Question on hearth construction

[adm]

I am still at very early planning stage, currently deliberating between modular and brick built oven. I am leaning heavily toward full brick build though and am planning around that....

I am currently trying to draw out my base and hearth, so could do with some advice....

The base itself will be on a poured slab reinforced with rebar. I will put membrane down under the slab. Walls will be concrete block filled with concrete and rebar.

On top of that, I will pour another slab for the base of the hearth. I am thinking of making this 100 or 150mm thick, then laying a course of thermalite blocks on their side on top of that. On top of those, I would put 50mm of the ceramic fire block board and then finally on top of that the firebrick for the hearth itself. I am leaning towards having the firebricks on their side as opposed to flat to get some extra thermal mass.

So - does that sound about right? Or am I missing some vital ingredient? I see lots of vermicrete used in bases, but if I use thermalite blocks, the ceramic board and firebricks on edge, do I need it?

I like the idea of having the firebrick hearth inside the oven dome, with a soldier course of firebricks around the outside rather than have the dome bearing on the firebrick floor....which gives me another question:

For the ceramic board - should that be cut to the same dimensions as the inside of the hearth itself with only the firebrick hearth on top of it, or should it cover the entire base of the oven with the dome itself also bearing on it? I am not sure how strong this stuff is in terms of compression and there would be a lot of loading on it from the dome.

Anyway, as you can see I am still at early planning stages but would welcome any advice!

Thanks,

Alasdair

[turkey]

Hi Alasdir,

how large are you planning on building?

Rebar in the base id good but I do not see you mention it for the hearth where arguably it is needed more.

Hollow block legs with rebar and filled cores, thats pretty industrial for over here, I think in the US you get a lot of builds round California which is an earthquake zone so they really over engineer their build for safety, this is not such an issue over here so whilst there is no problem with this approach it is more than is required and as such costs more but is personal choice.

Insulation seems suitably overkill and you should have no issue at all

The hearth firebricks do take quite a while to heat up, adding the bricks on their side will really take some heating to get going, for a pizza you want a hot hot base to get a good char underneath. Personally I would lay the bricks flat if making a pompeii oven for pizza.

it would be interesting to know others views on this.

[johnaero]

i made my stand with small foundation, think 6inch strong mix (no rebar as groud rock solid) then built up with 4inch block and brick giving total 8.5inch with wall. i found it easier and cheaper this way than having to buy hollow blocks and fill them whick i think bit much.

seen loads ovens build with either 4inch and brick or plain 6 inch wall around UK and europe.

as mentioned the USA versions seem very overkill but they dont use lot solids more cavity blocks in normal construction so guess that why used as cheap and common

[johnaero]

id be more concered about making a strong slab on top of the walls more than the walls themselves as this has to span a fair distance while taking over 500kg weight

[adm]

Thanks for the reply!

As for size, I am thinking around 900-1000mm internal oven diameter. Forgot to mention rebar in the hearth - but I am absolutely intending that as well. Also a DPM in the blockwork to make double sure of keeping damp wicking away.

Maybe I don't need the filled cores on the blockwork though - that's just what I have seen in many build diaries, but you are right - we don't get much earth movement over here and it would certainly save on concrete.

As for the firebricks on their side - another good point. I do want the oven to heat up fairly quickly, and I want it to be multipurpose so I can do pizza, roasts, a bit of bread making and some overnight slow cooking - but I probably don't need the extra thermal mass of bricks on their side so might reconsider that. I just thought it would look nice, but if it means the oven takes much longer to heat then I may as well save some bricks!

You know how it is when you plan things like this though - massive overkill always seems like a good idea at first!

On the ceramic board - I have no experience with this stuff, but always thought of refractory ceramics as kind of brittle. Does the board have enough compression strength to bear the weight of the dome and covering? I'm still trying to get straight in my mind how the board should be used - whether it just fits under the firebrick cooking hearth, under the hearth and dome, or whether there is any benefit to just putting it down over the whole base. I guess not on the last one....

Cheers,

Alasdair

[h12rpo]

The accepted wisdom on here seems to be that if you use celcon / thermalites on their sides on top of your slab and under your firebrick hearth then theres simply no need for the ceramic board - save yourself the expense, it will add nothing in terms of heat reflection, insulation or mass.