I am in the process of finishing last detail on the roof and don’t quite have a good guidance on roof base flashing. Basically, it is very clear how regular chimney is base flashed that has roofing on both sides of chimney - since when I bend the flashing on both sides, the flashing is nailed into roof deck through the triangular tongue of flashing that will be covered with shingle above and step flashing. In our case, the chimney is on the gable edge of roof, therefore only one side will have this “tongue”. The other side will terminate flush at a stop where shingles/roof decking ends on the gable. I refuse to nail through the visible flashing part as it will be a potential leaking spot. I was thinking about nailing into the “tongue” on one side and then using mortar screw, to screw it into the mortar joint on the other side, so that the flashing is fully adhered. Then I am thinking I am attaching one metal to 2 dissimilar substrates which will have differential movement. Should I forego tongue-nailing and instead screw both sides into chimney mortar? These screws will be covered by counter-flashing, so leaking here won’t be an issue and they will be attached only to the similar substrate. Or do as I planned and nail to decking on one side and mortar joint on other? There will be some ability to differentially move, but not as well as if I only nailed to roof decking alone. Below is the photo of my soldered base flashing (once again, not the prettiest of jobs, but will do the job okay). There is no “tongue” on there as I soldered another piece of copper to eliminate potential leak spot, but the area on the top right on the roof plane is what I consider “tongue nailing spot.” The other side terminates without it, so you can see there is nothing to adhere it to besides the chimney itself.