38" Igloo in Houston

[JoelTexas]

After months (years?) of lurking around these forums, I finally started stacking blocks this week. I wanted to start building in early March, but it took about 6 weeks to get approvals from the county and the homeowner association.

I'll talk through my thought processes, since it has been enormously helpful to me to read others'.

My goal was to expand my 8x12 patio slab a bit and put the oven and an enclosed grill on one side. Since I wanted to do a bigger patio, I decided to have pros do the slab. I had them lay concrete 6" thick on the third of the slab where the oven would be with the rest of the slab 4" to save cost. The slab is reinforced with #4 rebar spaced 18" on center.

I wanted the oven to be convenient to the kitchen, but also I wanted an open view of the back yard from the kitchen. So, I settled on a patio layout that has the oven at an angle to the back door. I put a curve one side of the patio so that the corner of the angled oven wouldn't put extra stress on the side of the patio.

I was originally aiming for a 36" oven, but the size of the blocks forced me to add a few inches to the width, and the ceramic fiber board insulation that's locally available only comes in 2x24x48, with 2 boards in a box. I decided to go with 4" of ceramic fiber all the way around, including 2 layers of 2" fiber board under the floor, and 2 layers of 2" blanket everywhere else. It's probably overkill, but my thinking was that you can't have too much insulation.

So, since I have a 48" square of insulation and a bigger platform that I planned, it seemed to make sense to go for a 40" oven. I decided to compromise on 38" in favor of quicker heat up times.

I started stacking blocks yesterday morning, and so far I've stacked the blocks, poured the cores, poured the hearth, cut and stacked the insulation, and laid out the cooking floor. I mixed all the concrete in a tub with a hoe. If I was going to do it again, I'd definitely buy a concrete mixer on Amazon for $140.

[JoelTexas]

The block cores and hearth slab used twenty-seven 80-pound bags of pre-mix concrete. I had calculated 25 bags and bought 26 to be safe (plus, I wasn't sure my rented pickup could carry more than that). So, at about 11pm, after hours and hours of mixing and pouring, I realized there wasn't enough.

I did my best to push the concrete I had to full thickness and left a hole with about 4" of rebar exposed. Then I got up early the next morning and rushed to Lowe's, bought another bag, mixed it and filled the hole. The finish on my hearth slab isn't as nice as I had aimed for, but it's square, plumb and level, and it will all be covered up, so all's well that ends well.

Cutting the insulation was a little bit more challenging than I thought it would be. I started by trying to cut it with a knife. That seemed to work, but it was fairly difficult to cut through the full thickness. I ended up using the cutting wheel on my 4.5" angle grinder. It easily cut halfway through, then I'd flip the board over and do it again from the back.

That worked great, except for one thing. I didn't notice it at the time, but the dust from cutting that ceramic fiber is awful. I should have worn long sleeves and a painter's respirator. I've been itching and coughing for hours after cutting that stuff.

[JoelTexas]

I got the floor bricks cut to fit using a 7" compound miter saw that I borrowed from my sister. The saw works great cutting the flat (stretcher?) side, but I'm not sure yet how it will deal with weird angles on the rowlock side.

I have the soldier course up to help visualize the floor, but I'm not sure yet whether to keep it or go with more of a conventional dome. Either way, the walls will be sitting directly on fiber board, and there will be a small gap between the floor and the walls. I'm thinking about filling that gap either with fire clay or with ceramic fiber rope gasket.

[UtahBeehiver]

Good start, with a full soldier, there will be quite a bit of outward pressure from the dome that may require some type of bolstering. Some builders have installed metal bands, others structural p or v crete. Just something to be aware of.

PS, all you builders out there, it is highly recommended that a Noish 95 or better mask be used when cutting ceramic fiber board or blanket.

[SableSprings]

... Either way, the walls will be sitting directly on fiber board, and there will be a small gap between the floor and the walls. I'm thinking about filling that gap either with fire clay or with ceramic fiber rope gasket.

No need to fill that gap between the cooking floor and the dome walls. In fact, you want a little gap there to accommodate heating expansion/movement. The gap will naturally fill with ash over time so don't worry about it.

As Russell noted above, the soldier base may need some buttressing to "contain" the outward/downward pressure of the dome. Since you don't mention a cover for the oven/patio area, you might consider creating a mortar/concrete dam around the perimeter to keep rain water from seeping into your bottom insulation. If you use this dam to secure chicken wire ends that will cover the ceramic blankets (and provide substrate for the outer oven render) it really helps (IMHO ). I think the dam attachment and tight chicken wire mesh/render provide a pretty good package combining form and structural support for the finished oven.

Also as Russell noted, make sure you wear a GOOD breathing mask and eye protection when you start working with the ceramic batting. You also want to plan to keep kids, pets, & varmints away from the batting (while protecting it from weather) until you've secured it and covered it with your render. As you've already found out, it is nasty stuff .

Looking forward to your progress!

[subzJC]

Wow I can't believe you mixed all that by hand! A recommendation for you brick laying adventures would be a cheap 1/2" drill and a paddle bit long enough for your 5 gallon bucket.
Go buy the cheapest harbor freight 1/2" if you dont have one that will work fine. Just add/rinse your bucket with water first before mixing each batch then the sand and so forth. I guess i'm saying dont start with a dry batch then start adding water. You'll be able to "rewhip" your small batches if the buckets been sitting to long with your new handy mixer. Retire that tub to soaking bricks!

One more thing I would add is to get a 2 quart plastic tub to use for measuring your sand, lime, clay and mortar. Keep it consistant!
Good luck you've got a good start.

[JoelTexas]

I took Russell's and Mike's comments to heart about the outward forces on the soldier course. I don't quite understand the forces involved, but in the meantime, I revised the soldier course to tapered half-bricks. I may also try to figure out how to snug a steel band around the bottom course, but hopefully this will do for now.

Mike, I like your dam idea. We get a lot of rain in Houston, and I've been puzzling about how to keep the water out of the oven. Putting a dike around the base sounds really smart. I was also thinking about something like Tyvek over the chicken wire and under the decorative finish, but keeping the flood waters out is probably a much bigger deal than deflecting the rain.

I started the day by tapering my soldier course all the way around. But then I worried about the instability that Russell and Mike brought up, so I tore everything down and started again. I settled on a tapered half-brick pattern that I could use for the first three courses including the soldier course. The brick is cut at about a 12 degree angle, which works out to be just right for the oven diameter and for the slant to put the second course perpendicular to the center. I cut about three courses worth of bricks, and checked everything by dry stacking with shims. Then I tore it all down again and started doing the mortar.

Today was the first brick I ever buttered in my life, so I am not confident about the mortar. I really struggled with getting clean seams on the inside, because little pebbles kept getting in the way. In retrospect, I think I erred on the side of too dry, plus I think the sand I used was too coarse. The mortar had the consistency of Play-Doh. After awhile, I gave up on using the trowel and just used my hands to mold the mortar onto each brick. That worked okay (maybe), but tomorrow, I think I'll go buy some finer sand and use a little more water. I'm really not sure if the mortar will hold or just flake off. I'll check it in the morning.

I also had trouble keeping the seams from lining up. I added some brick ends here and there to try to create offsets so that bricks would span seams, but for some reason it seemed a lot harder with mortar. Anyway, I'm learning as I go, so maybe it will get easier with progress.

[JoelTexas]

You know, JC, I had the same idea. I bought a 1/2" hammer drill from Northern Tool (closer to me than Harbor Freight) for $25 on sale and a paddle attachment from Lowe's for $10. I was able to mix one bag of Sakrete in a 5-gallon bucket, but the drill was spewing white smoke. I tried different speeds and switching it back and forth between hammer mode and drill mode, but I couldn't tell a difference. I did start by measuring the water into the bucket, then adding the concrete somewhat slowly. The drill was super fast at first, but it bogged down quickly on a few inches of gravel. I finally gave up on the drill and did the rest with a hoe and a mixing tub. I guess I needed a drill with a little more power for concrete, but at least I'm ready the next time I need to mix up a bucket of paint.

I thought about trying the drill method again today when I was mixing up 3:1:1:1 mortar, but I decided to skip it and go straight to the tub and hoe because I wasn't even sure how much water to use.

[UtahBeehiver]

With a half soldier you should be fine now. It is almost a full hemisphere now with outward forces closer to vertical now.

[JoelTexas]

I got a few more courses mortared in yesterday, then I started working on the entry.

I wanted the entry to have a few features: a lip for the door to seal against, a thermal break, and structural continuity with the main oven courses.

For the lip, I made several sets of weirdly shaped bricks that can fit together and tie in with the main dome courses using additional tapering cuts. This might be overkill, because it seems like some people just build the arch separately and butt it up against the main course bricks with mortar. Hopefully the extra effort will be worth it.

I haven't quite worked out the thermal break yet, but my general idea is to have an inner and an outer arch with insulation between them. I was thinking about using insulating fire bricks to create the break without giving up structural integrity, but I'm not sure the extra complexity is worth it, especially since the outside arch wouldn't be supporting much weight.

[UtahBeehiver]

Do watch your dome joints so they stagger. Notice on first pic that the left side has 4 courses with all the mortar joints lined up with each other.

[david s]

I got a few more courses mortared in yesterday, then I started working on the entry.

I wanted the entry to have a few features: a lip for the door to seal against, a thermal break, and structural continuity with the main oven courses.

For the lip, I made several sets of weirdly shaped bricks that can fit together and tie in with the main dome courses using additional tapering cuts. This might be overkill, because it seems like some people just build the arch separately and butt it up against the main course bricks with mortar. Hopefully the extra effort will be worth it.

I haven't quite worked out the thermal break yet, but my general idea is to have an inner and an outer arch with insulation between them. I was thinking about using insulating fire bricks to create the break without giving up structural integrity, but I'm not sure the extra complexity is worth it, especially since the outside arch wouldn't be supporting much weight.

I think integrating the arch with the dome is a great idea. Using IFB’s as a thermal break however, not such a great choice. They are subject to knocks and abrasions which entry bricks are subject to. They are also pretty weak so not too structurally sound. A thermal break’s primary function is to act as an expansion joint, so you may be better off placing it between the flue gallery and the outer decorative arch wher the difference in thermal expansion is far greater. If you insulate the flue gallery well there shouldn’t be too much of a heat sink loss effect.

[JoelTexas]

Okay, I'll kill the IFB idea. Thanks for the feedback.

I made some progress on the entry and flue gallery. To complete the lip for the door to seal against across the top of the frame, I cut an arc groove about 75% of the depth on two firebricks and propped them across the entry to make an arch. This made the transition side of the vent pretty oddly shaped, so I'll have to cut some puzzle-piece bricks to make sure the vent is well supported.

I'm playing with the design of the front arch. My thinking was that the back isn't really a focal area, but the front should be prettier, maybe. I cut a pile of shims at different angles until I settled on this. I don't think the shims are strong enough to use as is, so I'm thinking I'll trace them to make the same tapered shapes about 2" thick. Then I'm thinking I'll have to do something to square up the front and back to support the chimney. I'm also concerned that there isn't much lateral support, especially to hold up the front arch, so I may need to find a way to beef that up.

[UtahBeehiver]

If you want to see some vousoirs on a build go to this link, it was done by Tscarborough for Austin. His build is a great artistic brickwork oven.

https://community.fornobravo.com/for...is-begun/page4

[david s]

Okay, I'll kill the IFB idea. Thanks for the feedback.

I made some progress on the entry and flue gallery. To complete the lip for the door to seal against across the top of the frame, I cut an arc groove about 75% of the depth on two firebricks and propped them across the entry to make an arch. This made the transition side of the vent pretty oddly shaped, so I'll have to cut some puzzle-piece bricks to make sure the vent is well supported.

I'm playing with the design of the front arch. My thinking was that the back isn't really a focal area, but the front should be prettier, maybe. I cut a pile of shims at different angles until I settled on this. I don't think the shims are strong enough to use as is, so I'm thinking I'll trace them to make the same tapered shapes about 2" thick. Then I'm thinking I'll have to do something to square up the front and back to support the chimney. I'm also concerned that there isn't much lateral support, especially to hold up the front arch, so I may need to find a way to beef that up.

You guys building brick ovens all beat yourselves up trying to build a complex form with brick units and end up with a less than ideal result that has vertical joints lining up, a heavy structure that acts like a heat sink and unnecessarily thick. I cast the whole flue gallery in one piece, but for simplicity at least consider a simple casting for the top which creates a smooth funnel to the pipe as well as transitioning from a rectangular section to a round one. I use castable refractory, but because this piece does not see temps anywhere near those of the dome or floor a home-brew mix is adequate. One thing to be careful of though is to cast the hole a couple of millimetres larger in dial than the pipe to allow for the sudden thermal expansion of the pipe against the casting.
The other thing I often see is folk drilling holes into brickwork to attach an anchor plate, often using fixings that will corrode. This is a recipe for problems as the holes and the expanding fixings within them even if they don't corrode are likely to cause cracking. I think my solution is far superior, simpler and cheaper (see pic)

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