Challenges and progress of building pizza oven on a slope

[Yokosuka dweller]

Hello members of this forum.

I am creating this post to document my own build and to get input and tips before making more mistakes!

My current challenges are: I am a total newbie with masonry work. I cast the foundation not level. It slopes down approximately 2cm or almost 1 inch from back to front. I thought I could easily remedy this in building the wall of the stand by having more mortar between the bricks as I moved from back to front of the stand. But it seems I won't be able to completely fix this - also because I partly use reclaimed and old blocks I've scavenged here and there, so it really is becoming a bit of a puzzle to fit it together plumb and solid.

I hope that I will be able to solve this 'level problem' when forming and casting the top of the stand. Additionally, since the stand is dug into the ground due to the rather crass slope I am building on I am considering to grout/pour concrete into all the block cells on the back of the stand - I also buttress with rebar every 30-40 cm. In comparison I only pour and rebar every 50-60cm across the sides.

Another issue: I plan to have the stand be 4 rows of blocks. The rebar I cut is about 40cm too long at the top. Would it make sense to try and bend the excess rebar to 90 degrees to create a reinforcing link between wall and top of the stand, so that they become covered in the concrete of the top when poured? Or would bending the cast vertical rebar be too much force for the wall and would it be easier to just cut the excess rebar and throw it into the form to create strength that way?

I haven't begun to seriously consider the actual oven yet. I have about 77 firebricks (from my father in law). Clearly that won't be enough for a brick dome. And fire bricks in Japan cost at least 2 dollars a piece. So to save on cost (and since I don't have any proven skills in brick laying), probably the safest and cheapest way is going to go for a cast dome. The size of my stand will be roughly 140cm by 160cm. Given that I have this area to put a dome, archway, and insulate the whole thing, what size of dome would I be looking to fit onto there? I been thinking of around 75-80cm diameter oven dome. Is that a realistic size given the overall space limitations I have, when needing several payers of insulating mortar/insulating blanket etc?

Finally, to create an insulating space on on the top of the stand, I am thinking to pour first 3cm of concrete reinforced with rebar. On top of that, maybe around 8-10cm of perlcrete mix in the middle roughly to the size of the dome and surrounded by normal concrete. Will that be sufficient insulation? If not, I've also found in the local hardware store that they sell 1cm thick fireproof aluminum silicate plates. I could cut and layer a few (3-4cm) of such plates under the perlcrete. It would make the perlcrete less thick, but maybe that is a more effective insulation than the perlcrete on its own?

I am attaching some picture evidence of the progress in this build. You can see some of the challenges and considerations I mention in these photos. Any inputs or tips would be super welcomed, since this is a pretty challenging but also fun project!

Greetings from Japan

[Yokosuka dweller]

Okay, weather has been bad with lots of rain etc. But I managed to create the form for the top of the stand and begin pouring concrete.



After pouring and running out of concrete mix, the top is horribly warped etc. But I still need to pour a few inches more and make a perlcrete center area. At that stage I will make the finish nice and level.

Is it okay to just pour concrete on top of the already cured concrete, or do I need to do something to make it stick?

Also - I am planning to drill a 110mm hole at either 10am or 2pm of the center where the dome will be. That's because firewood is very expensive here in Japan and I want to keep the option for adding a gas burner. If I do, I will get an avanzini or any other dedicated burner specifically designed for pizza ovens. The hole for now, I am planning to just fill with sharp sand so that I can refurb/install the gas burner when my budget (and my wife) allows the expense. Is that the way to go, any ideas or recommendations as to how I can insulate such hole, whilst keeping it easily accessible for retrofitting?

[Yokosuka dweller]

Alright, as time goes on some questions resolve and are replaced by new ones!

I've partly cast the top of the stand now, but am on hold while I research the options for drilling an access hole for a gas burner through the hearth. I plan to drill the hole and then fill it with a pipeform while I cast the rest of the heath. Then fill the void with sand and perlite and keep it closed below - so that if a burner becomes a realistic feasibility (and a need) then I can retrofit without tearing too much apart. I hope that won't compromise the insulation below too much.

Meanwhile I am increasingly leaning towards trying to build the dome with firebrick. There are so many challenges related to that, and I am playing around with creating my own Indispensable Tool (IT). I think it needs a few iterations before I can call it good. But it seems a really helpful tool not only for the oven walls but also to help find the right angles for the bevels and cutoff's from the inner arch bricks - if one chooses the ambitious and difficult but beautiful option of making the arch bricks become part of the dome inner wall!

I also cut a form to dry run half cut bricks. I only have my father in law's angle grinder and a mason chisel, but it works for straight lines at least. I was thinking to lay the wall outside the oven floor (not on). Reason being that my under floor insulation will be about 12cm (almost 5") perlite/cement/sand and the floor goes directly onto that. I also think I've finally decided that my oven is going to be Asian size - smallish only 80cm or barely 32" diameter. That's because space is a space rare commodity in Japan and my father in law is already thinking that my oven will be too big! Really it is also because my oven stand is only 54x64" and I can't really extend on the sides too much so there needs to be space for the half bricks (115mm), inswool or other blanket (70-80mm) and whatever other prelcrete or cement I need to cover that up with (haven't really thought that far yet).

The there have been some challenges in identifying the right ingredients to build the dome. Firebricks are relatively (if expensive) to come by, but the 'glue' between them isn't. In Japan it seems there are two options for this: (i) a firemortar and (ii) a castor firecement type which is mixed with fibres. The latter is like a pre mix - pretty pricey (around 40USD for 25 kg). But it's very difficult to find lime here or at least I haven't been able to find any slaked lime that isn't used for gardening. Language gaps maybe! Therefore, the FB home brew seems to be out, at least for now. The drawback about the fire mortar seems to be that it is only recommended to use with up to 2-3mm joints, and since the dome eventually will have larger gaps to be filled they recommend the fire-castor cement. I think it's like a ciment fondu. Another drawback about the mortar is that it is not water resistant, so that would be another risk of using it, if it ever gets wet it'll come sliding apart, is that so?

One idea I though was great is to mortar/cast a rebar into the top of the oven that goes diagonally across. Then one could hang meat or herbs or other stuff for smoking or cooking there. It's something I saw people do in Turkey with a dome oven. Seems a great idea, if only the steel doesn't melt or expand and causes cracks in the dome wall...Well, this is still far ahead, so I can try to figure out if that's a feasible idea or not.

Alright, I am attaching a few photos for documentation. Any comments UtahBeehiver on my thinking would be massively welcome and I'd be thankful.

Next week I have to go overseas for work, but my next steps are constructing a jig for arch bricks and play around with that. I think my arch is going to be 25cm tall (63%) and around 40cm wide. Should be within realms of normalcy. Then once I've cast the rest of the hearth I must jump in the fire and start making the dome.

[Yokosuka dweller]

A few updates, but not much. I'm still holding off with the actual build because I am waiting for tools ordered online. I also need to decide whether to use fire mortar or a ready-made refrac cement for the dome. If I manage to find clay and the correct lime I will go for the homebrew, but it seems difficult, at least in my neck of the woods.

This waiting time isn't wasted though. I am reading more and more build threads in here, which is of a great help. Especially figuring out the angle cuts of the arch bricks is taking a long time to really grasp. I think I now understand that one can score the cut lines on the TDC brick using a combination of the IT and holding up a half brick either left of right of the TDC brick. The upper cut line is correct angle when it is 11,5 cm's across so it can accommodate a half brick. What I don't get is though - how do you know that the TDC brick of the arch eventually will line up with a course of bricks and not slightly below or above? Please feel free to tell me that.

Meanwhile, I think I've figured out more details of my oven. Since it's going to be 80cm/31,5" diameter, I calculated the height and width of the oven opening/inner arch and made a form. I didn't realise that it's actually kind tricky to make a semi circle when its wider than its high. But luckily youtube is your friend and I think I managed to make fairly accurate cuts and put an arch form together. The arch form is 26cm/10.25" high (that's 65% of oven height) and 42,5cm wide. I forgot to accommodate shims for the form so will have to cut off a tiny bit from the bottom or just make it a few mms higher.

I also made a quick and rough form for the oven floor. It is 105cm diameter (41,33"). That would give me 25cms, or a tiny bit more than a half brick on either side to sit on insulation. The idea is to put the form and pour regular concrete outside of it. Either to level of the insulation or slightly below. I haven't concluded on that yet. Then I would have a 105cm space for insulation. Still have to make a form for the landing, but haven't figured out exactly how long is good. Need to read a bit more on that.

On insulation, there are lots of threads here discussing that and the overwhelming majority of FB members seem to prefer CalSil or AlSil boards. Initially I was going to just pour 4-5" of perlcrete for the insulation layer under the oven floor. But as I am learning that there isn't such thing as too much insulation, I've found CalSil boards online. I can get 120cmx240cm of 20mm thickness for about 35USD (less than 4000 JPY). So now I am debating whether to put 40mm of CalSil board and then pour perlcrete on top of that for the remaining 3". Surely that combination would provide better insulation than the perlcrete on its own. The only fear I have is moisture, whether the perlcrete would smother the CalSil board and create a pond of water under the dome. Or should I pour perlcrete first and then put CalSil on top of that? Not sure what's the best option here.I have trie to find foamglass which some members have used to fend off water, but I haven't been able to find it yet.

I also found a furnace store not too far from here, I plan to go and talk to them to see what advice they have on refractory mortars/cements and those type of things. Has to wait till some time next month though.

If you have read this far and you know what I'm doing wrong, or what I haven't thought about here, please let me know.

Cheers,



Simon

[Yokosuka dweller]

Time for an update. Winter is here and temps are dropping but still in the 40s in the daytime when it's good. I've been away for work so finally I can get back to this project.

Finished the concrete slab on top of the stand now. Next step will be to create the insulating layer in the forms. I've gotten 7 cubic feet of perlite and vermiculite and will mix those with portland 6:1 to do the insulation layer. It will be 12cms thick, or almost 5 inches. Hope that'll do.

[david s]

Usually your better quality insulation should be facing the brick floor so perlcrete or vermicrete first with cal sil board on top would be preferable but it probably doesn’t matter that much. Builders have found that a 5:1 v or p crete provides adequate strength, anything leaner does not. Adding sand will make it stronger but reduces its insulating capacity. If creating a basin for the insulation space you should drill some holes through the slab to allow moisture to escape.

[Yokosuka dweller]

Usually your better quality insulation should be facing the brick floor so perlcrete or vermicrete first with cal sil board on top would be preferable but it probably doesn't matter that much. Builders have found that a 5:1 v or p crete provides adequate strength, anything leaner does not. Adding sand will make it stronger but reduces its insulating capacity. If creating a basin for the insulation space you should drill some holes through the slab to allow moisture to escape.

Thanks David. I've two questions on immediate next steps:

1. I drilled two 'weep holes' in the supporting slab at a low point. Is it enough that these holes stay in the supporting slab or I need to ensure the hole goes all the way up through the insulating layer until the actual oven floor? I'm wondering if the insulating mass (p/v crete) is permeable enough to let any water/moisture find its own way?

2. After filling the 'basin' with p-crete how long should I let it dry before starting the actual oven build?

[david s]

The holes only need go through the concrete base, not into the insulation. It is a pretty good idea to glue some insect screen over the holes to prevent the vermicrete from falling through and to prevent insects getting in and eating your insulation to make a nice warm home for themselves. see attached table for vermicrete strength and insulation value. Perlcrete the same.

The p crete takes way longer than you'd think to dry and builders often report that their ovens performance keeps on improving for months. This is because the underfloor moisture is the hardest to remove. The holes will assist it greatly. See also attached experiment on perlcrete drying.

Vermicrete insulating slab copy.doc.zip

[Yokosuka dweller]

Thanks a lot - and good advice to cover the weep holes with insect net. I hadn't thought about that at all. We have lots of earth wasps and critters that would probably love the p-crete basin as a nice nest. That would be pretty difficult to deal with in hindsight. So definitely prevent that. The experiment on drying insulation slab is also very helpful. Looks like I will have to put the breaks on and wait after pouring the insulation slab for at least a couple of weeks to allow some drying out of the insulation. And yep, 5:1; not 6:1 seems the way to go.

[Yokosuka dweller]

For arch bricks that taper into wedge shape I made some paper forms to see how many bricks I'd need to cut and if it fits my arch form.

Oven opening is 26cm high, and 42,5cm wide. Height is 65% of oven radius, which should be about okay. I estimate 0,5cm (0.2") mortar joint here.

As per my experiment, the tapered paper bricks stop too short. What am I missing here? Is my taper form wrong, or can I wing it like this. I have 1 'normal' uncut brick on each side horizontal at the edges and then the rest are tapered. Shouldn't the first brick on each side go a bit below the form...

[UtahBeehiver]

JR Pizza has an Excel spreadsheet that will help determine the size of the arch brick. Look at his thread, or you can look for a program on the Forum called Anglelizer that does the same thing. The the appearance in the pic, The paper moc ups seem to vary is dimensions.

[Yokosuka dweller]

JR Pizza has an Excel spreadsheet that will help determine the size of the arch brick. Look at his thread, or you can look for a program on the Forum called Anglelizer that does the same thing. The the appearance in the pic, The paper moc ups seem to vary is dimensions.

Thanks Russell, I found the spreadsheet - according to which the brick should taper down to 1,66"/4,2cm for the ID of the arch. I redid the mockup and it still doesn't match up entirely - about half brick thickness horizontal (1,28") is missing at the bottom of the arch. I guess in that case I can cut a brick in half and use that to get the sides to match height wise. I could also make the opening higher but given I am already at 65% I think I'm at the limit there. In any case, weather is cold so I'll have some time to figure this out. I often use your build pictures to get info, and for your arch it seemed to match up perfectly. My lack of attention to detail is coming back to haunt me!

[JRPizza]

How many bricks are you looking at using for your arch and what were your oven dimensions? When I put in the numbers above for door dimensions from post #10 using 15 standard dimension fire bricks I get a joint thickness of .18 and a small end brick dimension of 1.56. We are likely using different numbers. Let me know what your oven height is and your brick dimensions and I'll see what I come up with.

[UtahBeehiver]

Also look for the Anglelizer program, it is no longer available commercially but someone on the forum posted an archive freeware link to the program. You will use what is call an Axed Arch in the program.

[Yokosuka dweller]

How many bricks are you looking at using for your arch and what were your oven dimensions? When I put in the numbers above for door dimensions from post #10 using 15 standard dimension fire bricks I get a joint thickness of .18 and a small end brick dimension of 1.56. We are likely using different numbers. Let me know what your oven height is and your brick dimensions and I'll see what I come up with.

Hi JR,

Yes, I thought of 15 tapered bricks sitting on 1 standard brick laying horizontally at each side left/right edges of the arch. Oven will be 80cm diameter or 31,5" and the plan was to make 42,5cm (16,5") wide/26cm (10,2") high opening. Width can be adjusted more so than height because its already at 65% to dome radius. I did try to run your program with my dimensions, but my numbers came out differently. I guess excel has never been my strong side. You're saying tapered edge of 1,56? There is a pic showing the excel with those numbers here - what is t-joint btw?




UtahBeehiver , Thanks.

I also found the Angel-izer available for download, but I'm a mac user, so no cookies there. There is an Android App also called voussoirs. My wife had an old android phone and I could download it. Adding to my confusion, the numbers were again slightly different from JR's program, at 1.54 tapered bottom.

DavidS When pouring insulating layer would it make sense to pour two different densities/strengths? I'm thinking for example to pour p-crete at 1:5 under the center of the dome/cooking floor and then change to 1:4 for edges of dome - since these are load bearing areas and I was wondering if compression due to weight would be an issue.