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Nick P 03-26-2013 10:51 PM

Alternative to concrete hearth
Hi All,

Been reading for a while now and now it is time to start planning and designing my oven.

I have a small issue where I can not be 100% sure of the load bearing capacity of the slab that exists where I want to put the oven.

I am looking at building a smallish 32" oven. From what I can guess the oven will weigh about 500kg (floor, dome, insulation and render finish).

From my rough calcs a 4" concrete hearth would add about another 500Kgs.

Are there any alternatives to a concrete hearth? I have seen the Better homes and gardens plans where they have angle irons across the gap with 12mm cement sheet onto.

Would something like this work?

Thanks for any advice, Nick

col555 04-05-2013 05:16 PM

Re: Alternative to concrete hearth
Depending on the angle iron and the spacing im sure you can get it strong enough, im new here but im thinking the concrete is used for heat retention purposes as well as strength.
Someone more knowledgable would have to answer on that:)

hlcr 04-12-2013 01:06 AM

Re: Alternative to concrete hearth
Hi Nick,

Did you ever get a satisfactory answer to your qtn? I am also using the BH&G pattern for the foundation only and having the same issues with not being sure of how sturdy my foundation is.

I was going to use more angle irons, a thicker sheet of cement, insulation board on top and then some 50x50 paving slabs as I read somewhere that the crushed granite and sand would not work at all for support or heat retention - would appreciate any feedback! :)

cobblerdave 04-12-2013 08:28 PM

Re: Alternative to concrete hearth
Lose the paving slabs and the gravel. It's the top of your stand first. Yes on a smallish oven compressed concrete sheet with angle iron should be good. If it was me 300mm spacing at the largest. Then comes your insulation then your hearth brick that's your thermal mass. If you don't isolated the stand from hearth and oven it will only chew up fuel and your oven will never work properly.
Yes I have seen an oven with deco granite under the hearth it doesn't work well and is hardly used.
Go to the forno shop and drop a free set of plans in your basket. You'll find its more than a set of plans it goes right in to how the ovens work.
Regards dave

david s 04-12-2013 09:34 PM

Re: Alternative to concrete hearth
You could also use some Hebel Power Panel which has some steel reinforcing in the centre. The stuff is about a third the weight of normal concrete so expect about a third it's strength also. The manufacturers, CSR, can tell you the kind of loading they'll take. You can place the piers under it in such a way that you don't have too much of a span, say an extra one in the middle and/or cantilever the stuff over the outside, where the oven is lighter anyway. It is also a pretty good insulator and could save you by reducing the insulation under the floor. Don't know how big an oven you are planning, but another way to reduce weight is to use 6" rather than 8" concrete blocks for the stand. Apart from being lighter because they're not so wide, the voids in the 6" blocks are around half the volume of the 8's, a big saving in concrete and weight.

cobblerdave 04-12-2013 10:58 PM

Re: Alternative to concrete hearth
Gudday Davids
You used hebel power panel under your portable oven on a trailer didn't you.
How do you think it proforms as an insulation. I have it under my oven with no probs to date, but am reluctant to recommend it in case it does cause problems for someone else. I really value your answer on this
Regards dave

david s 04-12-2013 11:36 PM

Re: Alternative to concrete hearth
I've built a few ovens using 75 mm Hebel as the insulating slab over an existing concrete supporting slab, although not having the floor sitting directly on the Hebel. I used 25 mm of perlcrete between them.Not having ever demolished one of them I couldn't tell you how well it stands up, but I suspect ok. My mobile is slightly different. I was after strength and insulation all in one and just used the cooking floor straight on the Hebel, but again with 25 mm of perlcrete separating them. It has cracked, but having been over corrugated roads (no shocks) and a severe speed bump, I'm not sure what caused the cracking.
Artisan glass slumpers use Hebel as moulds in their kilns and the stuff lasts around 5 firings. They take it to around 900 C
When I did an experiment with foamcrete, essentially the same stuff, i placed a sample on the stove hot plate for 30 mins. The surface of the Hebel that was in contact with the hot plate was quite powdery but only the surface, presumably deteriorating the portland there but not penetrating deeper.

cobblerdave 04-13-2013 02:26 AM

Re: Alternative to concrete hearth
Gudday David's
Thanks for the detailed reply. It's basically what I expected and I sorry if I appear to be just validate my build. I too don't feel like pulling my oven apart just to find out what is going on in the depths. It hasn't fallen apart, cooks like a champ and has been doing that for for two yrs now.
I expect to see a few more stands built from hebel .... It's just so easy to use.
Again thanks for the more than detailed reply

Regards dave

hlcr 04-14-2013 04:43 PM

Re: Alternative to concrete hearth
hi all,
thanks for the responses, my dome is only 25-30inches on the inside so quite tiny...thanks for the tip re plans, i'll do that now :D

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