Using layered cement board for hearth subfloor

[ftaylor74]

Hi all,
Has anyone used several layers of cement board to form the sub floor of a WFO? I am considering using 5 plies of 5/8 inch cement board, bonded together with Heatstop 50 mortar, and then cut to the shape of the oven floor.

This will lay on top of a 4 inch concrete hearth. Fire bricks will then be laid on top of the sub-floor.

Would this work?
Thx

[SevenAcre]

Re: Using layered cement board for hearth subfloor

While it sounds great in theory, you will still need insulation beneath your firebricks. Without insulating you are just creating thermal mass with your cement board that will draw heat out of the oven. Since they'd be resting right on the slab, the oven floor would never reach temp.

Unless of course, when you say hearth you are talking about the oven entry and vent which you intend to be entirely isolated from the oven itself via a thermal break; then I don't see why it'd matter.

Hope this helps,
-j

[jeeppiper]

Re: Using layered cement board for hearth subfloor

I agree with Sevenacre about the insulation layer. Another thing to consider is that cement board is that it is not really designed to handle excessive heat. Over time, the thermal cycling of the cement board may cause it to break down and crumble.....which may or may not cause problems since it is underneath the hearth firebricks.

I recommend sticking with the time-proven methods: either silicon fiber-board or insulating concrete.

[ftaylor74]

Re: Using layered cement board for hearth subfloor

OK, I understand the point. I thought that cement board would have some insulation properties (lots of air voids in the stuff I've used in the past), and that layers of HeatStop would further insulate.

In any event, there are better (and also easier) ways to accomplish this.

Thanks for the input

[SevenAcre]

Re: Using layered cement board for hearth subfloor

Yeah, like I said it was probably one of those ideas that sounds like a great new solution, but ends up being impractical in application. Refractory mortar like the HeatStop is not an insulator. When described as heat resistant it simply means it will retain its integrity when exposed to temperature rather than breaking down and crumbling like a traditional material.

Good luck with your build!

-j