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  • Building outdoors on ground in tropics

    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.


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  • #2
    You have a great resource in Townsville south of you. David S is our casting expert and one of our old time contributors to the forum. He can give you some guidance on your project, material resources, etc. You can PM or he may see your post. An unenclosed oven is more prone to water issues but there are plenty of builds done this way. IMHO, some type of waterproof render will be needed over the 100 mm vcrete dome insulation, you could make the ratio higher up to 10 to 1 for better insulation qualities. No more than your proposed 6 to 1 on the floor.I am not sure what load the cement sheet can handle with the tube spacing but these ovens can get quite heavy. Being recessed, the oven will be exposed to water so sloping of the floor will be critical. Floor height rule of thumb is elbow height. There are a few builds similar to yours where they have build ovens into slopes or recesses.

    I reread you narrative and I see you proposed tiles or render on the outer coat which were not on the sketch I saw. I know that David S suggest a 100% water down acrylic coating over render for water proofing.
    Last edited by UtahBeehiver; 09-26-2018, 08:50 AM.
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    • #3
      It looks a fine plan. I'd add to use an acrylic product or addative in the top coat or maybe paint it.

      I use plain cheap mortar under the fire brick.....others scolded me.... I've seen it done without issue many times but high temp is a safe bet if there is doudt in your mind.


      • #4
        Hi John,

        your plan looks reasonable if you must have your oven partially underground. The usual height is floor at belly button level. Another solution that I think would be better is to build a step in front of the oven so the oven wont be underground. You get about double Townsvilles rainfall so you will have more water issues than us. The best thing is to have a roof over the oven but this of course adds a lot more expense. Most of my ovens, my own included, are out in the weather. It is not only rain that is a problem, in the wet season when the humidity is very high a WFO will pick up sufficient moisture from the atmosphere even if it hasnt rained or its covered, to make the insulation moist enough so it wont work properly. The remedy is simply gentle prolonged fires. We dont use the oven much at those times, it being so hot and humid the aircon is a preferable alternative.
        From the bottom up, some holes through the foundation slab and drain pipes to the rock wall are essential to remove standing water and underfloor moisture.
        The underfloor vermicrete would be better at 5:1. 6:1 is really not quite strong enough. Do this slab in at least a couple of layers with a couple of weeks drying for each layer before building over it.
        You have planned 85 mm thick floor. Most ovens have around 50 mm thick floors which prove adequate for doing bread and roasts etc. Go thicker if you plan on cooking bread for the village daily.
        As you point out, castable refractory is expensive where homebrew castable, while not quite as good is far more affordable and also easier to work with for a home project.
        Again most commercial cast ovens are 50 mm thick and prove quite adequate for home use for both pizza and home roasting and baking. Remember that extra thickness (thermal mass) requires more energy and time to reach temp.
        Your vermicrete layer over the blanket at 6:1 will not be a very good insulator as the more cement in the brew reduces the insulation value. I find 10:1 is about as lean as you can go without making it difficult to apply.
        Over the top of the vermicrete, after it has been thoroughly dried you can use a cement render over which can be applied some kind of waterproofing.

        underfloor insulation is essential
        A vermicrete dome is far to weak to be serviceable for much longer than a year i’d guess. It needs to be made of dense (stronger) material.

        Last edited by david s; 09-26-2018, 12:59 PM.
        Kindled with zeal and fired with passion.


        • #5
          Hi Guys,

          Thanks for your replies.

          I am struggling a bit with the cost blowout, mainly for the refractory castable cement. I am thinking now along the lines of somewhere between my original plan of vermicrete dome and be happy and the one I posted yesterday with the all the bells and whistles. This should bring the cost way down to a few hundred dollars (i am hoping!) as well as being quicker to build. If worse comes to worse, I can always knock it down and rebuild another one in 4-5 years.

          It seems that most people out there that have used home brew as the castable are happy with it, and it would appear from what I have read that they last 3-4 years of more without any issue aside from some small cracks when hot.

          I'm not to concerned I guess about being able to cook roasts etc the day after without refiring (one of the reasons for going to refractory cement dome), we have tons of dead wood around to burn and I like fires anyway!!

          So I am thinking of changing my original plan a bit to use home brew instead of refractory cement. I also have a heaps of pavers behind the shed which I thought I would be able to use as the base layer in the ground instead of forming up a 100mm thick concrete slab. On top of the pavers I could use about 50mm thick of 20mm stone/metal aggregate or pumice stone and then pour the vermicrete (5:1) insulating base directly on the stones and fill up the side with extra 20mm aggregate, then put pavers on top of the whole lot butting up to the side of the finished oven. This I am hoping would still be enough to keep the water away from the base of the oven.

          I know a roof over the top would help with the rain somewhat, but I'm in 2 minds about putting one up. Just where it is I think a roof would look out of place. I will think about it some more though.

          So onto some more questions.

          1. If I use homebrew for the dome / hot face, Can I still cook 60-90 second pizzas like a refractory cast dome?

          2. With the oven being outdoors with rain, humidity, moisture in the concrete, etc, I shouldn't be too concerned so long as I use a small fire perhaps the night before (morning before) to dry it out a bit before stoking it up for the main event?

          3. Do you think It is a problem forming a 100mm vermicrete slab directly onto 20mm aggregate, or should I put a sheet of 20mm thick Fibre cement sheeting under the vermicrete to give some support and stop the mix combining with the aggregate.

          4. I assume with a homebrew made dome that after the fire has died out, that the oven would still stay hot for several hours with a door in place. Cook a roast or stew perhaps.

          5. Anyone (Dave S?) know a supplier in Cairns or close to, where I can get the little stainless needles to mix in the homebrew. I've googled it but only seem to be able to get it in capital cities? The freight kills for heavy low volume items.

          Thanks again.


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          • #6
            1. Yes
            2. In wet humid weather this is exactly what I do.
            3. Yes I think it is a problem. You want to start with a good strong stable supporting slab that won’t move or settle. You may get away with your planned lack of one but you also may not. Perhaps a compromise of 2” steel reinforced slab in addition to the cement sheet would work ok. Skip the pavers, they always settle unevenly from my experience, particularly with a heavy load.
            4. You’ll have plenty of thermal mass to cook roasts with a 2” floor and 2” thick dense homebrew oven dome.
            5. You won’t get ss fibres in Cairns or Townsville. Get them from Antec Eng in Sydney. They are not absolutely necessary. The more important fibres are the ones that will burn out to assist you in safer water elimination. You will be able to get these from your local concrete suppliers. They are the really fine polypropelyne ones that come as part of a two pack system of random concrete fibre reinforcing used widely in industry. Don’t add the thicker ones though.
            Kindled with zeal and fired with passion.


            • #7
              Thanks for that Dave,

              Yeah, I think I'll ditch the pavers underneath and go back to a formed concrete bed. Was only thinking about the pavers because I have so many of them.

              Would making the dome 3 inches thick instead of 2, make it any stronger, perform any better or reduce cracking?

              Thanks again,



              • #8
                Thicker would make it stronger, but also increase the heat up time as well as taking longer to dry it. Most manufacturers producing cast domes use 2” thick castings which prove to be a good compromise.
                Kindled with zeal and fired with passion.


                • #9
                  Hi Guys,

                  So I made a start the other day and went for the base of the original plan which was to form a 100mm thick reinforced concrete slab in the ground on which to put a 20mm Fibro sheet on some aluminium tubing. After some cutting and some digging and lots of crow baring, I ended up with a square of around 1200mm x 12000mm. Which leaves me an internal diameter of around 800mm. Slightly smaller than I was hoping. I also have a problem of having to shore up some fairly huge heavy rocks, to stop them collapsing down on my oven. Will work on that later.

                  In the meantime I now have some more questions.

                  1. Can I reduce the outer vermicrete insulation layer from 4 inch thick (100mm) to say 3 inchs (75mm) thick. Will this make a big difference to the heat retained or would it be minimal loss compared to a 4 inch thick layer. If I can reduce the outer layer by an inch, I can gain an additional 2 inches inside diameter.

                  2. On some videos I have watched, some companies only seem to put a thin (maybe 5-10mm) render on the outside of the ceramic blanket. See one here from Melbourne Fire Brick Company, ( or Forno Bravo ( . Is this as good as a 100mm vermicrete layer. Does it get a lot hotter on the outside of the render compared to the vermicrete. Could I do a 2 inch (50mm) vermicrete outer layer with 10mm render on the outside of that for similar results to a 4 inch vermicrete layer on it's own. Hope this makes sense.

                  3. I have also now ordered the stainless steel ME446 Melt extract fibres, however I can only order in 10kg batches. And yep, with freight, there expensive at $150. I have calculated the volume of concrete for the dome to be around 0.04m3 to 0.05m3 and at around 2500kg per m3, that equates to around 100-125kg of concrete for the dome. So at around 2% by weight for the extract fibres, I'm looking at using around 2-3 kg worth. Does this sound about right for those who have used them.

                  4. In regards to casting on a sandcastle. I have seen some posts where the results on the outside look great, but on the inside of the dome, after removing the sand, there are some gaps and holes. Would it be better to try to make a reverse mold with sand. ie, build a big box, fill with sand and create a "hole" for the dome and fill it with the homebrew, so casting a dish rather than a dome. Then when dry, flip it over and put on the it on the hearth. I Imagine this would give you a nice smooth interior finish to the dome and if the outside of the dome had some gaps and holes, this could easily be filled with homebrew without fear of it eventually falling through to the inside. Have people tried this and if so, did it work out better.

                  5. With regards to the hearth. As my floor bricks are only 35mm thick, I was going to add an additional 50mm thick layer of homebrew under them. Should I use the ME446 stainless extract fibres in this as well. I think I will have heaps left over, so probably cant hurt.

                  6. Also, I intend to build into the retaining wall for the entrance. I was going to cut out a section around 100mm deep, 600 wide and 1/2 way into the middle of the wall, to allow for the opening of the oven. Was going to put down a layer of vermicrete directly onto the wall and then build up with homebrew base, fire bricks and cast around these and eventually vermicrete outer layer onto the edge of the rock wall. Anyone see any issues with this. I was hoping there wouldn't be as much heat at the very front of the oven entrance. If so, I could remove the 50mm homebrew base and thicken the vermicrete sub insulation and lay the 35mm firebrick straight on top of the vermicrete. Would this be a better way to go.

                  7. Was going to use a 6 inch flue around 1 metre long. Would a 7 inch flue be better than a 6 inch flue for an 850mm (33inch) oven Internal diam. Similar cost.

                  Any advice/ suggestions appreciated.

                  Thanks again


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                  • #10
                    Originally posted by david s View Post
                    Thicker would make it stronger, but also increase the heat up time as well as taking longer to dry it. Most manufacturers producing cast domes use 2” thick castings which prove to be a good compromise.
                    1. Assuming you are not using an insulation blanket over the dome then a 3” vermicrete layer would be the absolute minimum insulation. The more cement you put in the brew the less it will insulate. I find 10:1 is about as lean as I can go without it becoming too difficult to apply. They say you can’t have too much insulation, but it is the old law of diminishing returns so there is some point where the cost and labour aren’t worth the small benefit gained.

                    2. The vermicrete layer over the blanket acts to provide both insulation and a firm substrate to apply a final dense render coat. A render coat containing perlite or vermiculite drastically reduces its strength and therefore needs to be much thicker. I prefer to use a dense, thin final render enriched with AR fibreglass fibres over a lean vermicrete layer. This removes the need for time consuming chickenwire reinforcing. Thick vermicrete layers contain an enormous proportion (about 1/3) of water that is difficult to remove, so if not using a blanket layer allow 1.5” vermicrete layers at least a week to dry before applying the next.

                    3. Yes that sounds about right.

                    4. You could do it that way, but building the dish like form will be much more difficult as well as having to flip the casting and bedding it into position. If you cover the sand dome in wet strips of newspaper you should end up with a very good inner surface.

                    5. 85 mm thick floor is very thick. The usual is 50. Use the ss needles if you have them. Best not to mortar the firebricks in so they can expand and contract freely. Also easy to replace if needed down the track. The gaps between the bricks will simply fill with ash.

                    6. Good idea, you don’t want extra conductive thermal mass in that area.

                    7. 6” is the correct diam for your sized oven. Added height will give you a little more draw, but you shouldn’t need it.
                    Kindled with zeal and fired with passion.


                    • #11
                      did you remember to provide the drainage for moisture accumulation under the insulation slab? If pouring a vermicrete slab over the cement sheeting put some holes through it and cover them with fly mesh so the vermicrete doesn’t fall through. The ratio for underfloor vermicrete is 5:1

                      Kindled with zeal and fired with passion.


                      • #12
                        Hi Dave,

                        Thanks for the reply.

                        Have been busy the last 2 days and have managed to get a bit done. I've been able to build a reinforced concrete wall under the large stones behind the future oven. Basically dug out about 100mm deep under the rocks and hoped they wouldn't fall down, which they didn't. So all good now!

                        Today managed found an old bench with aluminium legs in the shed and cut that to size for the floor frame. I Dyna bolted them to the slab and then cut a piece of 19mm Fibro Floor sheeting and screwed that down to the aluminium tubing floor frame. Ended up cutting the corners off so I had better access to clean out and blow / hose under the fibro floor if I ever need to.

                        Went and got some Vermiculite today and mixed up a batch for the 1st layer of the floor. Went with your suggestion of pouring in 2 layers to help drying out. I did think about some drain / moisture escape holes in the fibro flooring but as I was pushed for time to get it poured before dark I opted not to. I figured that I could come back later and drill a heap of 1/2 inch (12-14mm) holes in the 1st layer of vermicrete sub flooring through the fibro which would give me a lot more surface area for the vermicrete to dry out when heating from above as you would have the surface area of the drill hole itself in the vermicrete as well as the hole in the floor. So I think I'll drill about 30 holes in the next week before I pour the 2nd layer of vermicrete. I've made / mixed / and poured a lot of concrete in my time, but that vermicrete is some trippy stuff. Had no idea what to expect or what I was doing. I mixed it together dry in a cement mixer and "poured" the dry mix into a barrow to add the water and hand mix as was suggested elsewhere on this forum because the cement mixer apparently crushes up the vermiculite. I mixed 5:1 and kept adding water till I got some kind of formation happening. That's a new one for me. Hopefully all works out. Still feels like sponge cake after a couple hours.

                        So I now have the stainless needles coming, still got to source some polypropylene fibres, job for next week.

                        A question on the LIME for the homebrew mix. Today when purchasing the Vermiculite, the rural supplies shop had 2 types of lime in 20kg bags. One was "Hydrated Lime" for making mortar and the other one was I forgot,(also a 20kg bag of Lime). Which lime do I use, the Hydrated or the other one? Sorry for the vagueness, I will find out more info next week.

                        Anyways, Rum and Coke is calling.

                        Thanks again very much for your time and advice. I appreciate it and maybe one day I can help others as you and others have done for me.

                        Cheers. Here's some photos of the last couple days.


                        • #13
                          Ok, sounds like you have it well in hand. I’m still concerned about drain hole(s) under the vermicrete slab. Can you drill some from the front of the wall into the space making sure they slope out slightly? When mixing vermicrete you need approx 3 litres of water for every 10 litres of medium grade vermiculite. Fine grade slightly more, course grade slightly less. Hydrated lime is what you want. Te other stuff may be agricultural lime which you don’t want.

                          Attached is an experiment i did re drying vermicrete slab that you may find useful.

                          Vermicrete insulating slab
                          Last edited by david s; 10-06-2018, 05:53 AM.
                          Kindled with zeal and fired with passion.


                          • #14
                            Hi Dave,

                            Thanks again for your replies. I worked out I added about 3 litres of water to 10 litres of mix, so pretty much spot on with what you said.

                            I'm not sure I understand what you mean by holes in the front of the vermiculite slab, when I take the form work off, the entire perimeter of the vermiculite slab will be exposed and hence able to dry out if needed. I have now also drilled around 30 x 12mm holes through the vermicrete slab through the fibro base, so it can dry out there as well. I have been putting the hose in every day for a minute here and there to help keep things cool and hopefully slow down the initial curing. Then I was going to leave a few days before putting down my next layer of vermicrete on the floor.

                            I have a few more questions though. Sorry, they never seem to end.

                            1. What sand do you use as the formwork for casting the dome. I was thinking along the lines of a brikkies loam as I reckon this would probably compact a bit better than say a plaster sand.

                            2. Then the question is what sand do I use as the 3:1:1:1 ration in the home brew. I am guessing to go for a plaster sand (not the brikkies loam) as it has no clay in it.

                            3. I am reading up on curing and when to cure, I was thinking about curing the dome a week or 2 after making it, before adding the ceramic blanket and vermicrete insulation but now I believe I have to complete the oven and then I do the curing. Is this correct? Oh and then leave the final thin waterproof render until after the oven has completely cured / dried out.

                            4. I have also read of people leaving a "vent" in the top of the outer vermicrete insulation layer, to allow for steam to escape I'm guessing. How does one go about this. I thought about drilling lots of small holes in the top of the dome and covering with an old "farmers plough disc" over the top to prevent water soaking back into the vermicrete. Just a thought.

                            5. You mention in your design some kind of seal on the flue that allows moisture to get out from the dome and vermicrete but doesn't let water in, is this like a "narrow roof" around the perimeter of the flue just above the outer vermicrete insulation layer? I was thinking of leaving a 2-3mm gap in the dome and the vermicrete insulation for flue expansion and siliconing on a downward facing 25mm stainless steel "skirt" with a fire proof silicone just above the outer layer so water couldn't drip back in the gap between the flue and both inner and outer dome. This would allow any moisture or built up humidity in the concrete the escape, even after curing.

                            Thanks again.


                            • #15
                              Hi John,

                              the pics on post #12 didnt stick so i wasnt sure if you have included drain holes to allow water out of the space between the concrete foundation and the vermicrete. This is most important as you have built below ground level and you get heavy rainfall.

                              1. I don't think the sand type matters much for the mould although finer sand will compact and hold together better. The correct ratio is around 10% water by volume, but you will find that your sand is already damp when you buy it so just go by instinct. Adding a little powdered clay will assist in holding it together better.
                              2. Sand that has a range of grain sizes is better for making a castable. You will be adding the clay to it anyhow.
                              3. The home-brew castable should be damp cured for a min of one week. Are you planning on casting a flue gallery in front of the oven mouth? if so this should be done before removing the sand mould. Driving out the water is best done after the insulating layers are completed, but before the outer shell is done.
                              4. yes, but if you do it like i do and as you've described in the last point then you shouldn't need an additional one in the top of the dome. I make mine from terra cotta and fire to 1100C but you can fabricate a conical one from stainless and seal with high temp silicon against the stainless flue pipe. Permeated Ultra Copper is the highest rated one I have found.

                              Pics show how I do it. The last one shows a stainless collar that i use for my mobile oven with removable flue.
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                              Last edited by david s; 10-08-2018, 01:21 PM.
                              Kindled with zeal and fired with passion.