Concrete slab + insulation layer thickness

[Sonicbeaver]

Hey folks. I'm in the middle of building a base out of red bricks. The walls will be double bricked - the outer layer several courses higher to accommodate the elevated slab. My question is what should the thickness ratio be between the concrete slab and the insulation (vermicrete) layer above?

A course of bricks is 2.5" high. So two extra outer courses will leave a 5" cavity. Would something like 4" for the concrete slab work, with a 1" vermicrete insulation layer? Or should I add an extra course (7.5" cavity) and go 4" concrete slab and 2.5" vermicrete?

Cavity depth is constricted to multiples of brick height as this will make it possible to perfectly level things off afterwards.I'm in the middle of building a base out of red bricks. The walls will be double bricked - the outer layer several courses higher to accommodate the elevated slab. My question is what should the thickness ratio be between the concrete slab and insulation (vermicrete) layer?
A course of bricks is 2.5" high. So two extra outer courses will leave a 5" cavity. Would something like 4" for the concrete slab work, with a 1" vermicrete insulation layer? Or should I add an extra course (7.5" cavity) and go 4" concrete slab and 2.5" vermicrete?

Cavity depth is constricted to multiples of brick height as this will make it possible to perfectly level things off afterwards.

[SableSprings]

These brick ovens are heavy! Most builders go with a 4" deep, rebar reinforced top slab. Also, using vermicrete as your base insulation, you'll want at least 4"-6" below your entire oven structure for the best efficiency (you don't want the concrete top slab working as a heat sink!) We usually look at optimal height of the cooking floor as about your elbow height. That way you're not stooped over or reaching up to work the oven.

So, it depends a bit on your height. Your stand height + a 4" top slab + 1/2" tile layer + 4" vermicrete + cooking floor brick thickness should come up to your elbow height. (P.S. Don't forget your brick or block mortar layers as they add up!)

[UtahBeehiver]

Sage advice from Sable Springs. I would add, that the ratio for v/pcrete under an oven should be 5 to 1 for suitable strength.

[Sonicbeaver]

Thank you folks for the advice. 4" then for the insulation.

I've had a slight change of tact where instead of pouring the concrete directly onto the structure, I will be pouring it onto a form on the ground, in halves - and then using some extra muscle to help me lift these onto the plinth. The reason for this is that I will be able to extend the size of the slab over what is already quite a small structure. I'm also now using breeze blocks as it will take an age to build with red bricks. Four courses will bring everything up to elbow height.

I wanted to ask - as the plinth is only around 1m square, would a single walled structure of breeze blocks be enough to adequately support the concrete slabs, or should I double wall it? I was thinking that rather than double wall it all around i could just add columns at the quadrants. See pic for a top view of the plinth - the dotted lines are the concrete slabs placed over the u-shaped plinth, and the dark rectangles are reinforcements, i.e single columns of breeze blocks.

I will of course be using rebar in the slabs.

Also you mention brick ovens are heavy, but I'm actually building the dome from refractory cement (2") + thermal blanket (2") + vermicrete (2") + render (1") , thus reducing the weight load on the slab dramatically. In this case would you still recommended that I use a 4" thickness for the slab?

Anything thinner will.of course help to lift the slabs onto the plinth but if you still recommend 4"...

[david s]

I like your approach, I did the the same. By reducing the span between the supporting piers and cantilevering the slab over them, the thickness can be reduced. But remember that the strength of a concrete slab is proportional to the square of its thickness. So, halving a 100mm slab reduces its strength to 1/4. My slab is 910 x 910 mm and 60 mm thick where it needs to take the weight, but thinner where there's far less stress eg at the corners and in the centre. Pre cast steel and fibre reinforced into a pre made mould, cured, then lifted (2 man) the slab into position over the top of the mortared stand, with the small holes located and filled, so they sit over the cores of the brick stand. The slab then becomes the lintel for the structure. My ovens are small and weigh 250kg including supporting slab, but excluding stand.

Apologies, that pic is really old. I've since invented in addition to the centre under-floor moisture venting hole, 4 more cast into the mould, that locate into the dome insulation space between the dense inner dome and the outer rendered shell.