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900mm dome in Adelaide with semi-cut kit – maybe some help needed as I go!

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  • 900mm dome in Adelaide with semi-cut kit – maybe some help needed as I go!

    Some help needed regarding the oven floor and insulation – my kit came with some large insulation blocks to go under the oven floor but I don't have any info about them – I think I have a solution for how to lay them/the insulation for the floor but after some advice if possible?

    I ended up getting the Murex Forno pizza oven kit, 900mm internal. ( Before Christmas they had a good special and got the kit shipped form NSW to SA for $1100 which seemed incredible value. YES, there are tradeoffs with this kit but nothing I don't think I can work with. I do love the fact the kit has cast arch bricks, 5 large bricks to make the inner and 5 for the outer arch makes a much easier build for a novice like me (or so I hope will be the case). I also love the circular cast chimney brick that the flue will eventually sit in.

    The biggest tradeoff so far though has been lack of proper instructions. I keep just getting sent some scribbled notes and pictures of their own commercial builds at different stages of completion for reference, which gives me a little bit of an idea, along with all of the help I'm getting reading this forum.

    I'm at the stage now where I've spent the last few weekends building the base – and yesterday I poured the cooking top. I went with 130mm reinforced concrete for the bench top slab over dry stacked berserk blocks – core-filled with concrete – so I have faith the slab and base will will hold.

    My biggest question for next weekend is going to be how to insulate under the floor and the dome and landing area.

    The kit came with several 300x300mm x 70mm deep insulating bricks for under the floor (see photo). I don't know much more about them, and I can't seem to get any more info from the company. They just say they are what they use in their commercial builds and they recess the blocks into the concrete BUT say to cut the blocks to just fit under the oven floor not the dome. Which seems weird to me, they said they lay their dome bricks directly onto the concrete slab – which I don't think (from what I've read) is the right thing to do.

    So now I'm thinking of using the insulating blocks they have sent under the floor, recessed into a vermiculite concrete slab ontop of the 130mm slab I have recently poured

    I'm thinking of adding an additional vermiculite concrete slab height of 90mm on top – which would give me 20mm under the insulated floor blocks to allow me to get them perfectly level, that will leave the outer dome with 90mm vermiculite concrete under it. If I mix vermiculite at 5:1 from what I've read that will have enough compressive strength. Is this correct?

    I did put a couple of 15mm weep holes into the slab for drainage/drying.

    I'm trying to not have to buy anything additional that'll be super expensive – vermiculite I can spring $40–45 on a bag but around $100+ for CalSil board might be stretching the budget a bit far. Would my solution in anyones opinion still be enough insulation?

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    Last edited by BanjoOgre; 02-23-2020, 07:21 PM.

  • #2
    Today I made up the form for the vermicrete/perlite insulation layer. I figure that I’ll do a 50/50 5:1 mix for the insulation. The form is 100mm high. I’ve cut the insulation blocks but still in two minds whether to use them in laid into the vermicrete - with the 20mm of vermicrete under them (blocks are 70mm) or not and just pour the vermicrete into the whole form. I’ll make my final decision tomorrow when I plan on pouring the insulation so hopefully I can use part of the long weekend to start building up the oven. Still looking for any thoughts or recommendations!

    I think I have decided to also go with one layer of 25mm calsil board over the top of the poured vermicrete whether I use the insulation blocks or not.

    hoping the insulation pour goes as well as the bench top - never used vermiculite or perlite before so curious to see how that goes.

    Removing all of the forms from the bench top pour was a relief earlier this week - knowing that it is solid and stable. I’ve never poured concrete as a bench top so wasn’t 100% sure what to expect. Super happy though.


    • #3

      I'm by no means an expert in this but let me comment.

      Your stand looks good so far, although the side closest to the fence is going to maybe be a bit tight to but not impossible to work around. Keep in mind the extra space needed around all sides of the dome for insulation of the dome later on.

      Seems you've got it right with your thoughts on insulation. I was advised to mix perlite and vermiculite for a more workable mix rather than using either alone, but it's possible. Personally I preferred to work with mid-grained perlite; also dry mix the 5:1 first so the cement is evenly distributed and then add water; mix with your hands, its easy compared to concrete. As for amount of water, @ DavidS recommends to add little by little until you've got a slight bit of water pooling at the bottom of the mix. Also, when you pour it, at first it doesn't look like it will ever set, but it does. Allow it to dry for a few days or the longer the better before you build on it, since that's the easiest point for it to dry - later will be more difficult once the oven floor is on.

      For weep holes, glue a bit of mosquito net or fine mesh in top (or bottom) of the holes to keep critters from gaining access and nesting in your insulation later on.

      I'm not sure if you're going to create a recessed 'pool' for the insulation area or whether it will be raised above the base slab entirely. In any case, some people advise to water barrier the base slab so any moisture won't be drawn up into the insulation from below. Moisture barrier depends on your climate and whether you're planning to cover the entire thing later. As for roof or weather protection, I found even it seems a premature consideration at this early point in your build, you may want to think about it a bit since embedding rebar post anchors into the base slab would be easier to do before the base slab is completed (maybe too late or irrelevant in your case).

      As for your insulation bricks, I'm not sure. I'd probably try to use them since they're there, maybe pour a base layer of p-crete and then lay the insulation bricks on top so they're flush with the external base slab - that way you'd save on the v/p-crete. These insulation bricks how do they hold up to abrasion? If they're easily chipped maybe try not to have them too exposed. And maybe the reason the company advises not to lay them under the dome is that area will carry a lot of weight so maybe these bricks better not be exposed to that much pressure. In that case the ins-bricks could go under your oven floor diameter but stop short and not be where the dome walls are going to be, in that area, just the 5:1 p/v/crete. Dome walls on insulation and NOT on base slab, as that would act as heat sink.

      Alright, just a bit of input from my side. Hope some of it can be useful, otherwise maybe someone with more experience will chip in.

      My build:


      • #4
        Thanks for the reply Yokosuka,
        Unfortunately my phone wasn't letting me add any images to the post I put up yesterday – the vermicrete will encapsulate the insulation blocks and will be poured into a form ontop of the structural benchtop. I made the benchtop quite a bit wider than it technically needed to be so there is hopefully going to be plenty of room to get in and render and work on the fence-side – we will find out in a few weeks time no doubt.

        I'm really looking forward to mixing the vermiculite/perlite mix up compared to mixing the concrete for the benchtop by hand! Just picking up the massive bags so easily was much more comforting than staring down a trailer load of concrete mix!

        I've got some old shade cloth that I'll put over the weep holes to stop the vermiculite mix from falling through and I'll put some more under the base stuck with some liquid nails or something to keep any bugs out!

        We don't get a great deal of rain here – I'm not too worried about the base waterproofing. I can't imagine having any problems with water, because the form for the insulation layer is around 1250mm diameter that should just about be the same width as the dome with the insulation blanket layer, so when it comes time to add the 30-50mm of vermiculite render over the blanket it'll also encapsulate the base layer insulation – then I'll put a waterproof render over the top of everything which should hopefully keep the water out. I do plan on putting tiles on the benchtop when it's all finished and I'll grade them to the edges of the benchtop to keep the water away from the dome.

        I've got enough room on the benchtop to add some supports for a simple roof if I decide to add that at a later date, the biggest advantage I can think of for having a roof for the time being is going to be to support the flue to get the smoke up and over the neigbhbors place – so I've got that option if I need to do it. The base concrete layer is 130mm high so I have confidence I can attached some support brackets to that without too much trouble without having to tie it into any rebar in the benchtop – which is too late to do now.

        Hopefully seeing these images will give you a better idea of my plans! I'm going to be pouring the vermiculite/perlite concrete mix in a couple of hours and I'll add some more pics then.

        I did realise the other day when I placed all the blocks out that the landing bricks (that go under the outer arch) are 10mm higher than the actual cooking floor bricks which is annoying! Just means that I'll pour the main vermicrete mix at 100mm high then when its set enough I'll remove that piece of ply at the front and pour the landing vermicrete at 90mm to accomodate the difference in height. Annoying but manageable – and easier than trying to shave 10mm off of a 100x200mm fire brick.


        • #5
          finished the Vermicrete layer a little earlier tonight. That stuff is WAY easier to mix than structural concrete! It sure is weird stuff though. It goes from being just under hydrated to over saturated very quickly! the texture is so odd compared to regular concrete and even now a few hours later it just feels strange – but I'll see how it is by Sunday/Monday and see if its set enough to start the actual build. Looking forward to the actual building of the oven now the base and insulation is pretty much sorted!

          Tomorrow I'll remove the separation piece of ply at the front of the form and mix + pour the vermicrete for the landing at 90mm high to allow for the change in height between the oven floor bricks and the landing bricks. I'm not at all worried about the landing being 10mm less insulation as that really shouldn't get too hot anyway.

          Still don't know how to work with a few parts of the kit with out proper instructions but its all part of the fun for these projects. I'm going to grab some 25mm CalSil board tomorrow that I'll cut to size to fit under the oven itself and the dome bricks before starting.

          Must also to remember to trace around the floor bricks onto a large sheet of cardboard or 3mm ply to protect the floor while building. Much easier to trace around the floor tiles when they are laid on to the material rather than forgetting to do that first once they are laid!
          Attached Files


          • #6
            FINALLY got to start building 'up' this weekend. In response to the above about floor insulation I just couldn't have forgiven myself if the insulation of the oven floor just wasn't good enough after it got to the cooking stage knowing I could have done something about it earlier so I decided to get 2 sheets of 600 x 1000 x 25mm calcium insulation board to put under the oven floor only (what awful stuff that is to cut! Feels like a giant cuttlefish shell with a similar texture). If I were to have put it under the dome bricks I would have needed an extra sheet so figured the 100mm of 50/50 vermiculite/perlite at 5:1 would be enough for the dome.

            That makes 30mm of vermicrete, 70mm of random insulating brick supplied by the pizza oven kit provider (commercial grade they use in their commercial oven builds they say) and 25mm of calcium board under the 50mm fire brick cooking floor tiles. I'm assuming that should have the floor of the oven sorted and well insulated – it does make the oven quite high now but I'm 180cm (6") so its just about fine for me – if not I'll add a small step but from my previous limited experience with wood oven cooking I think it'll be just fine for me – I should be able to see into the back of the oven without bending over.

            The Brick work has started and I've assembled the inner dome. Nearly burnt out my angle grinder cutting the first keystone for the first course before I remembered reading somewhere it's easier to cut fire bricks when they have had a dunk in water for 5 or so mins first! Much easier after that, still a pain int he bum but doable for the few cuts I'll be making – makes me so glad that I didn't go for a 100% build your own, cut every brick build.

            I know the small mortar joints on the inside are not ideal and I've seen so many perfect ovens on this forum I'll forever be questioning if I made the right decision – but I think for my budget, and lack of time and means to buy a quality wet saw for the job I've made the right call with the kit I did get. It has compromises but nothing I can't work around. Yes, the lack of instructions has been difficult but again, nothing that can't be worked out.

            Even the opening is a little small for my liking at 250mm high and 500mm wide as a full semi-circle shape – but should just about hit the right ratio for the finished height or very close but it'll do everything I want/need. The pre cut arch bricks are much easier for a novice brick layer like me I think, and are a happy medium. I made up some forms for the inner arch and mortared that in this evening so thats coming along nicely too – the transition will be a bit difficult and take some creative angle grinding I think – we managed to get the first course tied into the arch and will have to custom cut each intersection – it's not as sexy as bevelled arch transitions which seem to be all the rage in the forums but again, it'll do what I need and I'm happy to compromise here.

            I do think I'll be questioning whether I made the landing space in front of the opening too deep until I start cooking, I've left 300mm between the end of the outer arch and the front of the slab, but I figure the space will come in super handy for cooking more than pizzas, I'm thinking of the space as a small prep/checking area for foods other than pizza. I plan to build up a nice decorative landing area with some kind of nice stone that'll sit flush and level with the front fire bricks.

            At the end of the day this oven will get me cooking and provide my family with some great entertainment over the years. I can't wait to get finished now its starting to literally take shape! Maybe if there is a next time (don't plan on moving from here any time soon!) then I'll think about using what I've learnt here and making a whole oven from scratch.

            For now – onwards and upwards. Hoping to have the dome ready for curing fires over the 5 day easter break!

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            • #7
              Note, you are at the point where you may see what is called "inverted V" mortar joints in the inside of the dome. It can be minimized by grinding the top part of the the brick to give the joints a bevel.

              Google Photo Album []


              • #8
                Hi Russell,
                Thanks for the reply – I've been wondering what people's talk was of an 'Inverted V' but I do get it now, I've already seen it starting to happen in the previous row. I'll have to experiment over the weekend to see how easy or difficult it'll be to bevel the bricks one-by-one with the equipment I have (an 1100w 125mm angle grinder with diamond blade isn't the easiest to make fine cuts with!)

                I can potentially hire a wet saw and do the bricks in bulk over a day but that'd be an extra expense that I don't really have – but I'll see how I go.

                Will an inverted 'V' have any negative impacts on the actual oven or strength of the build or is it just cosmetic?

                Cheers for all the input!


                • #9
                  You only need to bevel the top front, not the entire side. Smaller inside mortar joints are better, if you are using home brew, this will mean less direct flame contract. As you go up the invert joints will increase is size and get quite large, I think on the lower courses you should be able to knock of the top corners relatively easy with your grinder. It appears that the kit dome bricks are already tapered so just a bevel in needed.
                  Google Photo Album []


                  • #10
                    Thanks Russell,
                    That sounds very do-able. I was planning on laying a few bricks every night after work but will have to wait till the weekend to get the grinder out so I'll take some time off during this week – I could probably use the time off anyway, it's tiring work putting one of these together with lots of manual labour when one is used to sitting at a desk all day.

                    My goal is to have the dome complete and the insulation layer on by Easter so I can use the 4-day break to do my curing – we then have an Anzac Day public holiday here on 25 April so hoping to have friends over for a pizza party then!

                    I'm using an HT mortar that is pre-mixed and I just add water, I got it from a local pizza oven builder. It wasn't all that expensive and just seemed more convenient, it's been super easy to work with so-far so I think I'll get another tub of that and keep going with it.

                    My kit came with a bag of fondu but I think I'll use that at the end to fill all the gaps int he outside of the oven when finished, from what I've read it can be harder to work with laying bricks – and I don't have any fire clay to mix with it only sand.