New build started in Scotland

[jjnevis]

This may be a daft question, but I can't find an obvious answer: is it better to build the dome on top of the floor fire bricks (my current plan) or just outside the floor? Or does it not matter?

[MarkJerling]

The dome, if built on the floor, will work fine, but it will be very difficult to replace the floor should the floor need replacement in future. What I did was to build my first few dome courses and then I set the floor inside. After that, I protected the floor with a cardboard covering and carried on with the dome. Many builders seem to do it this way and it does mean that the floor is easily replaceable in future. It is rare to have to replace a floor with normal occasional household usage, but if the oven gets lots of use then the floor can, in time, require replacement of all or some of the bricks due to wear or damage.

So, in short, for most household use, it does not really matter, but I still think the better way to go is to have the floor inside.

[jjnevis]

Thanks Mark, that all makes sense. Since posting the question I looked at a few builds on Youtube and it's pretty 50/50, even some of the large very "professional" looking ones build on the floor (perhaps a school boy error?). Anyways I'm thinking I will tweak my design a wee bit and build around the floor. I kinda feel it will be more stable to do it that way, my only reservation with this method was that if the floor expands it might push the base of the dome out and crack it, but I suspect the tolerances won't be that tight anyway. I could well be overthinking things, but the more one invests the time and energy, the more you want to get it right.

[UtahBeehiver]

Couple things, if you cut the floor to fit inside you need to leave about a card board thickness around the perimeter as an expansion gap and also requires more precise cutting of the floor bricks. Placing the dome on the bricks allows you to rough cut the floor and leave proud. Either way works so up to you and where you best want to spend your energy and time.

[JRPizza]

The order you build in also depends on if you use an IT and how you attach it to the floor. I copied the method Gulf used which I called "the wooden brick" where the IT attached to a piece of wood in the center of my floor. This method would probably be more difficult if you tried to build a few rings and then insert the floor, as I used the IT to draw the circle to cut my floor bricks. You can see the marker inserted in the IT in picture #1. As Russell said above, read up and decide what build sequence makes the most sense to you.

[P3 Stoaker]

G'day jjnevis, welcome to the Forum, which I think your quickly discovering is a source of terrific information from a bunch of very generous people, and for me made a huge difference to the end result. I've only built one oven, and it's not quite finished but here are a few thoughts, or ways I built.
1 For a few reasons I was keen to minimise the overall thickness of hearth/insulation, in other words, everything below the oven floor bricks. My base outside dimensions under the oven is 2 metres square. I was a little concerned over the hearth span so added additional support (centre rear). I cannot recall what the max span ended up as (would be on my thread) I used 6100 mesh, (6 mm steel at 100mm centres) and 40 mpa concrete and this was only 80 mm thick. Whilst I'm not suggesting you use similar sizes, it may give you a little comfort in your larger sizes.
2 I ended up using 3 layers of calsil board 25 mm each, giving a total of 75 mm on top of the hearth and the oven floor fire bricks are on that.
3 Might be worth looking into your dome bricks plan to ensure you have enough thermal mass.
4 Given the opinions of having the dome on floor vs outside the floor was 50/50 I decided I liked the idea of the dome being on the floor, as that spread the weight a little across the floor bricks. Additionally my floor firebricks are more like pavers, 300mm square which I think would have also help spread the load.

As mentioned this is only my build and thoughts, and background as to why. Good luck.

[jjnevis]

Thanks everyone for your advice/experience/input/etc, as well as being great advice it's nice to know that there are people out there to "chew the fat" with, so to speak, especially in these strange times.

Update on where I'm at and decisions to date...

Regarding the concrete table top, I decided to thicken it a little to 5 inch (125mm) and I stuck with the original mesh rebar that I had (similar stuff to what P3 Stoaker used I think), partly because it was convenient (I had it), but also because I spoke to a civil engineer pal who said it would be fine, just to make sure it was in the bottom third of the cast. I went with 4 parts ballast to 1 part cement.

Floor insulation - I did want to add an extra 25mm of ceramic fibre board to the 50mm that I already had, but it was going to take too long to deliver and I also decided it was hard to justify the extra cost, so I just went with 50mm and am hoping that I won't regret that decision.

I've also decided to build the wall on the oven floor mainly because (based on the advice above) it appears to be the simplest option, particularly with regards to accuracy of brick cutting - which I am finding quite a bind and pretty hideous process, although I do have a solution for simple cuts going forward - a friend has lent me a brick cutter. I'll try and remember to put a picture of it and the results of using on this thread, but in short - it's bloody brilliant. The cuts are clean enough for me to use on the dome and it's very quick and simple to use. My plan is to use the 76mm dimension of the brick as the wall thickness (which P3 Stoaker thinks may be too thin to provide the thermal mass necessary, but I guess I'll find out [worried face emoji]). This will mean cutting lots of bricks into three and the cutter will be great for this. I was thinking of covering the dome in a 5-10mm render of fire mortar to finish it off / add some thermal mass / improve stability, but not sure if this is necessary or advised?

My next set of ponderments (that should be a word, if it isn't) are...

1) The age old question and much debated topic of my "homebrew recipe". I was going to go with 1 fire clay, 1 hydraulic lime, 1 cement, 3 builders sand (and a tiny bit of squeezy for workability), but would obviously appreciate anyone's strongly held views or opinions on the matter.

2) Interface between wall bricks and floor - should I just sit them on the floor, or is it better to mortar them in place? I suspect just sitting them on the floor is the answer.

3) Getting a bit ahead of myself here, but I'm considering building a little house around the dome and roofing it to keep the harsh Scottish weather off it, rather than rendering it and sealing/tiling it. I just think it would cope better with the weathering/freeze/thaw etc. It also means that I can stuff the structure with insulation, further improving the heat retention. The main reason not to do it would be covering up the lovely dome shape, which I'm hoping would be admired by all visitors! Any opinions on this also welcome.

4) I am thinking of putting an air vent just outside the door stop at the bottom on one side, by chopping of the end of the bottom brick. The idea being that you could crack the door open and air would flow in to feed the fire, cracking the door also exposes the chimney vent. Hopefully it would make it easier to control the fire. Thoughts on this appreciated too.

[david s]

A 76mm (3") wall thickness is plenty to provide enough thermal mass. Most cast builds recommend an ideal thickness of 50mm (2"), which is considered adequate to provide sufficient heat storage and a reasonable heat up time. However, when using bricks 3" in thickness is not considered adequate, even for the wall of a house where bricks laid on edge are a no no, let alone a brick oven which is subjected to the thermal cycling with the related expansion and contraction issues. It is the width of the mortar joint that is important to provide the required structural integrity.

Although cal sil board is dry when paced on the concrete slab, unless you have purchased the waterproof variety, its ability to absorb water can be a problem if water from the weather gets in. To allow the water to escape from the underfloor insulation a few holes in the slab will allow this moisture to have a pathway to the atmosphere.

The high clay and lime content of the home-brew mortar provides plenty of stickiness for workability and no additional additives are needed.

The first row of dome bricks are usually not bonded to the floor bricks to allow for some movement, although I can't really see any floor brick moving much with a heavy brick dome sitting on top of it. It is more important to lay the floor bricks without mortar though, to allow for the free expansion of individual bricks. Should a floor brick require replacement, and this will only happen to bricks in the centre of the floor, it is also far easier if they haven't been mortared down.

Your stand looks great.

[jjnevis]

Thanks David, the info about the wall thickness is reassuring. My reasoning at design time was mainly to reduce the heat up time, but also to reduce the cost of the bricks, although in the grand scheme of things, I don't think the saving is that much. I was also a little worried about the over all size and fitting on my stand. I've started building the dome now and I'm happy with the decision to use the 76mm thickness for the wall. Also, as you say, there's no need for additives in the homebrew, it is perfectly workable as is. (the standard mortar for building the base was transformed by adding a smidge of fairy liquid).

My dome form (plaster of paris cast over a gym ball) is working out great. It does mean that I can't clean up mortar squeezing out of internal joints, but that should be easy enough to clean up after. If I get my joints right, then there shouldn't be any mortar showing on the inside anyway.

Finally, just another shout out for the brick cutter, great bit of kit, well worth trying to borrow or hire one if you have lots of bricks to cut.

[david s]

Proprietary castable refractories contain burnout fibres which provide a measure of protection from steam spalling, but refractory mortars do not contain the fibres. I think that because they require extra mixing to ensure correct distribution and the fact that a mortar joint is pretty thin, the steam spalling is not an issue and this is probably why they don’t add them and why they all recommend that the mortar joints must be thin.. However, the very thick mortar joints on the outside of a dome build where those very wide gaps are created is a good reason to add them. If you have some polypropylene fibres at hand for either proprietary refractory mortar or homebrew, mix in a small amount.

[jjnevis]

Thanks David, but I don't have the fibres you're talking about so will just push on with the homebrew mix I'm using and hope for the best.

On a different note, I think I've made my first mistake and it's a consequence of not having detailed plans and kinda winging it as I go along. It's something I can easily live with (I'm not going to fix it), but I think I've made the door stop slightly too far away from the oven, approx 50mm. I had wanted the middle inside surface of the door approximately in line with the internal surface of the dome wall, but it's more inline with the outer surface. This makes the tunnel a little longer, the thermal leakage a little greater and the flue slightly further away from the fire. These are all minor issues, but if I had thought it through more carefully I would have set the door stop slightly back a bit. The actual door itself is another issue altogether, haven't come up with a design for that yet, but I want to make a fairly thick insulated one.

Edit - I've just been contemplating my door stop placement issue and have decided that it is probably in about the right place after all. I could have moved it back a little, but the arch bricks would then start to get a little too small (reducing the wall thickness). Also, having stood in front of it, I think the opening tunnel length will be just fine, so feeling better about things again. (I could have just deleted this post but decided to document my thought process instead)

[jjnevis]

OK, time for an update after a little progress. I'll let the photos tell the story.

My next challenge is to transition from the curved rectangular (if that makes sense) chimney slot (I don't know any of the correct terms!) to a 6 inch (150mm) circular entrance to a 1 meter twin wall flue pipe. I was going to try and do this in brick and throwing A LOT of mortar at the problem, but am now thinking that building a form and filling it with some sort of concrete mix would probably be easier in the long run. Again - advice and experience on this would be much appreciated.

I have settled on the idea of building a wee house around the oven and making it out of... don't laugh... wood. I'm pretty confident that if I have a well sealed dome, enough insulation and an efficient flue that will be correctly passed through the slated roof, all will be good and the weather will have no chance of getting in and causing problems. Does anyone know of a wood "clad" oven that has survived years of use?

And last bit of exciting news - a local smithy is making an insulated door, although he is renowned for taking donkeys (a long time) to get round to doing things, so it might be a while before I get to try it for size.

I've got a week at least to wait before I can start the curing process on the dome, pretty sure I can start that before the chimney is in place, although I am a bit nervous because of the thickness of my mortar joints, but I'll soon find out how things go [worried face emoji]

[Tintan28]

Jay,

What stage are you at now with your dome?

David

[jjnevis]

Image ending 619 is where I'm at. I'm scratching my head on how to get from the rectangular vent into my twin wall flue at the moment.

[jjnevis]

Decided to go with a concrete cast for transition to chimney. The form work was a real head scratcher, but in the end decided to just chuck gaffer tape and anything I could find at the problem, including a full roll of kitchen roll and an old sleeping mat chopped up. I think it should work ok and was much easier than trying to do it in brick. I also reinforced it so if it does crack, it shouldn't fall apart. The twin wall flue will just be mortared onto that.