Chimneys have always been a problem area because they don’t get flashed right or the covering is installed incorrectly. Like any projection through a pitched roof you must start at the bottom and work your way up.
Dormer flashing is the first piece to go on, then the shingle tins, and then finish with a saddle. All this is done while the roof is being laid. Copper and galvanized steel are typically used because they are solderable, which is the proven way to make the flashing watertight. If pre-finished steel is used, the seams are caulked and has to be maintained. Wall coverings must be kept off the roof a minimum of 1”. This will allow the water to flow around the chimney and down the roof.
Attention has been given to the top of the chimney recently. We have made and installed many decorative tops. Copper and pre-finished steel are the popular choices of metal with a wide variety of designs ranging from vertical standing seam to curved standing seam. Whatever style fits the architecture of the house, we can build what you want. One thing to keep in mind is that these do need an air space at the base and an open top so the chimney can vent.