38" Pompeii build

[Frenchie]

Greetings,

I would like to document my WFO & outdoor kitchen build here; for the convenience of compiling all my (probably numerous) questions in one place and to serve as future reference for another build. A good friend is also considering a WFO project so maybe some of the contents here will be useful for him down the line.

Anyway, here's the rough outline. First up apologies for mixing units of dimension - I'm in the UK so prefer metric system but have tried to keep any oven dimensions familiar!

There is some unused space at the end of the garden which catches the evening sun, where I'd like to build an outdoor kitchen, dining and entertaining area. Because of access constraints, everything must be hand-balled to the build site. I see this as a good opportunity to improve fitness

The garden is sloped 1:12 away from the house. So far I have excavated a 6m x 3m level for a slab which will measure 5.75m x 2.75m. I have put down 7t of MOT type 1 sub base (particles from fines to 40mm) to a thickness of 150mm after compaction. Next step is to lay on top of this a 150mm thick steel reinforced slab for the foundation.

On top of this I am planning a corner build of a 38" (internal) dia. dome, with worktop extensions either side to the extents of the foundation (minus reveal) - so one short and one long worktop.
38" is chosen to maximise the internal diameter of the dome while using all the available width of my ceramic fibreboard which is 1200mm.

Can anybody comment on these general oven dimensions please?

• Internal diameter 38" (917mm)
• Height (internal) 19"
• Thickness ~ 4.5" (will cut bricks into halves keeping the clean faces pointing into the oven
• Inner arch height 12.25"
• Inner arch width 19"
• 3" ceramic fibreboard floor insulation
• 2.5" thick floor bricks
• 3" ceramic fibre blanket exterior insulation, rendered
• Flue diameter 6" or 8" which would be best?

Other general features:

• Floor internal to dome so all bricks can be replaced
• Heat breaks:
  • Between floor and landing
  • Between inner and outer arch
• Vent w/ Cowel on top of dome
• Cowling on chimeney
• Ceramic fibreboard to be placed on upturned mosaic tiles
• Weep holes in hearth
• Roof over entire outdoor kitchen eventually

How does the above generally seem?
And one final question if you may - I like the continuous plane of a landing and pizza oven floor but are there any features which can be incorporated to stop driving rain from entering the chamber (until the roof is constructed). Will a door suffice?

I will populate the thread with pictures as I go, and general drawings when ready

Thanks and good evening!

[JRPizza]

Frenchie, welcome to the forum! It looks like you have done your homework. What you have outlined looks good. Would recommend an 8" flue as the "standard" is 8" for any oven over 36".
Good luck keeping water out. A door will keep it off the oven floor but you are likely to get soaked during construction and even with good waterproofing I think the oven will get wet.
I built my roof structure before the oven and since I didn't know exactly where the chimney would penetrate I just put a temporary cover over the shelter frame. Even with that I got my floor insulation soaked and the water stayed there for months until I started curing fires.

[Frenchie]

Hi JRPizza - thanks for the welcome and also for the notes re: 8" flue and managing expectations with respect to water ingress!

Also I have to say, I am glad you replied as during my homework it was your build thread, including your roof structure and overall 'finished feel' which provided much inspiration. Your build like so many here is very impressive - workmanship to aspire to! I trust that you find much pleasure in putting it to use too

[JRPizza]

Thanks for the kind words. Like some other builders, my oven is 100% functional and enjoyable, but alas only about 95% done as the decorative bits are not my forte.
You certainly have come to the right place - good luck with the build and ask away - the people here are great at helping out when needed.

[JoJoMojo]

Its an amazing thing to build. So much fun, SO much knowledge acquired and, realizing I & my kids can cook for 100 people during a 3 day cook, for the next 100 years.... pretty awesome

[mongota]

Frenchie, welcome!

Agree with JR, all looks good, and go with the larger 8" flue over the 6".

I had a temporary door, very simple, made from sheet metal, that had a sloped return or kick out at the bottom. The vertical part of the sheet metal door covered the opening and the downward slanting return kicked out enough to cover the outward most portion of the landing. It worked well.

[Frenchie]

Worried!

Yesterday afternoon I poured the suspended slab. My mixture was 1:2:4 using Portland cement, sand, 20mm aggregate respectively.

water volume added was approximately equal to cement volume.

clear sheeting was fitted to cover the slab about 1 hour after screeding.

now around 14 hours after finishing the pour, I am seeing multiple cracks developing in the surface of the slab.

I am concerned, maybe the sand was too wet, or too fine (it is builders sand), and now this work needs to be undone.
there is lots of steel within.

with the work and finances gone into this, obviously I will be gutted if it needs to be torn down and redone.

is it ok, salvageable, just cosmetic?

thanks in advance for any insights

by the way, in one of the pics you can see a weep hole with cracks radiating. The hole is deliberate, a plastic tube runs through the formwork

[david s]

Standard mix is usually 3:2:1 by volume. If the slab is exposed to the sun it can get too hot which increases the tendency for rapid shrinkage and development of cracks. Covering with the plastic sheet early with the sun still on it may also have increased the early shrinkage problem. So long as you have plenty of steel reinforcing in there you should be fine.

[Frenchie]

Hi David, thanks for your response.

Ultimately in a week or so I will strike the boards on the sides and assess the quality then, see whether cracks appear through full thickness. I will update pictures then without the sheeting and side boards. Regardless I am grateful for the quantity of steel in the eventual vicinity of the oven, and the fact that it is centrally supported by a wall under the slab.

in the meantime, should I do anything or leave it alone with the sheet on for a week?

does it need sprinkling with water?

this will receive direct sun and temps are due to be 18-28C through this week.

Thankful for your answers, as always! Very reassuring to have access to such knowledge, lucky bunch we are on FB!

[david s]

Probably not much more you can do now because early curing is done. Keeping it constantly wet is preferable to sprinkle dry sprinkle dry. Sealed up well will hold the moisture in. I’d be leaving it covered for a week.
In hot weather concrete is better covered with white plastic. Black or clear can increase temperature.
We live in the tropics and during summer I don’t lay up a casting in the morning because it gets too hot during the day and is prone to shrinkage cracks. If done in the late afternoon there’s no problem because the casting has time to set in the cooler evening and overnight.

[Frenchie]

Thanks to everyone who has offered advice - this project constantly reminds me to check before I act having made a couple of mistakes.
Anyway, all is well with the suspended slab, I think.

It isn't super tidy, but there is rendering and tiling to be done so I am not fussed. Besides, faults and imperfections have stories

Anyway, it is fun to share pictures so here is a micro-blog update.

Digging out the slope; My garden is on a hill and I have heavy clay soil. I excavated enough to find a level and put down 8" of compacted MOT type 1 aggregate. Done by hand (well, shovel and mattock).



Then the form, sheeting and steel reinforcement for 6" slab. It measures 6m x 3m



Then after pour and screed bar (pumped mix was a back-saver. Anyway, off to the pub):



Some months later, blockwork and formwork for suspended slab:



Reinforced, with plastic tube set in for weep holes:



And another broken back later: (I sustained quite bad cement burns and am scarred from getting mixture on me while lifting and pouring buckets of cement. The irritation and discomfort I thought was abrasion but was an alkali reaction as the stuff soaked through my clothing and stayed on my skin for about 4 hours. Be careful!)





Next step I have my mosaic pool tiles as a sub-layer. I am making a template to cut calsil board and tiles to match. Upcoming question:

[Frenchie]

Inner arch design!

My original plans were not optimal for incorporating a heat break. So, now I will have effectively 3 arches;
1) inner arch intersecting dome
2) Inner entry (and flue supporting arch) - butted up against inner arch via fibre rope
3) outer arch (and flue supporting)

For the innermost arch:
If my dome is built with half bricks, should this also be built with half bricks?
Is the distance from the centre of the oven to the inside face of the inner-most arch a function of arch height or arch width?
If creating a semi-circular arch, I presume radius is taken as 63% (thereabouts) of dome height?
If creating a truncated arch, with straight side walls, how is the radius chosen, while observing the above ~63% rule? Is it purely artistic?

There are so many variables for this critical part of the design, I don't wish to be integrating dome and arch on 4th course and realise a monumental error!

Thanks in advance for your pearls of wisdom. Always appreciated

[UtahBeehiver]

If you are doing what is called a "tapered inner arch" (recommended) the you start with "FULL" bricks which are cut using info from your IT. Search the blog, there are dozens of examples. Caveat Emptor, this is one of the most difficult concept to visualize but not too difficult to execute as long as you are patient. The 63-66 inner arch ratio is a rule of thumb, good to shoot for but not a deal killer if you are off a little. I have at couple pics to get you started. The tapered inner arch is used on both full arch and axe arch.

[Frenchie]

Hello, thanks Russell for your detailed response about the tapered arch bricks - i've finally (after nearly a year) got my head around it!

Only joking, it didn't take a year but life got in the way of this project in quite a large way and I'm just getting back into it now. Pleased to be back!

Progress is as per the photos from last year, suspended slab is ready for building the oven. Therefore in readiness to cut templates and start cutting materials, I wanted to check in with the experts and see if my plans stack up.
I have revisited the arch design; we prefer the look of a semicircular arch so this version is around 22" wide and 12" tall.
Considering the internal dome height is 20", the ratio is approximately 60% which should be ok?

WRT the arch design itself, I am opting for a heat break, therefore will be building 2 distinct arches; the first that intersects the dome and a second which supports the chimney/flue. The second arch is 1.5 full bricks long, so I can use half bricks each side to create the flue gas opening on top. The outer arch looks to me quite long, but to be honest I don't hate the 'long tunnel'. If there are practical issues here let me know and I could use 1 full brick overall length, creating the opening using 1/4s or 1/3rd bricks instead?

In between the flue-supporting arch and the dome-intersecting arch will be stuffed (and backed with gasket sealant) fibre rope.
Loads from the dome and the outer arches are therefore decoupled. This raises the question; is that a problem? From a thermal load perspective I suspect not, but structurally, will the outer arches support the flue sufficiently? I am worried about overturning caused by side loads on the flue, e.g. wind, or an adventurous child perhaps. Is it good practice to tie back the outer arch to the main structure?

Thanks in advance, cheers all (pics / plans attached I hope)

[Frenchie]

got a cool component for my IT, it’s a tiller extension hinge for small sailing boat. £20 delivered