Worth the hassle to pour 2" of concrete, 2" of vermiculite, then use 2" of fb board?

[jas0ny]

Im having a hard time figuring out how I want to pour the hearth. I have the frame built ready to pour. I wanted to know what are your thoughts on me pouring 2" of concrete, let it cure, then pour 2" of vermiculite on top of that to finish the hearth slab, then use a 2" fb ceramic board? Is it worth that extra insulation? Or have most of you concluded that the 2" ceramic fb is well enough? thank you folks!

[RandyJ]

For one thing you will need more than 2" for the support slab. I would go no thinner than 3.5". As to the insulation that is more a factor of how much insulation you either can afford or are willing to afford. You can get away with 2" and have a fully functional oven. If you want to have the ability to do extended baking or roasting the more is better. I chose to do 4"of ceramic board and did not want to wait for the vermicreet to dry as it can take a while like a week or 2. I do not regret going that route but the cost does go up each time you make a change it is amazing how a little here and there add up. I hope this helps.

Randy

[jas0ny]

Randy, so the 2" i would pour on top of the 2" of concrete would not make it a 4" support slab? or does it need to be a minimum of 3 1/2" of pure concrete? I might go with your idea of 4" of ceramic board if its worth doing instead of pour the vermiculite

[SableSprings]

Randy, so the 2" i would pour on top of the 2" of concrete would not make it a 4" support slab? or does it need to be a minimum of 3 1/2" of pure concrete? I might go with your idea of 4" of ceramic board if its worth doing instead of pour the vermiculite

The vermicrete has compressive strength but is not going to provide the kind of total support you need in the top slab. As Randy indicated, 3.5" - 4" of reinforced (rebar) concrete is considered a minimum for a masonry WFO build. Many builders here have done the 4" concrete slab and then poured an insulating layer of 5:1 vermicrete or perlcrete under the cooking floor. The ceramic board is roughly twice as insulating as either 5:1 mix you do yourself with perlite or vermiculite. So 2" of ceramic board is approximately equal in insulation value as 4" of vermicrete or perlcrete. The trade off is that ceramic board is a bit more expensive but putting down 4" of ceramic board will give you serious heat retention (as long as you do a good job of making heat breaks, insulating the dome well, and making an insulated door). Both the ceramic board and insulating cement mixes will absorb water readily and can easily be damaged by abrasion, so they shouldn't be exposed in any way to the elements or day-to-day wear & tear with pans & peels.

If I had it to do over again, I'd put down 4" of ceramic board on top of my 3.5 concrete slab (underneath my cooking floor). My current 4" of perlcrete underneath my cooking floor allows me to cook for two days after a firing, but I'm down to 250F at the beginning of the third day. The mantra of this forum is to put in as much insulation around your oven build as you can afford and after 7 years of using my oven I can't think of better advice.

If you haven't already, do spend some time looking at and reading about some of the well documented builds mentioned and linked in the Newbies forum (2nd sticky thread -- A Newbies tour of treasures hidden in the FB archives by LBurou). Looking forward to your build!

[david s]

You haven't said what size oven you are planning to build so the supporting slab suitability is a debatable issue. The thickness and therefore strength of the supporting slab is dependant on the weight it has to support as well as the span between the supports on the stand. For a small oven with the slab cantilevered so the mass of the oven is over the supports then 2" is sufficient. Concrete stair treads are usually around 2" thick, but not designed to hold the weight of a one ton oven. But if you are expecting a 2" slab to support a 42" oven with the supports at the perimeter, I think you are in for some problems. The slab should be reinforced regardless of its thickness. You can consider a 5:1 vermicrete slab a negligible strength contributor, with good compressive strength, but poor flexural strength. It would probably break if you put your full weight on it, likewise cal sil board.

[SableSprings]

As David S noted above, the oven size (and type) is a huge factor in the slab requirements. I certainly missed that basic fact in my post and am really glad he has pointed it out for your decision making process. David's elaboration/clarification in post #5 is really good info for you (and all of us) when planning a build.

[RandyJ]

David you are right there are other options depending on how you are building and what size as well. I just went on the assumption that the walls will be along the edge and the oven in the middle. Then in that case I feel 3.5 is the minimum. I know mine is over kill but the support slab is 6" thick. I did this to regain a few inches I lost by dry stacking the base.

Randy

[jas0ny]

You haven't said what size oven you are planning to build so the supporting slab suitability is a debatable issue. The thickness and therefore strength of the supporting slab is dependant on the weight it has to support as well as the span between the supports on the stand. For a small oven with the slab cantilevered so the mass of the oven is over the supports then 2" is sufficient. Concrete stair treads are usually around 2" thick, but not designed to hold the weight of a one ton oven. But if you are expecting a 2" slab to support a 42" oven with the supports at the perimeter, I think you are in for some problems. The slab should be reinforced regardless of its thickness. You can consider a 5:1 vermicrete slab a negligible strength contributor, with good compressive strength, but poor flexural strength. It would probably break if you put your full weight on it, likewise cal sil board.

I am building on a 36" plan.
My objective is once everything is finished I want the look of the cooking floor flushed with the hearth while keeping the hearth thickness at minimal. What I gather so far is I would pour 3 1/2" of reinforced concrete slab with 4" of ceramic boards. Once I build the WFO what is the recommended material to pour around the 4" ceramic fb boards. I am afraid concrete will retain and soak up the ceramic boards if i pour around it. Thank you for your help and insight guys. Been lurking around the forums for sometime, but I catch myself overwhelmed with all this information that I read on here.

[david s]

If you want to do it that way a 5:1 vermicrete would be suitable, but you would need to stucco and seal over it so moisture doesn't get into it. It's a good idea to slope it away from the oven to discourage moisture collecting against the oven.I'm presuming you are planning an igloo style rather than an enclosure.