30" cast dome design

[sergetania]

Hello, everyone!

I am very new to the forum and to building WF ovens but I've got an itch that needs to be scratched. Pizzas do not spring enough in my kitchen oven. I think I got pretty good at making them but I don't know how I can make them any better in the home oven.



Hence I am here after seeing on YouTube how easy it is to build a castable pizza oven. Yes, I am thankful those videos exist or I would not be here. After reading the forum and other resources, here is a design I came up with. Could someone with experience review it and comment on it and also answer a few question below, please? I am open to any type of critique because this is not a simple and inexpensive project. Thank you very much in advance!

Foundation

4’ x 4’ 6" concrete slab reinforced with rebar and mesh

Stand

Built of pressure-treated lumber, just like the deck the oven will stand next to. Supports the base from underneath like a flat cabinet top. Legs – 6x6, support for the top – 4x4 and 4x8. The idea is to overbuild the stand. Legs height around 32”

Base

Built of 5000 PSI/countertop concrete similar to how concrete countertops are built. 3/16” rebar/mesh. They build 2” thick countertops out of 5000 PSI concrete. Obviously, this will require proper support so the stand should be very sturdy and flat on top. Do I need to build thicker than 2”?



Hearth

2” fiber insulation board under the dome/gallery with cooking surface made of firebricks. 2" board may not be enough but this is a cost-based decision. No plans to heat it for long periods of time but there will be wood underneath of the base plate. Is 2" of insulation underneath enough or asking for trouble?



Dome

Cast of castable refractory concrete over a 30” sand mound, 50mm thick.
The dome is insulated with 3” of ceramic insulation blanket held in place by chicken wire with a final 1” waterproof render
The height of the opening will be 63% of the dome height – 9.5” high and 19” wide. An edge for the door will be ½” and located in the entrance area between he vent opening and the dome (see the picture).

Vent/Chimney

The vent is a part of the cast dome/entrance structure, 6” diameter. I plan to cast a gallery separately and try to form a flat square area on top with built-in bolts for a chimney adapter like the DuraTech chimney anchor. If I don’t find anything less expensive I will use DuraTech anchor/pipe/cap. The picture below shows bricks on top of the flat area to hold screws. I don't think I need the bricks if I can insert bolts into the cast.

Arch

Mostly cosmetic, sits in front of the cast dome/entrance structure (please-the-wife is not optional so needs to look decent)

Overall, this is mostly influenced (except the stand and base - thank you handycrowd ) by two builds - the Newcastle by mesoiam and the Longmont by cnegrelli. Outstanding job, gentlemen!

A very optimistic price estimate:
Materials:
Concrete slab - $250
Stand - $100
Base - $100
Insulation board and blanket - $300
Castable refractory - $300
Firebricks - $150
Top layer render - $100
Front arch/door - $150

$1,450 + teenage labor costs (my back is seriously messed up) . Does it make sense to cook a few pizzas? I can't help asking myself that question but I will try to stop it.

Besides any problems with the design that need to be corrected, the questions are

1. Do I need to build a concrete base thicker than 2"? It will be supported from underneath and a flat insulation board will lay on top. I am not relying on it to bear weight. I really do not know about this. I want to keep it thinner but definitely have no clue what needs to be done.

2. Am I OK with the fiber board insulation only 2" thick under the firebrick hearth? The stand is built with wood. Asking for trouble or acceptable to save money?

Please help and review!
THANK YOU FOR YOUR HELP!

Sergei

[WoodywWun]

Sergie,

I write with not dissimilar experience as you so what I've done so far has been guided by several forum contributors which has been very helpful. I'm UK based.

If you are not relying on your standard concrete base to take any load then 2" (50mm) will be thick enough. I'm guessing that you have some sort of material under this such as old bricks or rubble or maybe (as part of my base does) sits directly on a garden earth base? Part of my base does take some load and I've made it 3" (75mm) thick with some steel reinforcing bar (10mm") cast into it.

My understanding of the ceramic fibre board is that it's the bees-knees and 2" (50mm) from a reputable source is all that is needed as under-cooking floor insulation. I have used 4" (100mm) of Vermiculite concrete for this layer to keep costs down.

Have you decided whether to cast in two parts or just one? From what I'm picking up, a two-part approach giving a casting for the dome and a separate one for the door/vent is the better way to go as cracking is less likely.

[sergetania]

Thank you for your reply! The stand will definitely support the concrete base well to allow making it thinner. However, that stand will be built of pressure-treated wood so that was one of the reasons I am worried about the 2" insulation on the bottom. I think wood begins to char at around 250F/120C so I hope this somewhat thinner insulation will still prevent the wooden stand from the heat.

Right now, there are two interesting builds going on, the Newcastle and West Midlands builds, one with the gallery cast separately and another cast all as one. Currently, I am leaning towards casting separately but this decision does not need to made now. I still have time to evaluate and decide.

[WoodywWun]

Sergie,

A mate of mine from up the road constructed an oven and cast a base out of ordinary concrete, which was probably a little thicker than your proposed 2" (50mm). He left the plywood mould in, under his concrete. Not sure why but I guess he probably hadn't planned it well! He has no insulation in place at all! He had concerns about the heat igniting his stand but still went ahead and fired it up. It is there to this day and the fire brigade has not been called, to my knowledge. He makes a very good pizza. His oven is simple but he can do bread in it as well. I've attached some photos and you will see that he has a stand made of wood posts, about 4" x 6" I think. Given that you have a good insulation layer, I don't think you will encounter any problems.

I've been tracking the West Midlands and Newcastle builds as well and I have seen your posts pop up. I am going for a two-part casting with an expansion gap, filled with Vermicrete between them. Worth the extra effort I think.

[sergetania]

That is a rustic but kind of a nice oven! Of course, I have a please-the-wife clause added to the permission to build so rustic won't do! But other than that it is very neat. It seems this type of oven may be more popular in UK. Here in US, everyone has a BBQ grill and maybe a firepit. Just one guy on the street has a WF monstrosity the size of a house which takes a day to heat up (not a dome oven) and it is still known as a pizza oven. I have seen it use for cooking once or twice.

Obviously, I can't tell whether a separately cast gallery is superior and by how much to a cast-as-one construction. In my mind, casting a gallery separately would allow a more complex form and a simpler casting process as the result. Then I am worried about joining a gallery to the dome. I don't understand the role of vcrete in that joint other than just closing up the gap somewhat. It will not join the two pieces together, neither will it seal the cap completely... but maybe it is not even needed. Can I just use a fireproof rope instead then?
Casting it all together could simplify producing a shorter gallery because I think I can just push a gallery form into the sand dome while being able to see how far in it will go. It is harder to visualize that with a separate form, at least for me. That's all I have gathered so far on the subject but I still have some time to think it over.

[WoodywWun]

Yup, his oven is rustic but functional! It does show that even with this simple construction, the temperatures generated in the concrete base were not sufficient to make the wooden stand burn. In the case of my mate, it was more luck than design! UK designs seem to vary a lot with some new interest in cast domes, some with fire-brick construction throughout and some using reclaimed heavy clay house bricks, which produce a very traditional oven.

So far, the lady of the house here is watching my build with curiosity but no real complaints. I've done a nice little brick arch in the stand which I think is aesthetically pleasing! I think you guys in the US are working to a very different game plan than most builders in the UK and your outdoor kitchen ideas sound very impressive. I suspect you have some rivals in Oz!.

Gleaning what I can from different sources, I understand that the differential expansion of the dome (basically a hemisphere) as compared to the door/vent gallery (an arch) is what potentially causes cracking. It would be good if one of the casting gurus could shed some light on this.

What I plan is to construct a wooden 'former' for the gallery and go to some length to ensure that the inward-facing radius is as close to the outer radius of the fire-clay dome as I can get it. The gap, I understand needs to allow for some expansion so a Vermicrete filler would permit this as it holds some flexibility, even when it is cured. A 4:1 mix has been suggested, which is quite a strong mix but should do the job of sealing and joining well, I hope. I think it will fill the expansion gap nicely and will allow any inconsistency between the two casting shapes to be filled. I also think, that if the Vermicrete will bond to both castings. It should provide a complete seal. I can see your idea of fireproof rope working but you would need to ensure that both castings were very well matched for their shapes.

[sergetania]

My problem is the oven will be right next to the deck, impossible to miss. That's gotta look decent. On the other hand, I have not yet adopted the mentality of building everything bigger, powering to the max, upping up the caliber, etc. I prefer to keep it small but functional. I am building the stand out of wood because I can't do the concrete blocks with my back but also I don't want to add all that weight on the foundation. I am a woodworker and I hope I can get away with it on this project.

Let's say we have two different structures next to each other that will expand differently, and use non-elastic material that allows for expansion of the joined parts to close the gap. All we will achieve is that the gap will become the crack that might have developed in unpredictable place instead. We will be able to control where the crack is. I don't think it will bond or seal but that may not be important. Am I wrong?

[david s]

Yes you are correct. Some manufacturers of cast ovens offer a single one piece dome casting for mobile ovens. This is because at the other end of the scale a brick oven on a trailer tends to rattle to bits. The one piece casting is subjected to uneven thermal expansion and invariably will develop some hairline cracks and usually a larger vertical one at the back opposite the oven mouth. I’ve seen this enough times to conclude it’s a normal characteristic. Most manufacturers, me included, cast the dome in a few pieces so expansion forces can be accounted for and also to make handling and placement easier. For a one off casting the extra work required is just not worth it and casting in situ overcomes the problems of placement and handling a large heavy casting.

[Nick J C]

From my reading (and coming from someone who has chosen a single cast dome) - it is not primarily (or even) about the risk of cracking - the risk of cracking in a well built single structure dome is almost zero if built with a properly constructed homebrew mix - The main reason for considering a separate gallery is to prevent heat loss when using the dome as a heat retaining oven. If there is an effective heat break, then when the door is closed and the fire is out, there will not be the slow conductive loss to the gallery. To my mind, this is only required for an extremely small cohort of people, who want their oven to retain its heat for days. Mine will be used primarily for pizza, with a bit of roasting/bread making - like 99% of people.

[david s]

An insulating gap between the dome and gallery to prevent conductive heat flow is overrated imo. You wouldn’t expect1/2” of insulationover the dome to reduce heat loss to much extent. The gap is primarily to function as an expansion joint as the dome will get substantially hotter and therefore subject to more expansion than the gallery.

[sergetania]

David, I hoped you would stop by! Now this thread is legit! Thank you for sharing the information!
Nick, love the build, thanks for replying!

Yes, the purpose of the oven is exactly that - nothing that requires longer attention span and can't be enjoyed with a drink of choice (recognizing that extreme heat is dangerous!)

So what I am hearing is that separate casting is too much work for 30" dome out of castable refractory(the homebrew is intimidating at this point) done in place. Then the only reason that it is worth considering is to make the casting process simpler by splitting casting in two but definitely requires more preparation. I will need to wrap my head around that. Thanks again!

[WoodywWun]

Can I just check something? Sergie's build I think is a 30" (760mm) build and the advice from experienced builders is that a two-part cast, whilst it has some heat retention advantages, is not really necessary for the use that most of our ovens will get. I expect many of us have been tracking the excellent 'Newcastle' build where a two-part cast is taking place. The Newcastle build is slightly bigger (an ID of 31" or 800mm, I think?).

So a two-part build can be avoided (much less complicated in construction!)? More important to the build are factors such as including the correct homebrew mix ratio and including melt extract and polypropylene fibres? Also, having patience for the curing/drying of the cast is key as are a careful series of curing fires?

Can I just chuck in another question... Would drilling a hole in the cast to accommodate a probe thermometer be a bad idea? One supplier I spoke to does recommend this.

[david s]

Casting the dome and gallery in one piece is actually harder than casting the dome first then casting the gallery up to the oven mouth. This is because it is difficultl to get the oven mouth truly flat and vertical if done in an all in one single casting. If the oven mouth is not truly flat and properly aligned it can create door sealing problems.

Drilling a hole for a thermometer probe is probably done best after the outer shell has been completed. You need to select a thermometer with a probe of the appropriate length. I like to insert a thin stainless tube into the drilled hole then the probe sits inside it. Done a few this way.

[sergetania]

David

Thanks! The question I am trying to figure out for myself is how to fasten a gallery form to keep it in place in one-piece cast to guarantee the door rebate is flat and vertical and the form does not move. It's all on the cooking surface, can't drive a nail through it.

I wonder if I could glue the form from inside to the bricks with silicone than remove the caulk to release the form.

[Mullster]

sergetania just found your thread - good luck with the build. I’ve had quite a few problems with my cast - worth you checking out my build thread.

Feeling how I feel today - I would definitely be advising you to cast the flue gallery separately - I think I would have been successful doing that.