hearth thickness

[mrgweeto]

After a whole month of rain in CT we have the foundation poured. We are going to put up the walls this weekend and by next week hopefully willl do the hearth pour. I have seen 7"thick and 4"thick hearths. We were thinking of 4" with an inlaid 4" of insulting cement. Then the floor of firebrick over that. I think that should be more than adequate. Does anyone have any other suggestions that they have found from building their own?
G

[DrakeRemoray]

Re: hearth thickness

I think the supporting slab (what I think you are refering to as the hearth) should be 4" thick. Then the insulating cement needs another 4" for a total of 8". The word "inlaid" gives me concern. If you have a 4" supporting slab, you cannot inlay 4" of insluating concrete into it, there would be nothing to support the insulating concrete...

If height or thickness is a concern, you can use fornobravo board (or some other fancy insulating board) in place of the insulating concrete....

Drake

[mrgweeto]

Re: hearth thickness

Drake,
I guess I shouldn't have used the word inlaid. What I meant was 4" on top of the cement hearth but not all the way to the edge. Just where the dome floor will sit on. Actually I was thinking that I could get away with 4" of concrete and then 1-2" of insulation board that they use in cruciable applications. I just think that 8 inches in total plus the thickness of the firebrick floor is overkill. Of course this is my first endeavor and that is why I refer to those that have built WFOs. I don't mind the work but if the difference in heat retention is minimal between 4" and an insulating board and 4" of concrete and 4" on vermiculite and portland cement then I'd go with the former.
G

[james]

Re: hearth thickness

Hey Mr. G,

A 4" slab with rebar should be fine. On top of that you can either pour 4" of vermiculite concrete or use 2" insulating boards. Both will work fine thermally, so it is really a matter of choice; easy of installation and cost. In terms of efficiency, 2" of FB Board is equal to 4" of vermiculite concrete.

I always think that the hearth only has to do two things: hold up the oven and stop heat from leaking out the bottom.

Have fun and good luck with the weather.
James

[mrgweeto]

Re: hearth thickness

James,
Thanks a lot for the info. I agree. I think the most inmportant thing is to insulate the dome and the total enclosure between the outside walls and the blanket/refractory covered dome. Rain is coming in for next week also so we are going to have to work in between the rain drops. It's amazing how as we are building this we can taste the pizza.
Once again thank you for the input.
Guy