I'm replacing the barge boards of a concrete tiled roof. The flaunching is rock solid and would be difficult to remove from the tiles so I'm intending to put a bead of mastic between the flaunching and the new barge boards to seal the joint. The problem is that there are some hairline cracks in the flaunching at right angles to the barge board edge.
the cracks occur at the narrowest points where the tiles step. I don't know if I can rely on finding some kind of paint-on sealing solution to fill these cracks, as it waterproofs the rest of the mortar, or whether to carefully enlarge and dovetail the cracks so that I can fill them with mastic more effectively. Any advice on this?