Another Hearth Question

[dalucca2003]

Originally Posted by 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

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]

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]

(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

[maver]

I guess for the straps I'm more concerned about how the corners are fastened, and the straps are so easy in that regard. I don't think the straps do much to prevent bowing (they might even worsen it if your wood has a lot of bow, think violin string) but if you have a 2x6 or 2x8 that has some twist to it you can drive it square with the straps (I used 2 to bring both top and bottom corners into alignment). You certainly can use the straps later when you pick up materials for the finishing of the project with your neighbor's old pickup . However, conventional concrete form nails are cheaper and should hold.

[christo]

It seems like we are talking about several different issues on this thread.

It sounds like the 2x8 is great if you are pouring a 3 1/2 thick slab followed by 4 inches of insulating concrete. - but you won't have any overlap down on the blocks of the base.

If I'm using the super isol, then a 2x6 or 2x4 is fine (5 1/2 or 3 1/2 thick pour) and can wrap the outside of the base blocks if I wish.

Now the question for MArcel, did you go with a 5 1/2 inch thick pour for a little extra support? In the wisdom of the list - is 3 1/2 enough without the insulating concrete - I did not think the insulating concrete mixture added much if any strength.

I have paint on the ground and am digging footings this weekend!!! - I'm behind schedule already.....

[CanuckJim]

All,

One comment about strapping forms from an old hand at concrete pours. The rule is: if you don't do it, it will bow. If you do, it won't. I'd take the extra time to support the centres of the 2 x 8s to counteract any possible distortion. If the form boards do bow at all, it will create a lot of headaches down the road with facade treatment, etc. Not worth the risk, and you can't do it once the concrete has been poured, either.

Jim

[Marcel]

Originally Posted by christo

It seems like we are talking about several different issues on this thread.

(C) It sounds like the 2x8 is great if you are pouring a 3 1/2 thick slab followed by 4 inches of insulating concrete. - but you won't have any overlap down on the blocks of the base.

(C) If I'm using the super isol, then a 2x6 or 2x4 is fine (5 1/2 or 3 1/2 thick pour) and can wrap the outside of the base blocks if I wish.

(M) It sounds, just above, that you would be resting your form boards *ON* the concrete blocks if you were using less than 2x8s. If so, and you are OK with the consequent lip, go for it. My choice would be the 2x8s because I would only pour a 5" or 5 1/2" thick hearth slab and the additional cost for the deeper form boards is probably less than $20.

(C) Now the question for MArcel, did you go with a 5 1/2 inch thick pour for a little extra support?

(M) I went with the 5 1/2 inch thick pour because that was the recommendation in the original Forno Bravo plans. I first poured about 2" of perlcrete and followed that with about 3 1/2" of refractory mortar.

(M) I'm happy with the thickness and the traditional order of the layers: Insulating on the bottom, refractory layer on top. I feel the order of the layers depends on the builder's priority concerning the emphasis on either pizza, or bread. I was OK with waiting longer for achieving the high heat for pizza but retaining that heat for baking bread and rolls.

(C) In the wisdom of the list - is 3 1/2 enough without the insulating concrete - I did not think the insulating concrete mixture added much if any strength.

(M) "Wisdom" is in my view and in this context, a subjective term. I agree that the insulating layer, irrespective of whether it is on top or bottom, lends relatively little strength. Perlcrete is filling the role of the insulating boards and certainly those boards add almost no strength.

(C) I have paint on the ground and am digging footings this weekend!!! - I'm behind schedule already.....

(M) Then you better stop reading my meanderings and get to work.

Ciao,

Marcel

[dalucca2003]

I am glad I asked the question about the frame....the consensus certainly looks like it is best to place the frame for the hearth around the block stand versus on top of it. If I do go with the straps I have seen posts mentioning they can be purchased at WalMart. Is there any particular strap or straps I need to look for and how many are needed?