36" build in coastal VA

[nlinva]

Hi everyone,

After years of thinking/dreaming about building a pizza oven (and much reading of this forum!), I have finally started. My initial hope was to build the oven as well as an also long-planned pergola over an adjoining patio, both over the course of the summer. I built the pergola first and that took much more time than anticipated, so I'm only now ready to get started on the actual oven. The advantage of the pergola being finished is that I have been able to put up the roof over the oven already, as you can see in the third picture. (The roof will eventually get shingled, but for now it is just plywood being protected by a tarp, since I'll need to cut a hole in it for the chimney to go through).

I am building a 36" interior diameter oven, which will sit on a 5" thick hearth slab measuring 60" x 82". The support structure is cinder blocks in an H shape, or, perhaps more accurately, like a digital 8 with an opening on either end of 1 cinder block's width. This makes for a fairly narrow entrance into the wood storage, but I've tried and it seems easy enough to reach in and grab wood.

The hearth slab is cantilevered about 6" beyond the H frame on all sides. I like the look of the cantilever, but more importantly the idea is that this will mean that the cinder block walls are more or less directly under where the oven dome rests on the slab (the horizontal part of the H is closer to the front than the back, for the same reason).

The slab has 5 weep holes cast in it with 1/2" pvc pipe, and I have already bought el-cheapo tiles at the big orange store to put between the slab and the insulation. My next step is to purchase the insulation.

Here I have a question for forum experts: it looks like Distribution International has two different kinds of CalSil boards, from Johns Manville: Super Firetemp L and Super Firetemp M. Meanwhile, Johns Manville's website lists only Super Firetemp H, S, and X, so they're of no help in distinguishing between L and M. I assume the difference has something to do with how well it insulates vs. how well it resists compression. On the DI website, the specs are:

L - Compressive Strength : 450 psi, Density : 20 pcf, Thermal Conductivity : 0.54 to 0.73 Btu-in/hr-sq-ft-deg F
M - Compressive Strength : 900 psi, Density : 28 pcf, Thermal Conductivity : 0.61 to 0.8 Btu-in/hr-sq-ft-deg F

My complete layperson's guess is that L seems the better one for my purposes: that psi number seems fine, and the thermal conductivity is less. Am I right? In addition, they also list Foamglass; are there benefits for doing 2" foamglass and 2" CalSil over just 2 layers of 2" CalSil? I understand Foamglass is better at not absorbing water, but with the weepholes and the tiles underneath the insulation, that ought not to be an issue, I think.

Any thoughts/suggestions on which insulation to get would be much appreciated.

Maurits

[nlinva]

The other thing I am thinking about is the thickness of my oven floor and dome. The firebricks I was able to get locally are 9 x 4 x 2.125 inches, which is both a bit thinner and a bit narrower than what seems to be most common on the forum.

For the dome, I could cut them in half and have a standard 4.5" dome thickness, or I could cut them across (as most people do, I think) and have 4" thickness. I'm leaning towards the latter, to reduce the total thermal mass of the oven, without sacrificing much by way of dome strength.

For the floor, I can put the bricks flat, but that would make for a comparatively thin floor, or on their sides, which would make for a balanced thickness between floor and dome. Here, too, I am leaning towards the latter; I like the idea of even thickness throughout, and the alternative (2 1/8 inches thick) makes me worry a bit about heat retention in the floor. I know other people have considered a thicker floor and decided against it because it would increase thermal mass too much, but since I'm reducing thermal mass a bit with slightly thinner walls, that should cancel out the increased mass of a thicker floor. Am I overlooking something here?

[rsandler]

Welcome Maurits!

As far as floor insulation, Distribution International also carries a Johns Mansville CalSil product called Thermo-1200, which a number of builders on the forum (myself included) have used under our ovens. Compressive strength is lower (around 100PSI), but the insulating value is better as well (0.344 to 0.647 Btu-in/hr-sq-ft-deg F). As heavy as these ovens are, you're ultimately distributing something like 1000 pounds of dome bricks over something like 140 square inches of dome circumference, so you don't need crazy-high compressive strength. The Thermo-1200 is only rated to 1200F rather than 1800F, but your floor should never get anywhere near 1200F anyway.

As far as foamglass, some builders swear by it, but the Thermo-1200 is billed as being water-resistant, and I can verify from experience that it does seem to meet that claim. Droplets of water spilled from e.g. mortar mixing just sit proud on the surface without soaking in. Combined with a tile separation and weep holes, I would say that's probably sufficient. Can't speak for the water resistance of the Super Firetemp product though.

[david s]

The water resistant cal sil is not available to me, but I’d be using it if it were. How is it for price in comparison to foamglas?

[nlinva]

Thanks for the welcome and the pointer to look at the Thermo-1200! That does look better. I had overlooked it because it does not come up when you search for boards and the image that goes with the product shows half-round pipe coverings. But they do indeed also appear to carry "blocks" of 1.5' x 3'. I have emailed to ask about the cost of various products; forgot to ask about the foamglas, but will also ask about that when they get back to me.

[rsandler]

The other thing I am thinking about is the thickness of my oven floor and dome. The firebricks I was able to get locally are 9 x 4 x 2.125 inches, which is both a bit thinner and a bit narrower than what seems to be most common on the forum.

For the dome, I could cut them in half and have a standard 4.5" dome thickness, or I could cut them across (as most people do, I think) and have 4" thickness. I'm leaning towards the latter, to reduce the total thermal mass of the oven, without sacrificing much by way of dome strength.

For the floor, I can put the bricks flat, but that would make for a comparatively thin floor, or on their sides, which would make for a balanced thickness between floor and dome. Here, too, I am leaning towards the latter; I like the idea of even thickness throughout, and the alternative (2 1/8 inches thick) makes me worry a bit about heat retention in the floor. I know other people have considered a thicker floor and decided against it because it would increase thermal mass too much, but since I'm reducing thermal mass a bit with slightly thinner walls, that should cancel out the increased mass of a thicker floor. Am I overlooking something here?

I wouldn't worry too much about the mass in the walls. The Pompeii plans even say that the 4.5" thick walls are on the thick side of what's recommended. Most cast ovens are 2" or so. My understanding is that we build our domes 4.5" thick because a brick dome needs to be around that thick for stability, and 4.5" specifically only because that's the most common size of firebrick. If your bricks are 4 inches, and you plan to cut them with the original long sides facing in (as tends to work well with a cutting jig), just roll with it.

I would be cautious however about putting the bricks on edge in the floor. Not all thermal mass is created equal. Because heat and flames want to rise, it's a lot harder to get heat into your floor than into the dome. A 4" thick floor is more than 50% more mass than the 2.5" thick most of us use, and I would think will take significantly more fuel to get to pizza temperatures. Unless you plan on baking a lot of bread (multiple batches of 6-8 loaves), you're best off laying them flat. The Forno Bravo cast oven kits include a firebrick floor that is just 2" thick, so I would think your 2.125" bricks will be just fine.

[nlinva]

I don't plan on baking bread quite at that scale (multiple batches of 6-8 loaves); on the other hand, I do like the idea of being able to cook with the retained heat for another day or two. I take the point that a lot of the heat from the fire rises rather than going into the floor, but on the other hand the floor is obviously closer to the fire. I feel like I've seen some posts on the forum suggesting that the floor was too hot, cooking the bottom faster than the top. If that is a common experience, wouldn't it be better, almost, if the floor takes a little longer to warm up?

I suspect you're right that 2.125" is just fine; I just find the idea of even thickness all the way around quite appealing, although not if it systematically costs a lot more time to heat up to pizza temps. The total mass for 4" all around would be similar to the more common 2.5" floor & 4.5" dome most people have; if heating time is linear to the amount of brick mass (is it? I actually have no idea), then the difference between putting the floor bricks flat or on edge would be about 15% in heating time (rough estimate); that doesn't seem terrible to me.

This just made me think of something: I think it is recommended to have a thin space between floor & dome (if dome is not resting on floor), to allow for thermal expansion; does that mean there is effectively a thermal break between the floor and the dome? I was kind of assuming heat would spread fairly evenly through the bricks all the way around, but if there is a thermal break, however small, that might not work as well and it might make more sense to think of floor and dome as two separate things to be heated. I suppose it depends on the relative contributions of conduction, convection, and radiation to spreading the heat around

[nlinva]

This week I'm going to practice with the wet saw (I got a Kobalt one used off Craigslist). If it turns out to be doable to slice a brick in half, I think I will go for a 3.187" floor thickness -- less than if I were to put the bricks on their side, but more than just putting single bricks down flat. I figure that will be a decent compromise between the two.

In the meantime, I've been working on my indispensable tool. I'll see if I can post some pictures tomorrow. The plan is to attach it to 1/2" plywood, with the point of rotation being about 1" above the bottom of the plywood. If I put the IT straight onto the insulation (i.e. without the brick floor), that should automatically make the dome a bit lower than a full hemisphere: a height of maybe about 16-16.5" for the dome. I was thinking of covering the whole floor with jigsawed pieces of plywood, and lifting those out once the dome is complete, replacing them with the firebricks. That way the firebricks don't risk getting chipped or getting mortar dripping down on them. Do other people have experience putting the oven floor in once the dome is complete? Is it hard? I assume that if all the bricks are pre-cut it shouldn't be a big deal, but I might be overlooking something.

[MarkJerling]

I think it's far easier to place the floor, leaving out one brick where the IT is attached, than to build the whole floor in later. I simply covered my floor with cardboard. That protected it from mortar spills and dropped bricks.

[SableSprings]

As Mark noted, the best way to place your IT is to simply set your floor & pull/replace the center brick with a wood block. Much easier to attach the IT at the proper height that way. As for your floor bricks...I do think you are making a lot more work for yourself cutting the cooking floor bricks. I think that a good base (and dome) insulation with a proper door are key factors important for next day cooking. If my oven needs a little bump in temp for the next days bread bake, a minimal "recharge" fire is needed. I'm very happy with my flat brick floor for its heat retention & versatility... and I don't have a great floor insulation base. Just my 2 cents worth...

[nlinva]

I bow to the experts... I think Mark and Mike are right that I'm making more work for myself than necessary. So 2.125" (a single flat brick) it is for the oven floor.

My IT's point of rotation is at 1.5" so if I put it at ground level (taking out one more more bricks to make that possible) that puts the pivot point at 0.625" below the surface. With the IT arm at 18" that puts the dome height at 17.375" above the floor. With a door of 18"x11" that puts the dome height to door height ratio at 63.3% which I'm very happy with.

I attach a pic of the IT. I'm using a tripod ball head and column extender, plus a simple right angle bracket. It's very low-tech, but it seems to do the job nicely. The pictures are still with the original plan of a jigsawed plywood oven floor and putting in the oven floor later; now I'll just use a smaller piece of plywood instead to replace the center brick

[nlinva]

I took some time to figure out the wet saw today and cut the first course of bricks. My goal was to taper them slightly, leaving about a 1/4" opening on the outside (for the mortar joint) when the faces are touching on the inside. I over-tapered a little for an average opening of 3/16". Is that too little for the mortar? I could alternate tapered and untapered bricks to make the outside gap slightly larger.

Speaking of mortar: where do people usually buy the lime and fireclay? And does it matter what type of fireclay? Sand and portland cement are easy enough to find, but the other two are harder.

I also made the inner arch form and worked on figuring out where to place the inner arch so the top-dead-center brick reaches far enough into the dome. Then I did a simple mock-up of where the outer arch/chimney structure will be and how much space I'll have on my front landing. I am planning to do a 1.75" reveal. With about 2" of insulation (on the outer arch; it'll be 4" on the dome) and 2" of outer covering (stucco/brick veneer), I'll have roughly 11.5" on either side of the outer arch. With an outer arch 13.5 inches deep (1 1/2 bricks) I could make the front landing 11.5" too, making it just over 2 feet from the front of the slab to the inner arch. I know most people don't have front landings that large, but I have long arms and I think I'll like having that space to move things in and out of the oven.

(In the picture, the bricks are positioned to show the 11.5" front landing and the outer arch, with outside of the red bricks showing roughly how wide the outer arch structure will be. The plywood cutout shows where the inner arch opening will be.)

[rsandler]

The dome calculator on the forum assumes a 1/8 inch mortar joint, so your 3/16 is surely fine. In practice, I found that on the small end, mortar joints are only limited by the fineness of your sand.

Brick yards generally carry fireclay; that's where I got mine on both builds. Check with wherever you got your firebricks. You can also use the brick dust from cutting your first course of bricks, though I liked making large batches of dry mortar mix in a big plastic bin an scooping a bit at a time, and I was never confident enough that the "cut" clay from the brick saw would be dry enough. I just added an extra scoop of brick dust to each batch of mortar. Brickyards also carry lime, though I found type-S lime at my local big box store too (same aisle as sand, portland cement, concrete etc.).

[mongota]

The firebricks I was able to get locally are 9 x 4 x 2.125 inches...For the dome, I could cut them in half and have a standard 4.5" dome thickness, or I could cut them across (as most people do, I think) and have 4" thickness. I'm leaning towards the latter, to reduce the total thermal mass of the oven, without sacrificing much by way of dome strength.

Your first few courses of dome brick will be half-bricks. As you go higher and the diameter of the rings decrease, most find it easiest to transition to third-bricks. If you plan on the same, then for your first courses cut your dome bricks in half and orient them so the wall is 4" thick, not 4-1/2" thick. When you go to third-bricks you can still maintain that 4" dome thickness.

Floor brick, I recommend laying them on the flat versus on edge, or cutting them.

Retained heat? I have 4" of board under the floor and 4" of blanket over the dome. I can still cook on the third day (with an insulated plug door, or sometimes I may need a small fire to boost the temp up a bit) and on the 4th use the oven as a dryer or dehydrator.

Mortar joints, there's what you plan for and what you actually build to. Your mortar and your ability to handle it will guide you during your build. As my rings got higher I held brick in hand and eyeballed the taper cuts. I'd adjust the tilt of the cutting jig on the wet saw table as needed then cut a couple on the wet saw and dry fit the bricks to see how they looked. I'd then batch cut about half a ring's worth and mortar them up. Never had a problem and the mortar did it's job, which is simply to fill the gaps between the brick. You can use a calculator tor your angles, but always be willing to adjust on the fly. Angles can drift as the build progresses. The build will be a fluid process.

Sometimes silky smooth sailing, sometimes a bit turbulent. But always fluid.

Enjoy the process. The curiosities that now are head-scratchers (mainly arch/dome geometry) will soon make sense and solutions will unfold right before your eyes as the dome goes higher.