one of the most easily solder able metal available.
Flat lock soldered copper roofing is excellent on low slope roofs where a long lasting durable roof is desired.
Picture taken out copper and common sense by revere.
Make your seams tight clean clean and tight seams are essential.
Assuming the copper is relatively clean and bright first brush or squirt on the liquid flux hold the hot soldering iron on top of and against the seam then add additional solder when the metal gets hot enough to melt it until the joint is filled.
This is the one of the most comprehensive resources available on metal roofing.
Lap seams should have a minimum 1 inch overlap and no gaps larger than 1 16 inch.
It may help to pre tin the sheet edges to facilitate soldering.
Check the slate roof warehouse for tools materials and supplies.
Solder much prefers bright copper with a very close fit.
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The solder should be 50 50 tin lead solder in either 1 pound or 1 4 pound bars.