Building outdoors on ground in tropics

[Johnnycantplaytoday]

Hi All,

Newbie here - Just chasing some advice / feedback on my Outdoor In/On Ground Pizza Oven plans before I start building. Started out just wanting to build a simple cheap vermicrete dome over a sandcastle with no insulation to make some Friday night pizza's to go with my cold beer. But after lots of homework and many sleepless nights my build plans have taken on a life of their own.

So to my situation. The only available practical place to build near the house is on the ground behind a concrete rock wall about a metre high. See below photo. This is out in the open and as we live in the tropics, it would be subjected to weeks of heavy rain at a time, not to mention it's in the garden and would cop additional water from hosing. Not really practical to build a roof over it and it wouldn't suit the immediate surrounds.

My concerns were none until I started reading all the posts on this forum about moisture, curing, cracking, lack of heat retention, homebrew etc etc. Didn't come across too much info on building in the ground tho.

So after many beers of thought, my plan is to excavate about 300mm into the ground behind the top of the wall. Form a 100mm thick slab with about a 250mm deep, 1200mm-1300mm square "recess" for the oven base with drain holes to the front of the retaining wall. Oven would be approx 850mm-900mm Int dia) On the concrete slab in the recess will be some 25x25mm aluminium tube to mount a 20mm Fibre Cement Sheet to above the concrete floor. On this sheeting I was going to form a 100mm vermicrete slab (6:1) for insulation, then on that pour a 50mm Refractory concrete slab for additional heat retention. Then on that would be the 35mm thick Fire Brick oven floor (same height as top of retaining wall). There would be approx 100mm perimeter around the oven in the recess to keep moisture and water away from the build which would all be covered over by pavers butting up to the edge of the completed oven. If dirt, leaves "stuff" did, make its way into the recess it would simply be a matter of removing some pavers and either hosing out or blowing out. The idea is that the oven is permanently off the concrete floor so never can absorb moisture. Also will have a 150mm flue and be covered either in small tiles with grout or some kind of waterproof render.

My basic motive was to cook some pizza, but then we added maybe cooking a roast the next day, or baking some bread and cakes. So chasing some advice / feedback

1. Is this overkill? Considering I was only originally happy to spend a couple hundred dollars, now I'm up around the $1400 mark.

2. Would a simple vericrete dome with a layer of ceramic blanket covered in more vermicrete on a vericrete slab with some firebricks inside suffice.

3. Is poor mans home made concrete dome mix (3,1,1,1,) much worse than proper refractory cement. Is there more chance of cracking. Is there less heat retention. Cant believe it's $90 per bag of the good stuff. and I need around 10 of them.

4. Is the 35mm Fire brick floor thick enough without the 50mm additional Refractory layer beneath it or should I keep the refractory layer there.

5. Could I ditch the 100mm vermicrete insulating layer under the refractory cement flooring and just have the Fire bricks on refractory cement on Fibre Cement sheeting as insulation as it is off the ground.

6. Floor height will be around the 1m mark, I am 6 ft, Minister for War & Finance is quite a few inches shorter. Do you think this height is OK, or should I lower it a bit. Seems about right for me, but maybe a bit high for her. Should I lower it a couple inches.

I would rather build it properly than have to rebuild it again. I think I've just answered my own questions.

Thanks in advance.

JB

[Johnnycantplaytoday]

Hi Guys,

Been a couple weeks so I thought I'd give an update. Well last weekend it rained most of the weekend finished off with 80-100km Gale force winds and rain coming sideways. So the tarp I put on the oven blew away and basically the oven got saturated and water went down the flue wetting inside. So I did a couple smaller fires a couple days ago to slowly dry the inside out and to help dry the outer insulation layer as well.

I finished my Internal Insulated door, but havn't had a chance to test it's effectiveness due to wanting to dry it out slowly. So I think I'm good to go with the internal door and a larger fire however this weekends looking like the same story as last week, so maybe not.

Anyway, I've attached a couple photos of the inner door and will get back with it's effectiveness when I get a chance to fire the oven up again.

Thanks again for all your help and advice.

I've also now put on over 2 kg's in the last few weeks from eating pizza every Friday, Saturday and Sunday & I personally hold each & every person on this site responsible for my increasing waistline!! Just Kidding! No really!

Cheers

JB

[UtahBeehiver]

That crack is just a wisdom line, I agree with David, all ovens have cracks somewhere.

[Johnnycantplaytoday]

Thanks Dave,

Not to much room to make a separate decorative arch and I sort of like the way it looks at the moment. Was hoping not to get any cracks but at that location and the way it is, I guess I shouldn't be too surprised. I will keep an eye on it though to see if it spreads or gets bigger.

Was wondering if it was worth poking around with a hacksaw blade or similar between the vermicrete and the casting to create a 1-2mm airspace at the front of the oven where there is no blanket. Or is this just wishful thinking. Could even cut a bit of vermicrete out of the inside of the flue gallery and get a blade in there also down the side of the flue gallery casting to create a small gap as well. That way when the inner casting dome expands, it will have a couple mm to expand into thereby reducing any stress on the outer vermicrete layer.

Could it also be worth trying to pour some kind of bonding/glue agent in between the crack while its hot, something like Bondcrete or high temp glue maybe to fill the gap and dry when it cools

Just a thought!

Regards

JB

[david s]

If you render the outer shell over the vermicrete and onto the outer arch of the flue gallery you are likely to get cracks there because the inner parts of the oven will be expanding against the cooler outer shell. These are only cosmetic and don’t really present too much of a problem because the outer shell is only a protective cover. But they do provide a spot where water can get in. If you build a decorative arch in front of the existing casting with a small expansion gap the outer shell can be rendered on to this allowing the hot oven to expand freely without placing stress on the cooler decorative arch or outer shell.if I recall you said you haven’t got room to do this so so don’t worry about it. Remember that there are two types of oven owners, one lot who say their ovens have cracks while the others lie about their ovens having no cracks.

[Johnnycantplaytoday]

Hi Dave and Russ,

Thanks for the replies. Well another weekend and great results and more questions

As for the inner door, I used a thick sheet of metal as that's what I had lying around at the time but I hear you with the S/Steel. So I did a scrounge at the local rubbish tip buy back shop on Friday and picked up an old double stainless steel sink for $20 that I will use to make a 3 inch (75mm) thick perlite/vermiculite "sandwich" inner door with some timber handles. It may not be the best, but it would have to be better than just a metal door. I can use this for now and experiment later when I get back some motivation and when the weather cools down again. This weekend was 42C and high humidity at our place. Was on my 3rd saturated work shirt by 2:00pm. Then gave up and grabbed a beer and jumped in the pool!!

I also knocked up a nice exterior timber door from left over hardwood floor boards in the morning. I intend to attach a sheet of s/steel sheeting on the inside of that to help reflect heat while the oven is firing up. I use the exterior door as a bit of a barrier to prevent excess heat escaping and wind blowing in. Sort of like a window slide across the main entrance.

As for curing, I finished my last curing fire Saturday night and the inside got up to 500C for a while here and there. I buy no means had a large fire, which was surprising as I've seen ovens with like huge roaring fires inside. Mine have just been some piles of wood with flames just reaching the ceiling, certainly nothing what I would call huge. When it does get a bit out of control I've been splitting it into 2 fires and moving around to the edge of the dome to help warm up the sides instead of just the ceiling. So all good and cooked my first pizza's that night that came out awesome. So very happy indeed!

As per Dave's advice I put on a sheet of clear plastic while It the oven was working on Saturday night and there was no moisture coming out of the insulation at the time or when we finished up. Exterior temp was basically 36C all around, inside temp was 450-500C. However the next morning, after around 8-10 hours there was moisture trapped under the sheet, not a lot, but enough to make the outside of the dome insulation look wet where the plastic was. So obviously still more water to come out.

We had the oven going again last night at the same sort of temps, upwards of 500C to low 500's C at times. All good. After a couple pizzas later when the fire was in the Pizza cooking stage (inside dome temp around 460-480C, hearth temps around the 350C, I noticed a hairline crack in the outer insulation layer coming from the front opening of the oven up 45 degrees to the side of the flue and then stopping around 20cm back from the entrance. I checked the external temp and it was 40C and the temp at the oven opening on the actual home brew was 190C.

Not sure if I should be concerned? Basically the vermiculite/perlite insulation sits directly on the inner dome flue gallery in that area, i.e. there is no blanket for about 10-12cm from the front. Refer photos. So I'm guessing that it could have been caused by expansion of the flue gallery pushing against the vermiculite/perlite insulation. This morning, when it all had cooled down you have to look hard for the crack.

I don't think it was caused by moisture steaming. Of all the places for moisture to escape, its the front section due to the large surface area exposed to the outside that would have been easiest for the moisture to get out.

So question, do you think that's the likely cause (flue gallery expansion) and should I be concerned. All I can think about at the moment is what I'm putting on my next pizza and when I'm going to get time to cook it!.

Thanks again guys.

JB

[david s]

Stainless steel is way Less thermally conductive than mild steel. It also has the advantage of not being as susceptible to corrosion. It’s disadvantage is that it warps quite badly under heat. This can be offset by using thicker material but that simply increases the amount of thermal mass. Aluminium easy to work but is even more conductive. I opt for a timber door because I like the look of timber, but it can only be used if the oven temp is below 300C or it will char. Making a suitable insulation panel is quite difficult. A 10:1 vermicrete is not strong enough. If you make it stronger it becomes more conductive and less insulative. Some builders have made insulating panels from Hebel and some that totally encapsulate a lower density material like fibre blanket. Wanting to keep weight, material, labour and expense down, I use a 25 mm thick cast panel of vermicrete, perlite and castable refractory that is bolted to the 25 mm thick timber door, but stands off the surface of the timber with high temp silicon to reduce thermal conductivity transfer. Check the attached link for thermal conductivity of materials. Also search the forum for doors, there’s been plenty of discussion, examples and solutions.

https://www.engineeringtoolbox.com/t...ity-d_429.html

[UtahBeehiver]

It makes a lot of difference if you want to be able to use your oven heat over several days. Ideally SS is better than carbon steel. You can find scrap SS from old BBQs or dead appliances, ie refrigerators, etc.

[Johnnycantplaytoday]

Hi Dave,

Thanks again. I will try the plastic over the top. So far the outside surface appears dry as a bone, no change.

I had another fire in there last night for around 3 hours, outside temp of oven was 36C all around including top while the inside temps got up to 350C above the flames for 5-10 minutes until the flames died down, then I would add more wood and so the story repeats. Ceiling was still black with soot though. At the moment using lots of crap wood and cut up formwork and ply, so quite smokey! Made heaps of charcoal and moved that periodically around the perimeter including the entrance. After everything died down the inside ceiling temp was 225C, sides were 190C and this morning ceiling temp was 110C and sides were 90C.

Started to make an inner door out of (I'm guessing) 2-3mm steel sheet. Was going to somehow put a 25mm layer of 10:1 vermiculite/perlite on the front of this as a temporary insulation for the steel until I can work out something better. The pizza ovens I see for sale at Harveys/Bunning etc just have a metal door with no insulation. Is it worth doing, would it make much of a difference or just stick to the steel door.

Thanks

JB

[david s]

All sounds like it’s going well. As the oven dries out it will fire much more efficiently and that is when the temperature can rocket up and do damage. Sounds like you are managing it well. Try the sheet plastic trick over the vermicrete to test for moisture.
The carbon burns off at around 300 C and you’ll see the crown of the dome on the inside go white. As it heats up more this will gradually spread down the walls of the oven until all the carbon has burned off. Then you’re done.

[Johnnycantplaytoday]

Hi Again,

Well I spent the weekend tidying up around the oven, I managed to get all the pavers down as well as getting the pavers "propped/supported" and paving over the "tub" that the pizza oven sits in. Worked out well.

Started my 1st curing fire Saturday arvo, has been 3 weeks since the dome was built. Started small and got the dome temp to 100C for around 4 hours.

Sunday I fired again for around 4-5 hours and the dome spiked at around 150-180 for short periods of time above the flames.

Monday, had a slightly bigger fire for around 4 hours, dome temp directly above the flames hit about 250C (for short periods of time) but then the flames died and the temp dropped quickly to about 150-170C.

So far there has been not a hint of moisture on the outside of the oven and the outside surface hasnt gone more than a few degrees above ambient temperature, eg the side of the dome outside was 34-36C before the fire started and didn't change during the firing, the outside surface of the dome on the very top was around 38-42C while the fire was going.

So reading up on curing fires and I am reading many completely conflicting ways or processes on how to do this.

Some posts suggest to keep the fire going as long as possible 3-6 hours, others say just get to temp and let the dome cool again.
Some posts say start at 300F and go up 50F per day for 7 days and then have the big fire, but that would mean your only curing up to 600F and then going to the big fire of say 1000F which is a 400F jump.

So far everything is performing well. After an hour or so I have been moving the fire/coals around to the edges of the dome and letting it sit in each location after restoking a bit for about 30 minutes, basically firing the entire perimeter as well as the front opening area.


Last night dome temp was 200C after the fire went out. This morning dome temp was 80C and the sides were 50C.

So I think I am going to continue doing what my gut tells me and to basically go every day for several hours a day jumping up 100F/50C each day for 7-8 days. That should get the dome up to around 800-900F after 8 days. By then I should be able to start throwing some pizzas in.

Dave, do you reckon this should be OK.

I also put some Permatex Muffler putty on the inside of the entrance, went on really well with a spatula, was like grey toothpaste. You can see it as the lighter grey in the photo below. Is hard as a rock now and appears to be bonded very well to the wall. I need to get some more as I didn't have enough. I will only put it on the outside of the hot chamber in case there are any nasty chemicals in there. Will keep you posted how it goes.


Thanks

JB

[Johnnycantplaytoday]

Will Do,

I intend to take my time drying the oven with many fires. Have put too much time and money into this sucker to stuff it up now!

Also got some Muffler Putty to try out as well. Will keep you posted.

Thanks

JB

[david s]

Yes, that’s correct, but a thick layer of vermicrete contains a lot of water. Around 1/3 of the total volume, so you’ve probably added somewhere in the vicinity of 100 litres of water i’d guess, that’s why allowing the sun and wind to do some of the work for you is a good idea. If too much heat is applied to the vermicrete suddenly it will swell and crack. That’s why I think it’s best to dry the oven out with a combination of sun wind and fire before rendering a hard cement shell over it. About the fourth or fifth oven I built, I cracked the outer shell because the vermicrete swelled from excessive steam pressure, so I now take it slower. The blanket acts to a certain extent as an expansion joint so I don’t want to scare you, but the seven fires in seven days should be enough to dry out your oven. You can feel the moisture on the outside of the vermicrete with your hand, it will feel clammy. Also you can throw some plastic sheet over the oven while firing. Water condensing on the underside will indicate any moisture present. Just try and get most of it out.

[Johnnycantplaytoday]

Hi Dave,

Wouldn't the heat from the fires dry out the vermiculite from the inside out fairly quickly. If the inside dome is hitting up to 400C, surely the blanket surrounding it must be over 100C which would translate to a lot of heat going into the vermiculite insulation layer. As the insulation layer is exposed to the side of the flue, the front of the oven and a hole in the top of the dome, I would have thought the moisture would escape after several firings. Or not!

Thanks

JB

[david s]

Ok. Just it’s like a pile of sand sitting on concrete that will look dry on the surface but will stay damp underneath for months. Because you have such a thick layer the moisture meter may help you.