Galvanized steel is the least expensive chimney flashing ... such as roof decking rot or ceiling drywall. The chimney may need to be rebuilt or replaced as part of the chimney flashing replacement ...

The caution seems to center around welding, though it's also one of the known dangers of cutting galvanized steel. However ..

Galvanized pipes, which were used for decades, are made of steel with a coating of zinc ... only to discover soggy walls, floors or ceilings. Some companies have built on the idea of blasting ...