Upgrading of existing doors
Years ago, it was accepted practice to improve the performance of an existing door to a half-hour fire-check or fire-resisting standard, although in some cases it was more economical to replace the door rather than alter it. The doors were usually panel type or a light core flush type about 44mm thick: they require a facing on the risk side with a non combustible board.
It is now the accepted practice to fit new fire doors and fire door sets in preference to upgrading them but this is not always an option, as in the case of historic/listed buildings, for example. These days you can configure and order entire fire doors with either 30 minutes (FD30) or 60 minutes (FD60) protection online. These fire doors can be configured with or without frames.