Fitting a gas fire surround

[Dimwit100]

This is my first post post so hello to one and all.
I am planning on replacing an old gas fire and fire place. The new one will be a wooden surround with a marble insert in which a gas fire will sit.
I know what size the opening in the chimney breast needs to be, but my question is when the marble insert is placed in front of the hole does it need to be sealed with something like a piece of fire board or fire proof sealant?

[htg engineer]

The marble back has to be sealed to the wall and the gap around the chamber opening between the marble and the wall must be sealed, you can use sand and cement or plaster.


htg