Challenges and progress of building pizza oven on a slope

[Yokosuka dweller]

Hi Danhem,

I did some curing fires before adding the p/v-crete - and overall I think I left the dome more than 1 month with just blanket and chickenwire on. This was more because I didn't have time to move to the p-crete layer earlier than that. In any case, I thought my oven was pretty dry when I began adding the p-crete layers and that also took a couple weeks to finish. BUT even then I discovered later on that I had a few small cracks. In hindsight I think I went up in temps a tad too fast, too eager you know...After finishing the p/v-crete layer I also let it sit for at least 1 month before adding the final render, and then let that dry for a couple of weeks before firing the oven. But even so, that render now has a nice long crack on the backside of the oven.

I actually think if I were to do this again, I would have added more insulation wool and just chicken wire and then proper stucco over that, and not go with the p/v-crete... why?

Because I think when the oven expands due to heat, the inswool layer is flexible right, so it wouldn't exert a strong push force against the final render even it expands a bit. But a cementitious p/v-crete is harder so not flexible and when that expands it pushes harder against the final rendering. Anyway that's my theory. I will probably hide the crack eventually by covering the dome in brick shims or something else.

I have made some progress with my area around the oven, but it's still a mess and a work in progress. Progress has been slow lately because I got hired full time for a job close to where we live. Can't say I love sitting in an office 8 hours a day, would much rather make stuff with my hands....but for now, we need to financial stability.

Cheers - hope things are fine down in Hua Hin.

[david s]

Your assessment is correct. Because the vermicrete layer requires so much water to make up the mix it takes way longer than you would think to dry. It may appear quite dry but deeper in it will still be damp even after a couple of weeks drying. Once you have rendered over it you have locked that water in. One litre of water creates 1540 litres of steam and this pressure is enough to create decent cracks. Too late now, but I’ve found, from making the same mistake myself, that driving the water out of the insulation space gently, before rendering the outer shell is a much safer procedure. Also a vent in the outer shell is also a prudent addition to relieve steam pressure. The problem is much worse if the insulation layer is entirely vermicrete because as you say there is less flexibility. The addition of the blanket acts as an expansion joint.
Too late now, what’s done is done. If it were mine I’d be firing it gently, placing some sheet plastic over it and when there is no longer condensation on its underside then you know you’ve dried it. Then you can fill the cracks with some more render. To get a really good bond dampen the cracks and paint them with a thin slurry of cement and water before filling them with render. Then try to keep the area damp for a few days. Hope this helps.

[danhem]

Your assessment is correct. Because the vermicrete layer requires so much water to make up the mix it takes way longer than you would think to dry. It may appear quite dry but deeper in it will still be damp even after a couple of weeks drying. Once you have rendered over it you have locked that water in. One litre of water creates 1540 litres of steam and this pressure is enough to create decent cracks. Too late now, but I’ve found, from making the same mistake myself, that driving the water out of the insulation space gently, before rendering the outer shell is a much safer procedure. Also a vent in the outer shell is also a prudent addition to relieve steam pressure. The problem is much worse if the insulation layer is entirely vermicrete because as you say there is less flexibility. The addition of the blanket acts as an expansion joint.
Too late now, what’s done is done. If it were mine I’d be firing it gently, placing some sheet plastic over it and when there is no longer condensation on its underside then you know you’ve dried it. Then you can fill the cracks with some more render. To get a really good bond dampen the cracks and paint them with a thin slurry of cement and water before filling them with render. Then try to keep the area damp for a few days. Hope this helps.

Hi David,

once the v-Crete is applied, it sounds like you are suggesting another round of curing fires to dry the layer out properly.

do you have any guidelines temperature wise or is the condensation trick the only way to monitor This drying process?

[UtahBeehiver]

David S always suggest putting a piece of plastic over the dome and if you see condensation forming under the plastic during the curing then the oven and/or insulation still contains water. Water that converts to steam increases volume by a factor of 1500 plus so need to get it out before you stucco and in an humid and tropical environment like yours you should put a vent at the apex of the stucco dome so steam can get out in the future and won't pressurize the dome and crack it.

Looks like David S already commented on this.

[david s]

A cheap garden moisture meter is another option to test for the presence of moisture. I don’t use this anymore because I’ve developed a fair bit of sensitivity testing the moisture with my hand held against the vermicrete to detect the claminess. Another way is to hold a mirror on the surface to test for condensation. The mirror will go foggy. Yes I did mean do more drying fires before repairing the cracks because it would be a pity to repair them only for them to reopen from more steam pressure.

[danhem]

Am thinking about a fat slab of granite for flue area/landing. I will put in a 1,5" steel pipe for heat break so hopefully granite on the outside of that would be able to accommodate the heat without breaking or spalling.

Looking at this part myself now and trying to get an idea of how it will work. The front of my landing still exposes the oven floor bricks and the CF board insulation.

You mentioned here that a 1.5" steel pipe is good to separate the oven floor from the granite. Am I right in thinking that the steel pipe should cover the 2.5" depth of the oven floor bricks and obviously run along the entire front of the landing, essentially covering the bricks?

I am also wondering what base material to use under the granite? Simply build up with regular bricks or is a more specialist approach required for some reason?

Instead of using granite I have considered using some nice tiles for this area. It was mentioned later in this threat that granite may crack due to heat - do you think granite would crack from heat radiating outside the oven, considering the steel pipe will act as a heat break? Seems unlikely to me as I have not noticed any exceptional heat beyond the flue area.

[Yokosuka dweller]

I was advised against using granite for the landing and debated what then to use. But I wanted a visual break between the oven inside floor and the landing. Eventually I came across a cheap piece of thick granite which had almost the right dimensions and just went for it. It is 3,5cm thick, compared to the thickness of the firebricks, which are 4,5cm. To accommodate the difference I put a 1cm wedge of steel and brick under the granite to the inside of the oven all the way across so the granite hit the exact right height of the oven floor and heat break steel pipe. I guess you can use anything, that works there. The difference in thickness between granite landing and oven floor then meant the granite slab just sloped gently down to hit the hearth floor at the outside arch exit, so that any water which might hit it would flow out.

I later polished it up to a good shine, and it hasn't spalled or broken yet. Maybe its due to the thickness and the heatbreak - the granite barely gets too hot to touch when the oven is blazing. Probably there's something to it and granite doesn't tolerate high heat, so I don't know how it will hold up over time, but if it breaks then I'll know more.

I think with tiles you'd have to be a bit careful, it does get hot there so I imagine you need smth pretty thick to absorb the heat. Maybe check for availability of soap stone or some other heat resistant natural rock, might be available in Thailand. I couldn't find anything suitable here in Japan even though there's mountains and rocks every where...but then again language skills weren't there to have a real investigation on it.

The heatbreak was just a 3cmx3cm square hollow stainless steel pipe. It was of course too thin to match the thickness of the firebricks. To get it to the level of the oven floor and to avoid having any lip where a pizza peel might stick I had to put two small firebrick squares under it on each side and then filled the cavity with perlite cement to bring it up, it has a bit of air gap under there I think is fine for insulation and I could remove the pipe if necessary. In any case the heat break really works-there's a 20-25% reduction of heat immediately beyond the steel pipe on the floor (and arch, but that's another heatbreak). I can see you've got CF board running under there, right? I didn't work with that material and don't know how compressible it is, maybe look at UtahBeehiver 's build I think he had the same design of the landing.

Cheers,

Simon

[UtahBeehiver]

If you go this route, you should use SS instead of Carbon Steel the K values are significantly lower for SS and it needs to be as thin as possible. Does it make a difference, maybe, maybe not. There was a recent cast build where they slice a wedge off the bottom of the ledger brick so only a minimal amount of ledger brick touch the dome floor bricks then filled the wedge gap with vcrete. That looked like a good option to me as well.

[danhem]

As for brick or stucco finish. I had not read up much on that yet. I was going to go with just stucco since I've got a lot of hydraulic lime and cement left.

Simon

Hi Simon,

What did you go for here in the end? Did you mix up your own stucco or purchase it from outside? I'm about ready to start the stucco layers and wonder if there are any do's or don'ts that can help here.

I was hoping to simply go a buy some product, mix it, then apply it. Seems that you were thinking of mixing your own with Portland and lime? Was that for any reason other than you had excess supply of these materials?

Thanks,

Danny.

[Yokosuka dweller]

Hi Danny,

I mixed my own based on advice from David S because I had material left over, and because I wasn't sure what kind of purchased stucco would be the right one. I ended up using way more sand than I had anticipated, probably close to 100kg or so, and around 10kg lime and 15-20 kg cement. I can't remember exactly the proportions. I mixed that dry and then added water proofing liquid and alkaline resistant nylon fibers to prevent cracking. I then stuccoed between 1,5-3cm thickness and when finished covered it in clingwrap to slow the drying and increase strength of the stucco.

I guess what I did isn't really a 'real'stucco. What I did was more just a single layer final render rather than a 3-coat stucco. A 3-layer stucco needs to be on a base mesh which could be good to also as expansion joint to prevent cracking. So depends on what you choose to do I guess. You can also consider to install a breather vent onto the oven? I did and I think that was a good idea - esp in climates with high humidity variation.

As I wrote to you some time ago. I am not really happy with the result of my 'stucco' effort, there are a few thin cracks. I want to fix those and then I will paint over it with a concrete primer and acrylic concrete paint to really seal it off. Then if I still don't like it I will make a final layer of brick shims to cover the dome. I like the brick look and it was a shame to have to cover it all up.

Haven't gotten around to all of this yet. We have a massive wet season at the moment (with bad floods and landslides in the south of the country), so the oven is mainly just covered and hibernating for now.

Simon

[danhem]

I mixed my own based on advice from David S because I had material left over, and because I wasn't sure what kind of purchased stucco would be the right one. I ended up using way more sand than I had anticipated, probably close to 100kg or so, and around 10kg lime and 15-20 kg cement. I can't remember exactly the proportions. I mixed that dry and then added water proofing liquid and alkaline resistant nylon fibers to prevent cracking. I then stuccoed between 1,5-3cm thickness and when finished covered it in clingwrap to slow the drying and increase strength of the stucco.

Okay great, thanks for that. I'll probably look at doing the same in that case. I dread the though of having to go out to try and purchase materials here as it invariably leads to huge frustration trying to navigate the language barrier here.

I'll search for David's recommended method on this part. Amazed that 100kg of sand was used! Any particular sand or will just a fine builders sand do the job?

You can also consider to install a breather vent onto the oven? I did and I think that was a good idea - esp in climates with high humidity variation.

Yes will definitely do that, seems like a simple and necessary task. I bought a pipe that should be fine for this purpose.

As I wrote to you some time ago. I am not really happy with the result of my 'stucco' effort, there are a few thin cracks. I want to fix those and then I will paint over it with a concrete primer and acrylic concrete paint to really seal it off. Then if I still don't like it I will make a final layer of brick shims to cover the dome. I like the brick look and it was a shame to have to cover it all up.

Haven't gotten around to all of this yet. We have a massive wet season at the moment (with bad floods and landslides in the south of the country), so the oven is mainly just covered and hibernating for now.

Well I'm literally having sleepless nights and have done for a while now as cracks and rain and failing domes have well and truly consumed me over these past couple of months. I'm normally ok with the rain here as it brings the temperature down and is great for the garden and golf course. This year's rain season is having a different effect on me .

I really like the brick shims idea. Gulf's looks great and as you said here the effect is very rustic - just how these ovens were meant to be I suppose.


Thanks again,

Danny.