Sizing Concrete slab for 42" internal diameter Pompeii

[Macrinehart]

Hi gang - I'm sure there's something on this and my search terms are bad, so sorry if this question is redundant. I'm just breaking ground with goal of building a 42" diameter internal dimension Pompeii oven. I saw UtahBeehiver post on corner oven designs, including hearth dimensions for a corner install, which is quite helpful. I'm wondering if most folks are building the slab to full exterior dimensions of the stand + finishing materials, or if there are tips/risks with building a smaller pad and having overhang of the hearth above the stand? I'm a total DIA guy - fairly handy and have done lost of projects myself but never a pizza oven and not a professional. My thoughts on this:

• plan for the stand walls (cinder blocks) to be vertically aligned with the firebricks, so that oven load is aligned over the stand walls at the Centerlines of the walls.
• Build overhang on the hearth to accommodate the full exterior dimension of the Oven (planning to do igloo style).

Why: first, engineering, so that load is over the walls. Second, setback zoning and space conservation in my yard. I'm hoping I could gain about 10-12" of space in front of my oven, which would leave a little extra room for the rest of the project. Basically, I need the pad to be 5' setback from the properly line, but if I make that 5' to the edge of the pad and then have the hearth overhang in the back, I gain a few inches in the front, which is kind of the difference between a 36" oven and a 42" oven. Overkill? Other suggestions?

[david s]

That is exactly what I did. By cantilevering the supporting slab over the stand, the span between supporting piers is reduced and the supporting slab can therefore be made lighter and thinner. But be careful as the strength of a concrete slab is proportional to the square of its thickness. eg reducing a 4" slab to 2" results in 1/4 of the strength.

Emu legs

[Macrinehart]

Thanks David, that's a creative build! For the sake of perspective, can you list the exterior dimensions of your pad and hearth, and the interior dimension of the oven?

Cheers! Mac

[david s]

Foundation slab 1000 x 1000mm supporting slab 910 x 910mm internal oven diameter 540mm

[Macrinehart]

So - I decided to download some CAD software and draw a 3D model to make sure I feel good about all the dimensions. I think this could also show if I'm missing something critical in the overall design. Please take a look:

1. Stand walls are 8"x8"x16" and 8"x8"x8" 2-cell and 1 cell concrete blocks, 4 layers high. I am not including grout in this view for simplicity
2. The pad extends 4" beyond the stand walls in all directions, total dimension 63" l X 56" w, the model includes a 4" footing around the pad that extends to 6" deep, the full pad is 6" deep
3. The hearth is 69" l x 60" w. for simplicity of drawing I just went 6" deep and didn't account for the depth of the oven floor in the design.
4. The Oven width is 58" (1/2" stucco + 3" insulation + 4.5" oven wall + 42" interior diameter + 4.5" wall + 3" insulation + 1/2" stucco), leaving 1" gap on each side of the hearth.
5. The Oven depth is 59" (1/2" stucco + 3" insulation + 4.5" oven wall + 42" interior diameter + 9" pipe landing)
6. 12" oven landing in front of pipe landing

I'm a little uncertain about the pipe landing design. Not a CAD expert and the drawing so far shows about 7" clearance for the lading where from the point it intersects with the exterior of the dome to the front of the door opening. I planned it like there would be 1 fire brick in length between the interior of the dome and the front of the oven opening (9"). Also the hearth depth is overkill, but I was just figuring out the CAD program and didn't have patience to plan out layers of insulation and oven floor. Same goes for adding the exhaust pipe. That will be v2.

So, I am looking forward to putting down the sub-pad fill and starting with forms, and wonder if anyone sees any problems with this plan before prepare forms for the pad. Thanks in advance for the feedback!

[david s]

If you are building in brick the normal thickness of a fire brick is 4"which is required for adequate bonding at the joints. If casting, the usual thickness is 2" because it does not rely on mortar joints except where multi piece casts are used. Some very large modular ovens are as thick as 3.75" thick. It depends on what you want from your oven, The thicker it is the longer it will take to get to temperature, but once there it will hold its heat for longer. Thick walls also increase fuel consumption.

3" of dome insulation is on the light side. You may want to consider reducing inner oven thickness, but increasing insulation thickness by the same amount, resulting in the same interior and exterior dimensions.
A 42" oven requires an 8" flue pipe or the equivalent cross section chimney.

"5. The Oven depth is 59". Is this an error? Was it meant to be 69"

It might be a good idea to increase the size of the supporting slab a few more inches in both length and width. It is a real bitch if you run out of room when applying the last layers or front arch.

[Macrinehart]

I plan to use fire brick. My understanding was the wall thickness is 1/2 length of standard 9"x4.5"x2.5" brick. Should I plan on 2 cuts/brick with some discard?

"5. The Oven depth is 59". Is this an error? Was it meant to be 69"

Might be a design issue? The join between oven dome and the chimney pipe landing a concern of mine. I thought the total cross section of oven depth should be longer but when I started adding up the dimensions came to 59", which seems short. The model I drew has the 9" fire brick intersecting with the interior oven wall on the inside corner of the opening. Maybe I need another course of firebrick cut in half for the pipe landing?

I could add a couple inches of width to the hearth, but if I add, 4.5" - 9" of depth (assuming I have a design issue on the pipe landing), then I start to worry about the depth of the pad and stand walls being insufficient, and leaving too much overhang. Current plan is 56" deep for the stand walls, 3 × 2-cell blocks + 1 x 1-cell. That leaves 4" of overhang in the back and 9" overhang in front.

If I increase the hearth depth, should I also increase the stand and pad depth for less overhang? My worry here, leaving too much unsupported overhang, and also cracking my head under the hearth while going for more wood.

[david s]

I plan to use fire brick. My understanding was the wall thickness is 1/2 length of standard 9"x4.5"x2.5" brick. Should I plan on 2 cuts/brick with some discard?



Might be a design issue? The join between oven dome and the chimney pipe landing a concern of mine. I thought the total cross section of oven depth should be longer but when I started adding up the dimensions came to 59", which seems short. The model I drew has the 9" fire brick intersecting with the interior oven wall on the inside corner of the opening. Maybe I need another course of firebrick cut in half for the pipe landing?

I could add a couple inches of width to the hearth, but if I add, 4.5" - 9" of depth (assuming I have a design issue on the pipe landing), then I start to worry about the depth of the pad and stand walls being insufficient, and leaving too much overhang. Current plan is 56" deep for the stand walls, 3 × 2-cell blocks + 1 x 1-cell. That leaves 4" of overhang in the back and 9" overhang in front.

If I increase the hearth depth, should I also increase the stand and pad depth for less overhang? My worry here, leaving too much unsupported overhang, and also cracking my head under the hearth while going for more wood.

Fire bricks are made in all kinds of different sizes and don't conform to the standard house brick size of 9' x 4' x 3". If yours are 9x4.5x2.5, then go with what you've got.
pic attached shows how you can maximise brick with minimal waste and minimum cuts. You can keep the cutoff wedges to fill the triangular thick courses formed on the outside of the dome.

[fox]

I have built them with quite a bit of overhang with no issues at all.

[Macrinehart]

That's really beautiful! What is the diameter on your oven and hearth? Hearth ooks like about 3" thick? Do you have any picks of the hearth prior building the oven? I am curious about the cross section including oven floor.

[fox]

Probably a few …. but that was five years ago and the pictures are on my old IPad, I just put it on charge and I will check what I have….

[david s]

Here is a similar solution. In this case the customer made a big blue and had a 1200 x 1200 mm steel stand fabricated for a 1600mm diameter oven without realising that the dimensions were internal not external. Rather than extending the stand they chose to place a 6mm steel plate on top of the stand and cast a reinforced round concrete slab 1980mm in diameter (just enough to accomodate the oven castings, insulation layers and outer render. Problems for me were to grind through the 6mm plate to accommodate a 100mm hole for the burner and working the outer layers over the oven with minimal clearance on each side and a highly inaccessible corner at the back of the oven.

Good planning is essential.

[fox]

I found a couple that might help you…..