Chimney flashing should always be installed as a two part system. The sub flashing and the counter flashing. The front of the chimney is the first to get sub flashing and that is called an apron. The apron flashing sheds the water that runs down the front of the chimney onto and over the roof shingles below. Installed wrong, this flashing can be one of the first roof components to fail.
The apron flashing shown here has been installed as one piece . The roofers have fashioned the flashing from a piece of copper sheet metal and secured it to both the chimney and the roof deck. The lower edge is nailed with standard zinc plated roofing nails and the top is nailed to the chimney with hardened concrete nails. The nails used to fasten the lower edge should have been copper which would have lasted the life of the copper flashing without staining the roofing. The top edge has been sealed with an asphalt caulking to prevent leakage.
The problem is that the top edge of the apron flashing has pulled away and broken the caulking seal. This is because the metal will expand and contract with changes in temperature. And on a roof there are large temperature swings, especially with a slate roof. There is also movement between the roof decking and the chimney. The chimney is a solid masonry structure with a firm foundation, and the house and roofing structure is built around but not secured to the chimney