Pizza oven build in the UK

[andyg]

I'm just starting to plan out building a refactory cast pizza oven. The access to my house is down a fairly small lane, and the garden is on a steep hill, so I figured casting something in place is going to be the easiest way to proceed. So far, I've cast the concrete slab next to the patio and almost finished the block-work for the walls of the base. I'm now just planning out how to proceed from here and finding I have lots of questions still.

Working from the bottom up, this is what I have planned at the moment:
Hollow concrete blocks sitting on a concrete slab as the base (completed now)
100mm reinforced concrete slab with weep holes
~10mm ceramic tiles with gaps between them for drainage
50mm ceramic fibre board for insulating under the floor
65mm fire bricks for oven floor
50mm thick inner dome, cast using heatproof screed (planning to use this product: https://www.vitcas.com/prd/heatproof-screed).
50mm ceramic fibre blanket
50mm outer render (either sand/cement or this stuff: https://www.vitcas.com/prd/outdoor-oven-cement)

My questions are as follows:

* Regarding the placement of weep holes in the base slab - should they line up under the insulation blanket? or it doesn't matter?
* Is it better to cast the slab with the holes or drill them afterwards? I can place in pieces of 10mm copper pipe when I'm casting, but then how would I level the slab if they're all sticking up?
* I'm assuming the idea for the mosaic tiles under ceramic fibre board is just to raise it off the concrete and allow any moisture to run out through the weep holes. I'm finding it difficult to source suitable mosaic tile sheets locally (and they're quite expensive), can I just glue down broken pieces of tiles with spaces between them instead?
* Should the ceramic board extend under the insulation blanket, or just the inner walls?
* Refactory cememt - I don't trust myself to make my own homebrew, so I'm thinking of using this product (https://www.vitcas.com/prd/heatproof-screed), they sell a bunch of similar looking products including one called 'outdoor oven cement', but I think the heatproof screed is the one to use for the inner dome. Has anyone used these before? Would I need to add fibres to this? Anything else I should consider?
* I've seen people mention some kind of vent/valve at the top of the dome, but I can't find which build this was on now - presumably this is just in the outer render layer (venting from the insulation blanket to the outside), not extending into the inner dome? what kind of valve is this, and where can I buy one?
* This seems a long way into the future, but for the outer render (on top of the insulation blanket), should that be a specialist mix (like the outdoor oven cement mentioned above), or just sand/cement?
* I don't really have the space to build a full enclosure for the oven, but assuming I have some kind of vent/valve between the insulation and the outside, is there any reason not to completely water-proof the outside (either with some kind of masonry weatherproofing cream, or tanking slurry or something else)?

Thanks in advance for any help and advice with this!

[SableSprings]

Welcome Andy! From your plan it appears you've been reading here already. For the weep holes, I think it's easier to cast in place rather than drill later. Set a pattern of five pieces of pipe slightly below the projected surface of the hearth slab and under the oven floor footprint. Put a cotton ball or paper towel plug at the top of each pipe to reduce/eliminate concrete flowing into the pipe when you pour. These pipe pieces would obviously pass through your form base. Doing it that way allows you to pour, level, and avoid hitting rebar if you'd chosen to drill. Once the concrete has set, you slide a piece of rod or sturdy screwdriver up the pipe tubes from below and tap out the plug and thin layer of concrete at the surface. This will also create a little divot around the weep hole to help any moisture gather & exit. If you can't find any porcelain/glass tile sheet squares, using broken pieces of tile would work just fine. Simply remember you want to provide a moisture block between the concrete slab and the insulation board (hence the use of glass or porcelain tile) and create both support for the board while allowing channels to the weep holes.

Your goal for the insulation is too completely isolate the oven. So, I'd be extending my board beyond the masonry outer dome and planning on the dome insulation setting down on that outer board rim.

Hope that helps get you through the next couple steps...outer cover & venting options can wait.

[david s]

My build employs a bathtub in the supporting slab and I was concerned about water getting in there so designed a drain in the centre, As the inventor of both under floor and oven dome venting, which seems to be now adopted by most nuilders here because it efficiently allows the removal of moisture, I have taken this a step further to use the steam pressure build up in the dome insulation space to force the moisture down and out. This is achieved by the placement of holes that locate through the supporting slab under the dome insulation. I use nerf bullets to cast around. with them well oiled 50% of the time they just pop out, but are also cheap and sacrificial. Mike's method may suit you better. The pics should tell the story.

You may want to consider applying a layer of vermicrete over the blanket because blanket will not act as a suitable substrate on which to apply render. The vermicrete will set hard enough to act as a firm substrate, even out lumps and bumps of the blanket to restore a perfect dome form and if it is made lean (10:1) will also act as additional insulation about as efficient as blanket.

Regarding the Victas screed you plan to use, screed implies a thin layer application and as such would be less susceptible to steam spalling. If that is correct then it may not contain any burnout fibres. The other product they list is Mouldable Firebrick which implies a much thicker application and as such probably does contain the burnout fibres. As they are a British company, phone or preferably email them with your plan and get their advice. I would assume that both products are calcium aluminate cement or sodium silicate based products. They will be able to confirm this.

There are quite a few well documented cast builds in the cast dome section under Other Oven Types that would be worth reading.

[andyg]

Thanks for the responses. I've been diligently reading through all the build posts on this fourm for a while now, there's a huge amount of great information and tips here, but it's sometimes a little diffused!

[andyg]

Right, I have the top slab poured now, and the drainage holes drilled through. So my next question is regarding the insulation board that will go under the fire-brick floor. I'd always assumed this should extend past the fire-brick floor, so it is underneath the insulation blanket. However in the presence of the slab drainage holes, I'm wondering how the moisture will escape from the insulation blanket layer. I was thinking maybe I should drill a few holes at the edge of the ceramic board where the blanket will go (see diagram). Or should the ceramic board end flush with the floor bricks? Or neither, and add a vent at the top of the outer render?
Does my diagram of how the various elements line up look correct? Am I forgetting anything major?

[UtahBeehiver]

You do not need a drain hole in the CaSi board. Some way of venting steam from the oven is a good idea, water to steam sublimation increases volume by 1500 times so a way to vent the steam will reduce pressure build up under render. Attached is what a few of us have used, you can get them at any car parts store for less than $10. Called a breather vent and screws into a 1/2" PVC bushing, Gulf adds a little wire mesh around the bushing to secure it better in the render. The floor insulation should be proud to the OD of the oven ensuring the none of the oven wall touches the hearth.

[andyg]

Thanks for the reply, I'l definitely look for a breather vent for the top of the oven.
When you say 'The floor insulation should be proud to the OD of the oven', does that mean that my diagram is largely correct, with the insulation board extending underneath the blanket, or not? (ignoring the holes in the insulation board)

[SableSprings]

Yes, that's what Russell means...the upper dome insulation rests on the ceramic insulation board. You are simply encasing the entire masonry structure with insulation as much as possible. The more complete the thermal isolation of your oven is, the more efficiently it performs.

[andyg]

Right, now I'm onto laying out my floor bricks. I feel like an idiot because I don't know anything, appologies if these are stupid questions or they've been answered elsewhere (I couldn't find them).

1. It seems my floor bricks are generally not quite square. Laying them out in a herring-bone pattern, there are gaps of ~1mm between many of the bricks. Is this a problem? If so, what's the solution!
2. I've seen some people laying tin-foil over the CaSi board (perhaps as a moisture barrier, plus a tiny bit of extra insulation). Is this a) a good idea, b) a bad idea, or c) doesn't make any difference either way?

[rsandler]

1. Always a hazard. Basically all you can do is play with the layout to get the unevenness to cancel out, avoiding the gaps where possible. When one builds the dome out of bricks you can select the flattest and squarest for the floor, but since you're casting your dome, you'll just have to make do. 1mm doesn't sound too bad as long as you don't go over that. You can sweep fireclay/brick dust into the cracks when you're done to hide the gaps.

2. Tin foil won't do anything good between floor and insulation. A moisture barrier isn't desirable (keeps moisture in, as well as out). Foil isn't an insulator--aluminum conducts heat quite well (typically we think of foil as being a heat barrier because it either restricts airflow, and thus convection or evaporation, or else reflects radiant heat; sandwiched between bricks and insulation it will do neither).

[david s]

1. Always a hazard. Basically all you can do is play with the layout to get the unevenness to cancel out, avoiding the gaps where possible. When one builds the dome out of bricks you can select the flattest and squarest for the floor, but since you're casting your dome, you'll just have to make do. 1mm doesn't sound too bad as long as you don't go over that. You can sweep fireclay/brick dust into the cracks when you're done to hide the gaps.

2. Tin foil won't do anything good between floor and insulation. A moisture barrier isn't desirable (keeps moisture in, as well as out). Foil isn't an insulator--aluminum conducts heat quite well (typically we think of foil as being a heat barrier because it either restricts airflow, and thus convection or evaporation, or else reflects radiant heat; sandwiched between bricks and insulation it will do neither).

Plus one on your explanation. Fire bricks and tiles will all vary slightly in shape, size and thickness because of clay body inconsistency and positioning in the kiln on firing which can result in a difference in shrinkage. Levelling then can become a problem. To achieve a nice flat cooking floor, this can be adjusted by a thin layer of 50/50 clay and fine sand, (best if dry,) in the same way concrete pavers are laid. Any small gaps will simply be filled with ash.