31"/800mm in Eindhoven, the Netherlands: Design critique and build topic

[Toiletman]

As discussed in my introduction topic I"m building a WFO on top of a 'technical space' for rainwater irrigation. The core idea is to combine a barbecue area in the gallery with a normal domed pompei style oven, kind of like Sixto has done, but probaby with a smaller gallery. Bricks will be cut using the google sheet shared here by Deeyayoh and an IT will be used. After going through the forums for some time and trying out some different brick designs in CAD I've got an 80% matured design in mind. Please find the drawings attached (using sensible units mostly, but 'freedom' units for some of the main dimensions such that the design intuitions of you guys living on the other side of the pond are also triggered ).

Construction wise I've poured the floating slab on which the oven will sit and I'm basically at the phase of finalizing the design and shopping around for materials. I'm not sure yet if I'll start oven construction this year as (wet) winter is already coming and it might make more sense to first finalize the tech space underneath the oven. What I'm looking for by posting this is:
- Critique/questioning of my design: did I miss things? Would you do things differently?
- Sharing my design/build experience such that others can also learn (like I have from on all the great posts here)
- Documenting the build

Base design:
My base consists of a single brick U-shape that supports a first 3"/80mm reinforced slab. On top of that is an open space of 120mm/5" to be used for tool/bbq part/wfo door storage. On top of that is the actual 5" slab that holds the WFO. The slab that will hold the WFO juts out from this significantly (see drawing). The main rebar in the floating slab is 8mmx150mm square (.3"x6") with a significant amount of local extra support welded in (see picture). According to my calculations this should all hold up fine (safety factors of 3-5x), but critique is welcome from any structural people about this as well on if the single brick U-shape should be improved. The center of gravity of the slab and the oven will still be well inside the U and therefore the foundational slab will also be loaded properly. Compressional strength of the bricks and the mortar is fine, and also buckling loads are more than 10x away from expected loads.

Hearth dimensions:
hearth (inner) diameter 31"/800mm
dome inner height 16.6" / 422 mm
Entry arch height 10.5" / 267 mm
Entry arch width 15.7" / 400 mm
Oven floor height wrt ground level: 41" / 104cm

The philosophy for these dimensions are mainly that I'm quite limited in the total size of the wfo due to the side of our garden and the location of the slab. The vent arch dimensions are such that our biggest pots would fit in for a roast/stew/bread baking etc. Height is based mainly on the fact that the higher you get the more convenient it becomes to look into the oven (+ the tech space underneath also needs room).

Gallery and insulation dimensions:
Floor:
slab: 5" /125mm with 4 weep holes underneath the hearth
glued broken tile for water channels
Calsil 2" / 50mm
1.6" 40mm dense fire stone tiles for the floor

Walls:
half-size firebrick (4"x4"x2" / 105x105x55mm) cut to size + homebrew mortar
2"/50mm insulation blanket
2"/50mm verticrete/render finish
Probably with a vent port on top (still to be detailed)

Gallery:
22" / 560mm wide (matching a grill size that will fit in setback mortar joints)
12" / 306mm deep ("")
13.8" / 350mm high (this might have to be increased by one brick for slighly better viewing angles, see section B-B in the drawings)
See the drawings for the changeover to the chimney: I'm not sure yet about this area. I'm mainly worried about the thrust since now all chimney mass is going to directly push onto the arch, instead of being supported on the sides of the gallery. I might add buttresses or widen the chimney (which would increase the weight, and would not work brick-size wise). In the end at least everything should go into a 6"/150mm inner diameter double walled chimney pipe. There's currently a WAF (Wife acceptance factor) here as she doesn't like the chimney not being 'in the middle' of the dome. I don't want to do it with bricks as it will significantly increase the weight of the chimney (and be very challenging to make), and doing it with 45 degree bends in the pipe to me seems just as ugly. I'll browse the forums more if I see any good examples here, but suggestions are welcome.
The idea would be to have an L-shaped thermal heatbreak filled with insulating rope and high temperature kit between entry arch and gallery, not sure yet if it also makes sense to make this in the floor (like with a piece of steel like Gecko has done). But I think it makes sense to have the heat break overall: the pizza oven will be much hotter than the gallery, and when using the bbq the oven will be much colder than the gallery.
For the BBQ inside of the gallery the plan would be to weld/order a sheet metal 'drawer/ash box' and just let that rest on the gallery floor, suspending two grilles/grates above that in setback mortar joints: the lower one to support coals, the top one to actually grill the meat.

Still to do in this area:

• Possible plans for a rotisserie still to be detailed, main parts should be located in the door,with maybe just an insulated bush in the inner dome wall opposite the entry arch.
• (double) Door design --> probably a detachable handle such that the BBQ can be used without interfering with a handle. Possibly also an outside door to entrap heat in the BBQ area.
• Possibility to add a thermocouple through one of the weep holes into the floor tiles --> IR gun is probably going to be better/easier

Questions:

• Tie-in the gallery to the dome? I'm slightly worried about the structural integrity and I"ve seen 'thermal tie-in's which allow movement (ie. with an U-shape in the middle). If these are made out of steel I'd say the heat leakage would not be significant if I add in 2 on each side of the entry arch while they would add significant mechanical anchoring?
• Chimney design as proposed: will the arch be able to support it? Butress the sides with a few more bricks to the sides?
• I've seen some people incorporate valves in their chimney. Is this useful/needed?
• Trade-offs on the height of the gallery: vision vs bending vs good draw
• Chimney pipe routing to middle of the dome?
• Base/foundation construction: do you see issues here?

And of course please find attached pictures of the work and the current status.

[Toiletman]

And the current status:

[Toiletman]

Update: formwork fully removed. Pretty happy with the result. One of the weep holes got moved though, so I have to see if I'll be able to drill it through and make it function properly, instead of as a 'concrete splitter by freezing'. I'll post a separate reply with some design pictures/questions later.

[Toiletman]

And here's some 3D views for the people that didn't view the pdf drawings, or find it hard to go from 2D to 3D:
As mentioned before, main area's where'd I'd like to get some advice:
- heat break between gallery and main dome: does this kind of design make sense? Should I put some 'expandable' tie-ins in?
- Should meaures be taken to lead away the thrust of the gallery arch more? Ie. add butresses somehow? (I kind of like the current elegance of a brick and a half in depth, and wouldn't know how to elegantly add a brick perpendicular to this?)
- Depth/height of gallery ok?
- To accommodate the grill in the gallery I'd like to user larger joints in the gallery than in the dome: would you see any risks in doing this or would the homebrew mix be able to deal with this without problem?

[Toiletman]

Winter is finally over! Time to start building the oven itself. Most materials are in, just waiting for the stones that should arrive on monday. I've been busy transferring the design onto the base and preparing jigs and tools and stuff. However, I've ran into a snag where I'd like your advice.

In my 'infinite wisdom' I thought it was a good idea to let the base slope outwards away from the pizza oven center a little bit, such that any water would run away from it. However, that gives some challenges with creating a level oven floor. I see two options to deal with this:

1. Using vermicrete as a leveling layer between mosaic tiles and Calcium silicate plates


2. Using a clay/sand mixture between the tiles and the Calcium Silicate plates


What do you think? Which option would work?

[MarkJerling]

Either method will work. In my case, I did the leveling with a sand layer under the floor bricks.

[Toiletman]

Either method will work. In my case, I did the leveling with a sand layer under the floor bricks.

Thanks for your reply. Any idea of how much unevenness you fixed in that way? It does seem to be the easiest way.

I think I have about a 1cm of difference between the highest point (center) and the outermost edges. During mock-up the calciumsilicate plates did seem to conform to that elastically, but I'm worried that they might suddenly crack or snap with the weight of the oven on top.

[MarkJerling]

Thanks for your reply. Any idea of how much unevenness you fixed in that way? It does seem to be the easiest way.

I think I have about a 1cm of difference between the highest point (center) and the outermost edges. During mock-up the calciumsilicate plates did seem to conform to that elastically, but I'm worried that they might suddenly crack or snap with the weight of the oven on top.

I used a thicker layer of sand than other users as I wanted the flexibility to be able to go to thicker floor bricks, once these ones wear out, in future. (Or if I found the ones I used were too thin, which they were not.) My slab was pretty even. So, I have a rather thick layer of sand and it's been absolutely fine. The sand works like another insulation layer in my case. Having said that, my oven floor is inside the dome, my dome does not sit on the floor bricks, so your experience may be different.

[Toiletman]

With my oven walls on the floor bricks and after measuring again I went the vermicrete route just to be sure. The equalizing layer of vermicrete is now sitting outside curing under plastic.

How brittle is the hardened vermicrete supposed to be? the leftovers that I made into a ball and left to dry (now 22hrs or so) are very brittle: Just a little squeeze and chunks start breaking apart. Regular mortar leftovers from sand/cement are much more robust? I used the usual 5:1:2 (vermiculiteortland:water) volume ratio's, so that should be ok no? Or could it have been to dry?

[david s]

As that layer’s requirement is to insulate as well as to provide a firm enough substrate to apply the render/stucco on to, I find a 10:1 ratio better. While there is no strength in a 10:1 layer its insulation value is about the same a blanket. Strength wise it’s very weak, but firm enough to support a render/stucco layer. A 5:1 is stronger but insulates less.The higher the cement content, the greater the density and therefore strength, but the lower the insulation value. If you have plenty of blanket then the insulation value of the vermicrete layer is less important.

[Toiletman]

Finally a few days of good weather and time off so I could finally make some progress (see pictures).

I went with vermicrete for the low spots, and some loose vermiculite (seeved) for the high spots. All on top of Mosaic tiles and insect screen:

Afterwards the calsil boards... what a job to cut them to size while wearing breathing protection gear... guess I can write off my 'shop vacuum cleaner'.



Finally a first stab at the floor, just to have an initial feel for it. It'll need to be redone and properly cut to size after our holiday.


Ow yeah, I did also finish a chipster cutting jig, with a nice 3D printed angle indicator:

As that layer’s requirement is to insulate as well as to provide a firm enough substrate to apply the render/stucco on to, I find a 10:1 ratio better. While there is no strength in a 10:1 layer its insulation value is about the same a blanket. Strength wise it’s very weak, but firm enough to support a render/stucco layer. A 5:1 is stronger but insulates less.The higher the cement content, the greater the density and therefore strength, but the lower the insulation value. If you have plenty of blanket then the insulation value of the vermicrete layer is less important.

In this case the vermicrete didn't have an insulation function: I just used it for leveling, as the real insulation comes from the CalSilBoards on top of it. It turned out that the vermicrete that was covered by plastic was more robust than the leftovers: although I must say I didn't try to break it, but it held up well during cleaning and fitting the calsil boards.