New dome build in Perth Australia

[Ricey]

Hi
I have been planning to build a pizza oven in my back garden for a couple of months, finally getting around to do it.
So far I have half built the frame, I hope to get that completed by the end of this week.
The more youtube videos and forum posts I read the more confused I get.
I was thinking of creating a 80mm slab with a styrofoam insert (40mm in the shape of the base of the dome). The idea would be to fill the styrofoam hole with Perlite where the firebricks would sit on top of. Then I would use Perlite for the interior of the dome, but now I see people saying Perlite is not good for the interior. So what is the best for interior? I would imagine using firebricks would be expensive and then what would you use as mortar to handle the heat?
With so many methods is there one this forum thinks is the best?
Also, anyone in Perth with recommendations for buying insulation etc.
Thanks
John

[MarkJerling]

Hi John and welcome!
Are you suggesting that your supporting slab will only be 40mm thick in part?
Firebricks are good for the dome, alternatively if you're cheap like me, I have found old night store heater bricks work well. Yes, you do need fire cement.
Kind regards,
Mark

[Ricey]

Hi Mark,
Thanks for the reply and for sharing your design,
Interesting to see you using attic insulation, is it as good as thermo blanket?
I was thinking of 80mm slab with 40mm of Perlite in the centre of it.
I see some people use perlite or fire board bedded into the slab, under the fire bricks and others just put firebricks on top of the concrete slab.
It seems you paid a lot more than I would expect for the build, what were the most expensive parts?
Ive bought my sand, cement and bricks from a salvage yard so not really spent so much so far to build the frame, I can buy a meduium size DIY kit for $1100, wondering would that be the best way forward.
Thanks
John

[david s]

Do not place the firebrick floor directly on the concrete supporting slab. If you do the heat will conduct into the concrete resulting in fast temperature loss.It is imperative to insulate between the brick and the concrete. Typically a minimum of one layer of 40mm calcium silicate board, but two is far better. The stuff is not cheap so an alternative is to lay up a 5:1 vermicrete, 100mm thick, but the drawback is that you’ll need to wait a few weeks for it to dry out.

[Ricey]

Thanks David!
I seen a post where you do not agree with Perlite for the inner dome, whats your recommendations for the inner dome?
John

[david s]

Something dense and strong like firebrick or castable refractory. If you don’t want to spend too much use the homebrew (see cast ovens). A perlite dome results in a weak structure which has low thermal mass and vulnerable to knocks and abrasions. Use perlite or vermiculite for insulation.

[MarkJerling]

Hi Mark,
Thanks for the reply and for sharing your design,
Interesting to see you using attic insulation, is it as good as thermo blanket?
I was thinking of 80mm slab with 40mm of Perlite in the centre of it.
I see some people use perlite or fire board bedded into the slab, under the fire bricks and others just put firebricks on top of the concrete slab.
It seems you paid a lot more than I would expect for the build, what were the most expensive parts?
Ive bought my sand, cement and bricks from a salvage yard so not really spent so much so far to build the frame, I can buy a meduium size DIY kit for $1100, wondering would that be the best way forward.
Thanks
John

Hi John

No, attic insulation is not nearly as good as furnace blanket and will almost certainly degrade at the heat the oven will be at. So, what I did was 25mm of furnace blanket against the bricks and fibreglass house insulation over that. Most people, of course, use only the furnace blanket, in which case 50mm will be so much better than 25mm. Under the floor, I used aerated concrete "Hebel" panel as the insulated board most people on this site uses cannot be easily obtained here. I was worried that the aerated autoclaved concrete may not cope with the high heat so I did a sand layer over that. So far, it all seems to work well.

Key for me was to create an oven that cools very slowly for retained heat cooking. In some respects, we did that almost too well as the oven is usually too hot the next day for retained heat cooking and reaches optimal temperature for bread and slow roasts about 18 hours after pizzas.

Don't compromise on your structural slab. Make that one thickness, in my view 100-125mm minimum and then place your insulation board / perlite / vermiculite slab over that for your oven support.

In our case, most materials were free / salvaged and free. So, I paid nothing for the hole in the ground, LOL, nothing for the wood storage base bricks, reinforcing steel and aerated concrete panel. (Well, cost me a few boxes of beer.) The night store heater bricks were likewise free, as was the stainless steel flue.

Main costs therefore was readymix concrete for the foundation and floor and suspended floor, sand and cement for the plaster render and regular mortar with the single biggest expense being the fire cement which is very expensive here. I think I used about 10x 30kg bags of pre-mixed furnace mortar, which I got directly from the manufacturer for about $45 per bag. Cutting the night store heater bricks cost about 10 diamond blades, so that would have been the next highest cost.

Most night store heaters have 12 or 16 bricks. Each of these cut in two, so that gives 24 or 32 usable bricks per heater. A few of the heaters had really nice big bricks, 300x220mm, and I used those for the floor. The regular size brick is 230x190mm, and about 40mm thick and I used those for the dome, the door arch and the base of the chimney. I needed about 160 half bricks, 40 for the door arch and about 20 for the floor, so 140 bricks all up, so quite a few heaters! (About 10 heaters) Two large night store heaters were of a different design to the others and they had very large bricks - 395x266x45mm. I used those big ones for the floor. The regular size ones will work fine for the floor too, but the bigger bricks gave me less floor joins.

The kit ovens probably work out cheaper but the inside of the kit ovens are quite small. The advantage of a built oven is that you can build it as big as you want! We can do 3x 12" pizzas at once where our friends with a kit oven can only do one at a time. Of course, I over-engineered the whole thing so we used quite a bit more concrete than needed, very deep foundation. But, we live in earthquake country and I did not want to risk having the thing tilt in an earthquake!

Regards,
Mark

[Ricey]

Thanks again Mark for your detailed response.

I promise I won’t compromise on the slab. I was telling the guy at the salvage yard why I needed the big square piece of thick MDF to form the slab. He was saying to just put the MDF on top of the base and pour the concrete in place, just leave the timber under the finished slab, wondering has anyone tried that, as 1.3m x 1.2m x 10cm of concrete would be sore on the back to lift into place, you could probably get the MDF out using a jack or just power hose it until it breaks down

10x 30kg bags of pre-mixed fire mortar seems a lot, was that all for the dome?
I was thinking of using something like this around the dome bricks as I can get it locally:
https://www.bunnings.com.au/lanko-5k...ortar_p0760382
And then perlite, insulation and chicken wire for the outer layers

My idea is to build something about 93cm (37’), would a 5’ Flue be enough?

[david s]

MDF would be ok but will get wet and rot.cement sheet is a better option because of this, but requires some temporary bracing from underneath until the concrete has set for at least 24 hrs. If age slab cantilevers over the vertical supporting piers the base formwork cannot be removed. I have done repairs on two ovens that used marine ply over steel steel stands for the base formwork and in both cases the marine ply had rotted from accumulated water.

A 5" flu pipe for a 93cm internal diameter oven is inadequate, you'll need a 6" one and make sure it's not galvanised or you'll be replacing it, use stainless.

That refractory mortar, apart from being very expensive is hard to work with as it goes off really fast. This is a problem, particularly if you are not an experienced bricklayer. The dome bricks require trimming and the work is pretty slow. The home-brew mortar is a far better option.

[MarkJerling]

Thanks again Mark for your detailed response.

I promise I won’t compromise on the slab. I was telling the guy at the salvage yard why I needed the big square piece of thick MDF to form the slab. He was saying to just put the MDF on top of the base and pour the concrete in place, just leave the timber under the finished slab, wondering has anyone tried that, as 1.3m x 1.2m x 10cm of concrete would be sore on the back to lift into place, you could probably get the MDF out using a jack or just power hose it until it breaks down

10x 30kg bags of pre-mixed fire mortar seems a lot, was that all for the dome?
I was thinking of using something like this around the dome bricks as I can get it locally:
https://www.bunnings.com.au/lanko-5k...ortar_p0760382
And then perlite, insulation and chicken wire for the outer layers

My idea is to build something about 93cm (37"), would a 5" Flue be enough?

I used temporary shuttering of thick cardboard, with plastic sheet over, set over pallets, on leftover bricks. Once I pulled all of this out, I had a smooth concrete surface under. Ply or any some such can work for removable shuttering, provided it's well supported as the weight of wet concrete can be substantial! For permanent shuttering, I'd use treated ply, as David has pointed out too.

Yes, the fire mortar was all for the dome and for the squirreltail chimney. As David points out, the homebrew mortar option will work out substantially cheaper. When working with the fire mortar I found that it was best to mix only enough for about four bricks at a time as the stuff sets almost immediately. So, how I'd do it is that I would pre-cut all my bricks for one course of the dome and then mortar 3-4 bricks at a time. Usually, I'd re-trim the last few bricks for a course as the addition of the (thin) mortar joints meant that the bricks sometimes needed adjusting in size for the last few.

Have a look at my drawings for size and height. Don't make the whole thing too low. It's easier to work in a pizza oven if it's set a bit higher.

[david s]

" I'd use treated ply, as David has pointed out too." No, don't use ply for the reason I've already given, unless it can be removed easily, If you have timber props to hold it in place until it has set, be sure to sit them on wedges for easy removal.

Refractory mortar (calcium aluminate cement based) is highly temperature dependent and goes off really fast, but working time can be increased by using chilled water. I always do this in the summer.

Set the cooking floor level around belly button height.

[MarkJerling]

" I'd use treated ply, as David has pointed out too." No, don't use ply for the reason I've already given, unless it can be removed easily, If you have timber props to hold it in place until it has set, be sure to sit them on wedges for easy removal.

Refractory mortar (calcium aluminate cement based) is highly temperature dependent and goes off really fast, but working time can be increased by using chilled water. I always do this in the summer.

Set the cooking floor level around belly button height.

Sorry David. I misread your first post.

[Ricey]

Thanks for all this detailed information guys. I am now going to get back to the build
Just one question more, is this what you are talking about when using homebrew mortar for the dome?
https://youtu.be/gR69JhL7_yQ
Would I get any of these ingredients in Bunnings?

[david s]

Yes that’s the recipe. 3:1;1;1 sand, use silica sand or fine brickies sand from landscape supplier, GP cement and hydrated lime from Bunnings, Bricklayers Clay from a concrete industry supplier.

[Ricey]

I’m up to slab level on my build and was hoping to share an image but it seems restricted to 2mb upload, I don’t know what camera would take a photo that size today!
My build is on Maxiblocks from Bunnings and I’m thinking of using this for the slab
https://www.westbuildgroup.com/wp-co...Concrete40.pdf
in my calculation for 1m squared by 10cm, that’s 10 bags, just wanted to check if that’s a good concrete and if the maxi blocks and 6mm concrete board will take the weight?