Fireplaces installed in most homes in the 80’s til now are no longer masonry. They are called zero-clearance or factory built fireplaces. They have a metal firebox and metal flue and typically a stucco chimney with a metal chase cover. In the business, they are often, derogatorily, called temporary fireplaces, especially if they are located near the ocean. What happens is that the salt air and higher humidity causes the chase cover and cap to rust and decay fairly rapidly. When they deteriorate they allow water inside the chimney, causing more problems. This is a replacement we did in Manhatten Beach, using copper which in time will patina that lovely shade of green.