Another Hearth Question

[mrpbjnance]

I am going to pour my hearth this sat. When I pour I am going to lay down the Durlock Concrete Backer board which will take up 1/2 inch. (I am not resting my forms on the board but I am leaving it there. I have framed with 2X6 which means my hearth will be about 5 1/2 inches thick. Plus I am going to put down the super isol board. Is tht to thick? should I reframe with 2X4s or would that be to thin?
I have plenty of concrete so the amount is not a concern. Will the thicker slab be to much to heat up?

Thx

[james]

A thicker slab would be fine and structurally very solid. Because the concrete is under the insulation, it will not get hot, so there are no thermal issues to is being thicker. The Super Isol will stop the heat and keep it in the oven.

The extra 2" in oven height also are not a big deal. Typical cooking floor height is between 38"-44", so you have lots of wiggle room.

James

[jwnorris]

Alternate to poured base

I will also be constructing my base this weekend. However, based on the heat transfer of the Super Isol board, I intend to place the Super Isol board directly on the backer board with no concrete base underneath. By base structure is constructed of square steel tubing, welded together, and supposedly engineered to hold in excess of 1300 pounds. Does anyone see any problems?

J W

[dmun]

?

based on the heat transfer of the Super Isol board, I intend to place the Super Isol board directly on the backer board with no concrete base underneath. By base structure is constructed of square steel tubing, welded together, and supposedly engineered to hold in excess of 1300 pounds.

I don't see any problem with heat transfer, or the strength of your steel base, but I do see a problem with the concrete board, unless we're thinking about different products. The half inch concrete-in-glass-mesh stuff is used for mounting tile, and such. I don't think it has any tensile strength. I've said here that a lot of the weight of the oven is transfered to the edges, but not all. I think unless you are planning on lots of tubular steel pieces supporting the oven, that you are not going to have enough support to prevent cracking.

In flooring, that concrete board is used with multiple layers of thick plywood, overlaid with a layer of concrete based mortar before the tile or marble is put on top. It never just floats between joists. You can't use plywood here because of clearance to combustibles, but maybe some kind of metal base would give you the support you need.


I'm not a structural engineer, but I think you need to think about how heavy your oven is before you commit to this.

[james]

Hey JW,

You need something under the oven that is rigid, to keep the floor and oven from flexing. The hearth and stand have to hold the oven up (and take the weight); they have to keep heat inside the oven; and they have to provide a rigid and stable platform for the oven floor and dome. I think you should add a simple concerete slab to provide the rigidity function.
James

[mrpbjnance]

How long to dry

Ok My hearth is poured. I used Durarock concrete board and had that sitting on the block stand. I poured about a 5-6" hearth and will put the super isol on top of that. I have been keeping it wet by spraying it done now and them but how long should I let it dry before I start building the oven?

Thx
Mike

[dmun]

go ahead

As soon as the hearth slab is solidified, (24 hr) you can proceed with the next stages, the additional masonry on top will keep the surface of the concrete drying out and weakening before it's cured.

There have been instances of failure of incompletely cured concrete, but mostly in fast-tracked apartment building scale projects. You are unlikely to pile enough weight on that slab to compromise it.

[james]

Go for it. Your hearth slab will have lots of time and air to cure naturally, while you are building your oven.
James

[mrpbjnance]

Alright!

Great I thought I had to wait a week! The super Isol will go on today!!!!

[jwnorris]

In my case, the base structure of tublar steet is designed to fully support 1300 pounds. There are several horizontal support pieces and a center support column. I am not worried about the base or the cement board.

On top I have put 2" of SuperIsol board. I will build the oven floor on that.

I intend to glue the SuperIsol to the cement board with the glue that James sells and set the oven floor on a bed or Refrax motor.

J W

[dalucca2003]

I am going to pour my hearth this sat. When I pour I am going to lay down the Durlock Concrete Backer board which will take up 1/2 inch. (I am not resting my forms on the board but I am leaving it there. I have framed with 2X6 which means my hearth will be about 5 1/2 inches thick. Plus I am going to put down the super isol board. Is tht to thick? should I reframe with 2X4s or would that be to thin?
I have plenty of concrete so the amount is not a concern. Will the thicker slab be to much to heat up?

Thx

I am getting close to framing my hearth....probably this weekend. I have a question in regards to the wood frame for the hearth. I will be utilizing 2 x 6's and in looking at some old instructions it states that the frame should sit in top of the stand. It further states to make the frame roughly 1/2" smaller than the block stand.

In looking at the most recent instructions, there is no mention about the frame resting on top of the block stand. My question is does the frame need to sit on top or what is the best method for setting up the form for the hearth?

[maver]

either

I made mine on top of the hearth stand for convenience, but that leaves me a small gap to either fill or make it look by design when I cover my hearth stand with bricks. No reason that you need to do it that way, you could certainly build an external support under the form and around the blocks. I think the external bracing is more important than how you choose to support the form. I used ratcheting cargo ties, which worked very well.

[james]

I have made my form the size of the stand (just a tiny bit bigger), and held it in place with 2"x4" supports. That way it is easier to make sure the hearth form is completely level.

Still, either way is fine.

James

[Marcel]

Consider 2x8 (EIGHT) for the small additional expense.

(M) I wish I had placed my hearth retention boards on the outside rather than resting on the concrete blocks. The lip turned out to be a lot bigger problem than I had at the time anticipated.

(M) If I were to build another oven I would opt for 2 x 8 (eight") form boards as that greater height would allow me to strap the boards tightly to the block stand, support the smaller edge from underneath, and still have ample space for the 5 1/2" of hearth slab. I'd vertically overlap the outside of the concrete blocks by about 2" so I could fill the form with enough refractory concrete and re-bar to be able to screed at the top of the 2 x 8" forms.

Ciao,

Marcel

[james]

You can see the 2"x4" support in the lower left corner of photo 3 on the online installation pages:

http://fornobravo.com/pizza_oven_ins...ng_hearth.html

We are definitely recommending a 2"x8", with the new hearth design that is 7.5" thick.

Also, I don't think you need the straps. 2"x8" is a hearty material, and I think it has the strength to not bow out under the weight of the concrete.

Would other builders agree with that? It saves a step if you don't have to do it.

James