Here in Pensacola, FL and the Gulf Shores, Al area many of the homes we inspect have prefabricated fireplaces and chimney units. It is very common after a short period of time to find them sagging and rust forming on the top. It doesn’t take long for that rust to work it’s way through and next thing you know there are holes allowing water to enter into the home.
A few repair people have had some success making repairs to prolong replacement. A few of those repairs even looked pretty good. It may seem rather petty but…The alternative is to spend $4-600 and have a new cap put on.