Finally decided on 32in castable dome

[Mullster]

Well after great advice and encouragement from DavidSUtahBeehiver ive finally switched plans for a 4th time! Started very unwisely with a gym ball vermiculite dome on a movable stand (hoping to do it on the cheap), to the same dome on a decent fixed base, to a brick Pompeii (despite lacking most of the tools) to finally deciding on a castable dome. I think I’m going to be able to make it 32 inch ID and still have sufficient space either side on the hearth.
Had the foundation slab poured today into my two section base - the front piece is simply for standing and because I didn’t want a step straight from the grass. It won’t hold any weight (other than mine).
Looking forward to sharing and learning with you all.

So a question - concrete was poured at 1030 Friday morning - how long do I need to resist before building the stand on top? A week?

[Yokosuka dweller]

Hello Mullster,

Looking good. I also did a 32" dome, or exact it was more like 31,5", plenty of size for my small family. I can't administer more than 1 pizza at a time anyway and it's big enough (oven opening) to do bread or dutch oven roast as well.

On your concrete, I'd say keep it covered with plastic (or indeed water) for at least a week, sprinkle the concrete from time to time so it stays wet, that way it cures harder. Once it changes colour from green to more grey/white it should be cured enough to start building the stand.

[Mullster]

So here’s the latest picture on the concrete foundation which was poured 4.5 days ago now - I’ve watered it each day and I won’t be able to get to the stand building until the weekend anyway. But - was wondering is there any benefit in me taking the frame off sooner? Will that help
or hinder the curing? It seems to be really solid already and there are no visible cracks right now....

[UtahBeehiver]

If not in way, just leave it for now. Concrete does not reach full strength until 28 days. PS, don't be impatient, the turtle wins the race when building these ovens.

[Mullster]

UtahBeehiver Thanks - I suspected I might be better leaving it. Can I unleash the turtle on Saturday though after 8 days if all looking good?

[aaronpizza]

Mullster - I'm in the same boat as you - just a few weeks behind - I'd love as much detail as your willing to give

Photos, where your source your home-brew components from etc

Good luck!
I'll be following

[Mullster]

aaronpizza Nice to hear - good luck to you too. Planning this weekend to build the stand - just keep flip flopping between 2 designs - feel like I need to get them off the graph paper and try them out by dry building them. Gets me my exercise lifting the 18kg + blocks!!

Im in the UK so the blocks I managed to get from Wickes, and the concrete was delivered by a local company that barrowed it in from a kerbside cement mixer truck.

i had the wood already for the frame and ordered Rebar from a supplier on Amazon. The wire mesh I got from BM Stores as well as the pea gravel.

havent looked yet for the home brew ingredients knowing that I still have stand building, framing the hearth and another concrete pour to get through - pacing myself

[Mullster]

I’d really appreciate any comments on my design assumptions here. I’ve tried to follow the Longmont castable threadDavidS shared but I’m still nervous about whether I am understanding the opening, the vent and oven landing dimensions well or not. I want to be confident before I make my final decisions on stand dimensions which I know will impact hearth dimensions and framing the concrete pour for the hearth, etc. it’s like that nursery rhyme about the knee bone connected to the leg bone or however it goes!!

anyway hopefully you can make out the dimensions on the picture - key ones are:

dome internal 32in
external after outer render 44 in
opening 18 inch wide 10 inch high
vent/flue castable opening 20 inch (to create place for door to sit up against the oven opening...)
8 inch deep to accommodate 5 or 6 inch diameter chimney
another 8 inch of oven landing area (not sure of material for that yet)
gives me 49inch from front of oven landing to back of oven inside (I think!!)

any glaring errors in assumptions?

[david s]

Hi Mullster,

Because ovens built with bricks require a decent bond at their mortar joints they need to be the full width of a brick (4"). Builders have discovered that if the bricks are laid on edge (3") or clay pavers (2") then they have all sorts of trouble with failing mortar joints. Fortunately the dome, being a self supporting structure prevents it from collapse. For a cast oven this problem does not apply. So most castable builds are around 2" thick with a 2" thick firebrick floor. This results in good strength and sensible heat up times with adequate thermal mass for extended retained heat cooking. By all means you can make it thicker, which will make it stronger, but will also increase considerably the materials required, the overall weight and realestate on your supporting slab as well as increasing heat up times and fuel consumption. Make it thinner and you will reduce strength, thermal mass. A one piece cast dome does have the disadvantage of being heated unevenly and is therefore more prone to cracking, and why most castable manufacturers offer multi sectioned domes, but again it won't fall over even if it does crack. The extra hassle of designing in multi sections for a one off cast in place dome is not worth the extra effort IMO. The flue gallery can quite easily be made much shallower by setting it back so it sits slightly on top of the oven mouth, see my pic attached. A shallow entry makes working the oven so much easier. To make it simpler you can use bricks for the sides then just cast a section for the top.

Hope this helps you,
#99

[Mullster]

DavidS that’s really useful and I had read that one of yours before. Just so I understand though - did you have anything in front of that then - an additional arch or anything or was that casting literally the front of your oven? Also where was the rebate (hope I’m using the right word) for your door - on the ‘join’ between the dome and the casting?

also I took my measurements for the dome from one of your other posts so my additional 6 inches either side of internal is 2in castable, 2in insulation blanket and then 1.5in vermicrete topped off with 0.5in render. Is that still what you would recommend?

thank you so much for the advice!

[david s]

Yes I have a decorative arch, in front of the flue gallery, which is 2.5" deep and separated by a 1/4' expansion joint. This makes the decorative arch and outer rendered shell independent from the inner oven parts that can expand freely within it. Pic shows 1"rebate for door. Your proposed numbers look ok. At 0.5" you will need to reinforce the final rendered layer with either chickenwire or random fibres. I use random fibres, much less labour.

[Mullster]

DavidS Thank you again! So in practice I need to think in terms of 3 key parts of the build (apart from the floor):

1 - the dome itself (made with homebrew)
2 - the flue gallery (made with homebrew)
3 - decorative arch (made with bricks or even vcrete)?

[Mullster]

SorryDavidS I’m confused (again and not for the last time) - based on the other picture you shared is you casted flue gallery mounted on top of the bricks that form your arch? or is your 2.5in arch a separate entity?.

plus - I’m a bit confused how the expansion works if the rendered shell and gallery is all one piece so to speak - there is no gap in the render layer I assume?

[david s]

These pics should explain it better. The arch has yet to be mortared to the base, but you can see the little gap which becomes the expansion joint.

[Yokosuka dweller]

Sorry for butting in to this thread. David your decorative arch is beautiful. I was wondering and wanted to confirm - so that arch is only mortared to the oven stand and doesn't touch the arch bricks at all? I guess that is the intention with the heatbreak there, but does it really get strong enough standing 'free' like that/without connection to the arch? Do you incorporate any rebar or wire to strengthen that slim piece?