Fire resistant timber for framing out fireplace.

[Glenbirnie]

At my last house which was a self build (95% by main contractor) I had an inset wood burning stove fitted into a concrete block chimney breast with Ankey liner etc

The block work was fixed with fire retardant 2" x 2" and then sheeted.

Does fire retardant timber have a code like C16 etc so I know I am getting the right stuff?

Thanks