We use a pyramid-tipped, acetylene-fueled soldering iron for our roof and gutter work; propane models and other types of tips are available. Regardless of the iron or tip used, soldering requires some prep work. Make sure that the tip is clean and that the four facets are flat and meet at a sharp point; otherwise it will be difficult to control the flow and direction of the solder. We use a grinder with a metal wheel to flatten the facets, then we sand them using an orbital sander with 80-to-120-grit paper.
Light the iron and keep it on a low setting so it heats up slowly. When it's hot enough, "tin" the tip on a scrap piece of copper. Spread flux, then melt enough solder to spread evenly on all four facets of the tip. Tinning makes for a smooth flow of solder when working a seam.