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mrpbjnance 08-01-2006 09:30 AM

Insulating slab with Super Isol
I have purchased the Super Isol board from James... (Thanks James! Your help is appreciated!)
My question is what thickness should I make my insulating hearth now so it is strong enough to support my oven. 2"...4" Do I still need to do 6"?


dmun 08-01-2006 09:40 AM

thickness of support slab
4" of reinforced concrete is what is called for: This gives you two inches on either side of the grid of re-bar. Remember to keep the ends of the re-bar away from the edge of the slab, but long enough to be above the source of support.

Alan 08-01-2006 11:19 AM

Are you using one layer or two if the Super Isol?

mrpbjnance 08-01-2006 12:19 PM

I just purchased it...I only bought enough for 1 layer.

Should the rebar be lon enogh to reach the blocks end to end? Or does that matter?


james 08-01-2006 12:24 PM

Thermally, one layer does it. 4" of vermiculite concrete has proven to be enough to effectively block heat movement, and the 2" Super Isol should do a better job than that. I think we're in a safe zone.

For the rebar, your lengths should reach to the top of block stand, but not go to the end. You want concrete "around" the rebar, with the rebar "resting" in the center of the slab, on the blocks.

From our concrete experts, structurally -- why is that true?


Alf 08-01-2006 02:49 PM

Reinforcing steel should be encased in a minimum of 50 mm (2") of concrete to protect the steel from the elements and thus rusting. Thatís according to our Uk building regulations and the fact we live on a generally wet / damp Island.


Alan 08-01-2006 06:46 PM

Rebar can be placed in different locations in the slab according to the size of the slab and the loading. It will generally keep the parts of the slab from separating if it cracks, and can make it stronger as well. A slab that's resting on its edges and loaded at the center will be in tension on the bottom and in compression on the top (think of the slab sagging a very tiny amount with the top concave and the bottom convex - this is how the slab will move, though the movement will certainly be too tiny to see).

Concrete is much weaker in tension than in compression, so if the slab were heavily loaded you'd put the rebar near the bottom where it's in tension but be sure, as Alf says, to have enough concrete to protect it from corrosion. I imagine that a 4" slab is very lightly loaded compared with its ultimate strength, which is why people do fine putting the rebar near the center. It still adds integrity and strength to the slab. If it were to begin to crack, the sections are held together and can't go anywhere.

mrpbjnance 08-01-2006 08:21 PM

adding a center section
I know its not necessary but I am leaning toward adding a center section of blocks to support the middle of the slab. I also plan on usig concrete backer board instead of plywood and just leave it there.
So I will pour the slab on top of the backer board and then put the super isol on top of the slab..then the oven on top of that.

dmun 08-02-2006 04:38 AM

A reminder; if you block in the back section completely, you need to put in a vent. If the point is just additional support, you could just add a stack of blocks on each side, and still have the additional wood storage. Remember: the oven distributes most of it's weight to it's edges. Just building a pier in the center wouldn't be that useful.

Other than that, your plan sounds good.

camarina73 08-02-2006 05:16 AM

layer of super isol and 2" of vermiculite
I wanted to put 4" of concrete super isol and 2" of vermiculite in that order. That will put my top of landing at 42.5". Two questions. 1. Is that to high? and 2. Can I pour the concrete slab, Put super isol on top with no adhesive and then pour the vermiculite over? I wanted to do this so I would not have to adhere super isol to vermiculite layer. Please let me know your thoughts thank you.

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