Hearth thickness

[P3 Stoaker]

G'day All,
Setting up to pour the oven supporting hearth. I have designed a reasonably big base meaning the hearth will span about 79" by 67". Looking to pour a 3 and half inch reinforced concrete slab hearth. Has anyone built a 3 and half inch hearth spanning that distance? I will have room on top for around 3.2" of perlite but I'm limited by the total combined thickness, if I make the concrete hearth thicker I loose insulation so It's not a simple case of making hearth thicker. Any thoughts appreciated.
Kind regards

[P3 Stoaker]

So I guess what i'm asking is what distances have you spanned with your concrete hearth and what thickness was the hearth?

Regards

[Chach]

you can recess the concrete into the center of the structure like a depressed pad and only leave 3 1/2" on top if you feel you need an extra inch but i think you would be fine as long as you have the rebar in there. If your pouring the cells of the block at the same time you can notch rebar into the cells from the added thickness or drill holes and insert rebar dowels if you poured the cells already.

[P3 Stoaker]

Thanks Chach for your info, the question didn't attract many replies.
In the end I thought best to position a permanent support under the hearth. I'll do that in the centre width ways so the span is less than half and protruding inside off the back wall 2 bricks long so the depth span will be similar meaning max span will now be less than 40 inch. I'll simply build a brick pillar and pour directly on/over that. The larger span would more than likely been fine but you don't really get a second chance and given the significant work involved I'd rather play it safe.
Kind Regards

[Chach]

Another option would be to get some structural pan decking that they use in new office buildings for the floor. They use this to support the concrete and usually pour 6" on top of this but I added rebar and only poured 4" I used this on mine and you could stand on this no problem without concrete. If you do the wall don't split it in half width wise split it depth wise. Your never going to crawl 70" into the bottom to get wood. I also built a wall not for support even though it does support the slab I really did it because I didn't see myself crawling that far in to get wood so I left enough depth for 2 rows of fire wood like 36" - 42" deep or so...I think this is what your saying your going to do. You will be more than fine with that for sure. When you pour your wall though you will not be able to take your form out that back side so you have to use no combustible material and that's why I used the pan decking do I didn't have to take it out.

[david s]

Thanks Chach for your info, the question didn't attract many replies.
In the end I thought best to position a permanent support under the hearth. I'll do that in the centre width ways so the span is less than half and protruding inside off the back wall 2 bricks long so the depth span will be similar meaning max span will now be less than 40 inch. I'll simply build a brick pillar and pour directly on/over that. The larger span would more than likely been fine but you don't really get a second chance and given the significant work involved I'd rather play it safe.
Kind Regards

Another option is to cantilever the supporting slab over the walls of the stand. This reduces the span, placing the support under the heavier part of the dome and also allows you to stand closer to the oven. The down side is that it requires some more complex form work.

[P3 Stoaker]

G'day All,
Thanks for the idea's. I have decided on a solution which in the end is relatively simple, and means the max span will be about 40 inch.I should add the base brickwork is complete and awaiting the hearth. A little background, the Pompeii Oven Instructions I downloaded talk about a 3 and half inch slab on a 65 in wide base that I'm sure will be fine if reinforced with steel mesh in the concrete. I have 2 relatives that built to the same instructions and both said build the base bigger. I always planned to build the base from bricks matching the house and double thickness with the inner layer being shorter and the outer layer. The inner layer supports the hearth and the higher outer layer of bricks forming the edge so the slab then perlite fills the cavity to be flush with the top of the outer bricks. I will then finish the top with slabs of granite, 40 x 20 inch up to the dome and then insulate to hide the cuts. This way the edge of the hearth is not seen and brickwork matching the house contains the concrete and insulation.
Those inner bricks are 6.7 inch below the outside row and this governs the available thickness as being three bricks lower was way too much. So I will have a 3 and half inch hearth and 3.2 inch perlite insulation layer.
My solution is to build a small wall dividing left to right extending forward from the back wall. It's easy, cheap and wont rust. I tried to load photo of the brickwork but it exceeds the MB limit of 2.0 and altering that exceeds my computer skills. I will try a few things when I get a chance to upload the photo. I'm ready to pour slab, little more form work but its pushing 105 deg f here often at the moment and getting to 115 at times. I'll wait till it cools a little, or lot preferably. I'm in Australia, its hot, winds and the country is literally burning. Our population is a little more than half that of California spread over an area almost as big as the USA and we have lost more than 2000 homes since the fires got bad around a month ago.
I will try to get the photo's up over the next week or so because I've not seen a big base like I'm doing and someone might be interested.
Regards to all.