Cement board under fire brick floor?

[microbe]

I have completed my hearth and have a 75mm perlite-crete insulating layer. It is pretty flat but not perfect.

A sheet of cement sheet (hardie board) on top of the insulating layer would be an ideally flat surface for my fire brick floor. It is supposed to be fire proof https://askinglot.com/is-hardie-board-fire-retardant

Any thoughts on my plan?

[UtahBeehiver]

You can level with a sand/fire clay mix. Those floor bricks appear to be what are called splits 1" thick or 25mm in your case. This thickness is not sufficient for retaining floor heat. A min of 2" or 50mm is recommended.

[microbe]

Thanks for that. I have sand and clay and was considering that solution, but the cement board seemed easier.

I am pretty much committed to the split bricks now. It was pretty much impossible to find 50mm bricks in Sth East Queensland (Australia).

[microbe]

UtahBeehiver - thanks for the link to your album, beautiful work. Is there a site with a step by step of your build?

[UtahBeehiver]

No, my only my pics document my build.