30" cast dome design

[sergetania]

Mullster thank you, I have been following your thread. I still have a plenty of time to decide on how to cast the gallery. I am going back and forth on whether to cast separately or together. What is the main reason that made you think it is better to cast separately? Thanks!

[Mullster]

Mullster thank you, I have been following your thread. I still have a plenty of time to decide on how to cast the gallery. I am going back and forth on whether to cast separately or together. What is the main reason that made you think it is better to cast separately? Thanks!

sergetania the main reason is I believe it’s quite a bit more difficult to make an easily removable framework when it is a one piece cast.Nick J C did a great job of it recently, but I made a mess of mine

[sergetania]

I just saw your post about the problems. Sorry about it and don't be too hard on yourself. Everyone can make mistakes. I hope you can fix that thing. I really appreciate you posting about the negative experience. I will try to learn from it.

[sergetania]

I can hardly believe it myself but I have actually started the build. Two steps are done, I have got a 6" slab and built a stand. Next, making a form and pouring a 2 1/4" thick concrete countertop on top on that stand out of a 6000psi countertop concrete mix with 3/16 rebar mesh. Please someone(David, Russell...) stop me now if I am doing something stupid that I will pay for later. Thank you and hopefully I will get to the fun part soon!

[david s]

You've already done it now, but I dislike timber stands, especially if they are out in the weather. A well built oven that doesn't contain steel or timber should be still operating well after your children have died. You can't say the same for a timber stand subjected to weather. Usually some diagonals will increase the stability enormously, enabling a lighter weight construction, although you've made it very strong.
3/16" mesh is very lightweight, you may want to add some thicker bar as well. You don't say how big your oven is going to be so it's difficult to estimate what weight the supporting slab has to support, but generally a 4" thick slab is recommended. Be careful the you provide sufficient support in your formwork, particularly in the middle of the sides to prevent bowing or at worst a blow out. Have you done a plan for the oven size to fit your supporting slab. Usually the supporting slab requires more room at the front to accomodate the flue gallery so it needs to be rectangular rather than square, although my build has a square footprint.

[sergetania]

David,

Thank you for responding. I really don't like doing things without knowing what I am doing that also go against opinion of people who are hugely experienced. So I realize I have a problem here. If you don't mind I will explain how I came to make these decisions though I understand it still does not change the fact that I am planning to do things that are not recommended in general. Please don;t view this as a sign of disrespect... I hope it is not.

The plans are in the first post of this thread but I am also attaching them to this post for convenience (some dimensions have changed but not drastically). I am planning a 30" cast dome. Insulation - 2" ceramic board on the bottom, 3" ceramic blanket over the dome. A layer of firebricks inside of the dome. Stucco over the blanket, no v-crete layer. Trying to stay on smaller/lighter side.

The wooden base - you gave the perfect reason to build it out of concrete blocks - so the kids can enjoy it too. I didn't think about it. Honestly, don't see the kids staying in the same house - hardly ever happens in the US. So planning for our lives only. It is a pressure-treated ground-contact 6x6 and 2x10 lumber - same as the deck standing next to it. It should last 20-30 years easily. If the deck and the oven go at the same time it is fine.
However, the main reason is that I could not build the block base. My back is a total disaster, two surgeries, not good... probably should not even started the build but sitting home for months gets to the best of us. Hired neighbor's kids to pour the slab and my daughter(enjoyed it!!!) helped me build the stand out of wood. No way I could build it out of blocks. I have been doing woodworking with my lousy back for years and years, so I knew I could build it out of wood but not blocks. Now everyone laughs because they think I have overbuilt the stand. Let them laugh... I just like the look of 6x6 lumber.

Now, the concrete countertop. Because of the wooden stand and smaller size oven I thought I could get away with a thinner (and lighter - funny how one thing leads to another) top. I hope a stronger 6000psi concrete with proper reinforcing will be enough. As you can see the wooden stand looks more like a cabinet base. The concrete does not need to span much at all. With the thickness of 2 1/4" I can't use rebar thicker than 3/16 mesh. Large size rebar causes telegraph cracks in thin slabs. The plan is to place reinforcement mesh at the bottom and also the top. The front and back of the countertop are cantilevers so running reinforcement lengthwise of the slab in the top part of the concrete should give it enough strength. There is a guy (of course, on Youtube, but at least it is a super boring channel - the Concrete Countertop institute) who teaches about concrete strength, and he built 8'x6'x1.5" slab and loaded it with 500 lbs of sand on top. It bent more than 1" in the middle but didn't crack because it is properly reinforced.

I know you have probably seen lots of smart guys like me who later regret "smart" decisions they have made. Any chance that you think my reasoning makes any sense at all? If the experience says I should go 4" thick slab maybe I should just listen and try to change my plans. Thank you again for your time and help! Again, I respect your experience and what you have done very much.

[david s]

My apologies, I had read your build, but had forgotten as there is so much traffic here it's hard to keep up. Although your supporting slab is well supported under, 2" is pretty thin and sufficient embedment of reinforcing starts to become a problem. If you can stretch it to 3 or at least 2.5" it would be better. Most concrete countertops also include a fair amount of random AR glass fibres for reinforcing too.
I respect designs that push the envelope, I'm more guilty of it than most, but there's a good reason overbuilding is a popular way to go as it minimises chances of problems occurring from things not being strong enough. 2" calsil should be plenty to keep the heat off the timber, but a slightly thicker slab would also help in this regard. Cantilevering is a great way to reduce the span and allows you to reduce the strength of the slab. Do yourself a favour and slope the slab down slightly at the perimeter so rain coming off the dome will be encouraged not to run back under the calsil insulation. And while you're about it cast a few holes in the centre to act as weepholes. Some polystyrene works quite well for this, you simply place it in when you've finished towelling off the top, then they're easy to knock out when the slab sets.

[sergetania]

David,

Seriously, you are not supposed to remember a design posted a month or more ago! Thank you for your time and expertise, again!
I like the idea of bumping the thickness to at least 2.5". It should make placing the reinforcement easier, especially with the two layers, in the top and bottom layers.Also, I have seen a number of pictures on the forum where the form sides get pushed out by the concrete so I will try to build a sturdy form.

I have been planning to install an aluminium flashing along the perimeter of the ceramic board sealed with caulk to prevent water going under the insulation board. I want to make a drawing of that so hopefully you can look over to see if it makes sense. Sorry, I do not understand what you said about the weepholes and polystyrene. Can you please elaborate? Thank you,

Sergei

[sergetania]

David,

Attaching a drawing of the oven layers as I plan it including the aluminium flashing around the ceramic board. Please let me know if any of these is a problem. Thank you! Sergei

[UtahBeehiver]

Looks fine, even though you are flashing, water has a way if sneaking in so make sure you have weep holes in you base so it has a means to egress.

[david s]

The weep holes in the supporting slab allow moisture to escape from the underfloor insulation space. They can be drilled after the slab has been cast but you risk hitting the rep and or damaging the slab. So when you cast your supporting slab it is easier to place something like polystyrene, I actually use packaging foam, in the wet concrete so there are a few holes left when they’re removed. I glue a small piece of insect screen over the hole to keep out creepy crawlers.

Regarding the foil, any moisture barrier prevents water getting out as well so may not be of much use. Do what you think best.

I think a layer of 10:1 vermicrete over the blanket about 30mm thick is a better solution than rendering directly onto the blanket as after 3 layers it will be a bit lumpy meaning you have to compensate by making the render inconsistent in its thickness. Also the blanket is somewhat springy which makes the job more difficult. If you do the vermicrete layer, as it’s quite insulative you can probably reduce the blanket to two layers.

[sergetania]

Russell,

Thank you! How do I make these holes in a concrete top? David has mentioned something about the polystyrene. It should not compromise the strength of the countertop, right? I am only going 2.5" thick with that.

[sergetania]

Thank you David, I was typing as you posted the reply. A couple of holes through the top? 1/2 diameter?

I am afraid of v-crete to be honest. It seems it adds weight and moisture but I guess it could make building the outer layer easier. Let me get to laying the blanket and then at least I have a better understanding of what I even want to ask about the outer layer construction. Thanks!

[david s]

Yes around 1/2”. I only have one, but two is probably better. Yes you can think it through as you go. It is a good idea to make sure you have enough room for the layers over the dome. You don’t want to end up not enough. You can either increase the diameter of the supporting slab or reduce the internal oven diameter to achieve this.

[sergetania]

Right, makes sens,thank you! 30" inner diameter, 2" dome thickness, 4" of thickness for insulation and outer layer. That's the plan.