Scratch build thread - 36-inch inner diameter dome oven

[mnagy]

Here’s part 1 - Building the stand:

Part 2 will be recorded at the end of March when I put bricks around the outside and add a 4” top layer of vermicrete.

[mnagy]

Here’s what will be accomplished in Part 2.

[mnagy]

Some questions for the pros out there:

1.) I intend to have the entire stand top (inside the outer bricks) be filled with 5:1 vermicrete. I’ve read different methods of making sure moisture can drain from this layer. The only disagreements out there seem to relate to whether or not you should be drilling holes in the stand top (and how many / how big the holes should be). My current plan is to drill 5x 1/4” holes in the stand. Then put 1/4” scrap tile randomly under the vermicrete to create channels. On top of this tile, I’ll put some fiberglass mesh to keep the vermicrete out of these channels. On the bottom of the stand, I’ll cover the holes with mesh to keep the bugs out. How does this plan sound?

2.) You can see from the video how I constructed the stand. I feel like it’s plenty strong, but the block walls (filled with concrete) are clearly the strongest part. Should I put a 2” I-beam in the front spanning the opening within the vermicrete layer (sitting on top of the reinforced concrete layer) to provide extra support?

3.) I’d like to be able to make a 19” NY-style pizza, however the largest inner diameter I can realistically put on this stand will be 36” to 38”. Would a 20” opening be acceptable for a 38” inner dome? Would a 19” opening be acceptable for a 36” dome? I realize it’s slightly more than half, but not by much. Also, is there any benefit to lowering the opening hight? Would that help compensate for my slightly wider opening?

4.) I was planning to pour a 2.5” dyed concrete countertop as the finished top (rising up flush with the oven floor). I think I’d like the way granite or some other stone would look, but I just figured cutting these big round wedges to fit around the dome would be either impossible or prohibitively expensive. Anyone know what four corner pieces of granite might cost for a crazy custom cut like that? They would essentially be right triangles with a concave hypotenuse to accommodate the dome (4 of them, with a 28” leg length).

5,) how high does my flue have to be? Are the dimension in the plans flexible there? Or will a shorter flue prevent proper drafting?

6.) any other strategies for making this oven more water resilient? I plan to have a custom cover made for it, but it doesn’t have a roof over it. I fully expect this thing to get rained on every once in a while. My plans to help prepare ot for that are 1.) use vermicrete for the insulation base rather than ceramic/foam type solutions, 2.) do that holes/tile/mesh thing, 3.) fire it in the fall before it starts to freeze and cover it (bone dry) for winter. Is that sufficient?

Thanks!

[david s]

Making a 5:1 vermicrete mix takes a lot of water. Around a third of the total volume in fact. That excess water is difficult to eliminate with heat. Additionally, adding cement to a vermicrete mix increases its thermal conductivity and conversely reducing its insulating value enormously. If you can use dry perlite or vermiculite (they are interchangeable) poured in between the dome and the outer bricks (the corners can be filled with empty plastic milk bottles to save on perlite) . See attached table. You will end up with a far superior insulating layer. After drying and some cooking fires the top can be sealed off with a couple of inches of 5:1 vermicrete over which a fully waterproof layer can be applied.

Regarding oven mouth, the greater the area, the higher the heat loss. On the other side the smaller the oven mouth the greater the difficulty in access for cooking.

Granite does not like heat, tiles are probably a safer solution.

The flue only needs to be high enough so you don't get smoke in your face. A 6" diam. flue is required for ovens up to 36", so if going for a 38" ID you should probably go up to 8" especially if it's a short length.

[mnagy]

Thank you for this response. This is a great solution for the space between the dome and the outer bricks. What do you think about underneath the dome/floor? Is my holes/tiles/mesh/vermicrete layer a good strategy?

[SableSprings]

We've normally advised larger drain/weep holes than 1/4" - it would be better to drill 1/2" holes. Five under the oven's footprint is perfect and the tile, mesh, vermicrete layering will serve you well. I really prefer to see the use of mosaic tile sheet squares for this application. Mostly because with the smaller pieces there are many more channels for moisture to pass through and they are mounted on a mesh backing. Flip the sheet squares over and the mesh keeps the vermicrete from "falling through". Often the box or tile stores (or garage sales) have discontinued/surplus squares for really cheap.

You've done a great job planning & building your foundation & hearth so far, looking forward to the rest of your build.

[Gulf]

Great advice. Since you are, in effect, "creating a pond" by recessing your vermicrete below grade, definitely create several large bore weep holes and go all out on the ceramic mosaics as Mike advises. You will also want to complete the stucco all the way down to tiles before you pour the counters. This should allow any water that enters that cold joint to shed away from the oven insulation and down to the hearth slab where it can find a weep hole to exit.

[Gulf]

I would also like to add that you should include a recess for a storm door on your outer arch.

[mnagy]

What do you mean by this? I was planning to have a door butted up agains the inner arch (with the flue outside the door). Are you saying that I should have a second door on the outer arch (in front of the flue)?

[Gulf]

Yes it is a second door. It is to keep out blowing rain, leaves, birds, cats etc. Having a recess for it, in what ever covers the face, will help keep it secure and help shed water from entering through the entry. it does not have to be fireproof since it is only placed when the the insulated door is closed and the oven is not in use.

[mnagy]

Cool. I can definitely do that.

[JRPizza]

A second door can really help with heat retention too. My inner door is a steel shell filled with 4" of CF blanket, but it still gets pretty hot on the outer skin. When I have my outer door placed the inner skin usually reads 120-130F, but without the outer door it will read closer to 100F. That tells me I am bleeding off less heat when the inner door is not exposed to drafts and the space between the two doors acts as some amount of insulation. My outer door actually fits up against the handles of my inner door and seals against the back of the chimney arch, so it also prevents heat from coming off the inner door and floating up the chimney. My shelter keeps all water out of the opening so I didn't need to have the door sit further out, and have not had problems with stray cats. I did have a neighbor's stray chicken roost on my dome one very cold night and it really made a mess. Chickens definitely belong inside the oven, not on top of it

[mnagy]

Here are the locations where I plan to drill 9x 5/8” holes for drainage under the insulation layer (vermicrete). The chalk lines are my attempt of mapping where the rebar is to try and avoid them when drilling.

[mnagy]

Here’s the current status. Question for the experts: I’m going to fill the cavity created on top by the outer bricks with vermicrete for insulation. Should I be concerned with thermal expansion for this insulation layer? If it pushes outward, I’m worried about it cracking the outer bricks. Is that a possible issue?

[mnagy]

I was also thinking about pouring concrete in the 1.5” gap between the front and rear outer brick and the block wall? Or is there a risk of my brick cracking from expansion/contraction?