31"/800mm in Eindhoven, the Netherlands: Design critique and build topic

[Toiletman]

We just came back from holiday, so it's time to start work on the chimney. I remember that we were having a conversation about vents or weepholes for the insulation blanket? It seems something weird has happened on the forum with a kick of 'Dogshark58 Build' getting some of these replies? Also it seems my questions about buttressing the side thrust of the chimney arch disappeared.

Anyway, I think it was being recommended to have the mosaic tiles under the calSil, and to use separate weep holes under the insulation blanket. I get the addition of weep holes under the insulation: to provide a path for any steam or recondensed steam to go. The additional argument was that the steam does not automatically go up to a vent on the top, as that's also where it's hottest and maybe the highest steam pressure is present.
What I don't get is why we wouldn't recommend to also put the mosaic tiles under the insulation blanked at (a large part of) the vermicrete? As both the blanket and the vermicrete would wick up water like crazy, it would be beneficial to 'lift them off the ground' away from any puddles, by using the mosaic tiles, like we do for the CalSil board right? By basically extending the mosaic tiles from under the tiles up to the last half inch/cm of the plaster this would also create a path for water/steam to get to the weep holes that are under the brick dome, instead of driving the necessity for extra weep holes in the insulation layer? The main function would be especially to prevent rain ingress in this case, not so much the one of major steam load of drying the brick dome.

Any comments?

Then for the sidethrust of the chimney: I did a small calculation based on the expected loads and the angle of the sidewall supports as they're currently designed. If I would consider the side walls as freestanding (ie. not attached to the floor) they would topple over. In reality they will be cemented to the floor, but as these are free floating bricks that would not significantly increase their footprint. In reality the whole wall will also be embedded in the vermicrete/plaster dome afterwards, but I don't think I should count on this providing structural support right? (+ it would mean I would have to support it in another way until the dome is in place).
To ensure the side walls do not topple over I came up with a buttress design as depicted below. This is only attached to the back of the sidewall, but I would imagine that the mortared bricks are more than strong/stiff enough to completely offload the sidethrust assymmetrically like that.

Any comments on this? I've seen a lot of people use 'full' arches, where there's basically no side thrust left anymore.

[david s]

Because the top of the dome gets hot first, the moisture present there is also driven out first.,as moisture will always travel away from the heat source. For this reason, the direction of moisture travel is towards the base of the dome, so any venting is better placed at the bottom. This situation can also be assisted by steam pressure build up in the upper part of the insulation space. The accompanying drawing explains this process.
Alternatively if you are still keen to provide a vent in the upper section of the insulation space, some holes made in the base of the flue pipe that sits in the insulation space, can allow moisture to escape into the flue pipe and exit there (second drawing)
Either of these solutions remove an exposed breather at the top of the dome's exterior.

[UtahBeehiver]

The vent on the dome is just not for drying ceramic/pcrete on dome. Water has a way if getting in the ovens especially those oven open to the elements. So if the blanket or the pcrete gets wet on the dome and you heat the dome up during cooking, physical water when it sublimates to vapor, increases in volume by1500-1600 times which can increase the pressure between the dome and the outer stucco layer. Unless there is a way to relieve this potential pressure their is the risk that this internal pressure could crack the final or outside render. A vent provides pressure relief.

[Toiletman]

The mosaic tiles are only intended for underneath the oven, separating the concrete top slab from the bottom insulation (board or insulated concrete).

Thanks for your clarification SableSprings, I do see that the mosaic tiles are mostly put under insulation boards. My understanding is that it is to prevent any direct wetting of them: the water can go around the mosaic tiles to the weep holes. However, I don't fully get the rationale for doing it only there? As all insulation materials would love to wick up that water, also the blankets and vermicrete layers would soak it up very fast right? Or is your explanation that they dry out much faster due to the vent port? Would there be any downside to extending the tiles under the blankets and vermicrete?

[SableSprings]

The mosaic tiles are only intended for underneath the oven, separating the concrete top slab from the bottom insulation (board or insulated concrete). Most builders consider adding a small vent for the upper dome that leads into the chimney area or is placed near the top of the oven giving any steam/moisture an escape route. This vent doesn't need to be large, just so it penetrates any outside shell of the oven and doesn't allow water a path in. In the US, there are a variety of small vented plugs used in the automobile industry called Breather Caps that work well for our oven builds. You can also make one out of some piping connections to simply provide a path out, but not in (upside down "U" shape with a longer end reaching into the insulation layer).

[Toiletman]

Two more questions that came up during preparations for the work after:

• Under which layers should I put the mosaic tiles? as they are there to protect 'wicking' insulation materials from water, I'm assuming they should also be under the fire blankets and possibly also the vermiculite stucco?
• I did some calculations on the side thrust of the chimney, and were the side walls freestanding, they would definitely topple. However, they're partly embedded in the stucco, and will also be attached to the stone floor (which consists of loose stones basically). I'll probably put in a buttress, as depicted in the picture below, but would welcome any comments/remarks about people's experience with similar arches.

Ie. with some of the casted chimneys I would imagine that they also take up the side thrust, that might be a solution I could also consider. The buttress I'm currently planning is also only supporting the side walls on the 'back'. I'm assuming that the walls are sufficiently short/stiff that this will basically support the whole sidewall, but also here: any experience/knowledge is welcome!

[Toiletman]

Thanks for your comments Giovanni Rossi and UtahBeehiver. Based on your info I also went for the Dremel and homebrew design. I chose the 'life, the universe and everything' theme, which also mixes well with my age, and with a little play, the year of fabrication and the number of our address.

Probably some final touches to the dome on wednesday: pointing work on the inside, the mandatory pizza oven eats man picture, and probably hand prints on the top. Then it's time for holiday, and when we come back I'll start work on the chimney. Happy with the result of the work so far, and already wondering about lighting it up.... although that's still quite some time away. Thanks for all the priceless advice, tooling, support and information here, making this oven would have otherwise really been impossible.

[UtahBeehiver]

I just used my homebrew mortar like GR did as well as a dremel tor the design. He is right, no one but you and the forum will know it is there but then again it is cool to do and you only get a chance one time. That said, I have also seen some nice placks made and place on the outside of the oven.

[Giovanni Rossi]

Looking good! you must be getting excited to drop in that keystone!

My mortar was dark enough to contrast with the lighter brick, so I just used that to fill in the design I carved with a Dremel.

I had to mortar 2 full bricks together to make my keystone. I used the time while the clamps were on and the mortar was curing to carve and work some mortar into the design. The next morning I double checked that it was holding well. I check it now and again and so far none has landed in a pizza.

I was inspired by RusselI's (UtahBeehiver ) cool design. I didn't spend a lot of time on mine because the artist in me has yet to emerge.

Unless you're going to take a pic and display it next to the oven, few are going to see your efforts; but, I admit to enjoying that I know it's there.

[Toiletman]

After a week of a lot of bad luck (dead car battery, broken bike gear cable, flat tire) it was finally possible to get some bricks in...

Two rows of quarter bricks. The issue of them being too wide didn't really lead to any problems... just that I had quite a few too many. The anchor brick for the last row is now in, so hopefully tomorrow I'll finish that and then it's time for the keystone.

I've seen some really nice designs with prints and engravings on the keystones. I was wondering: what kind of material could I use to color the mortar such that it stays in place and also keeps it contrast?

[rsandler]

Wow, that's pretty quick. I had to gently tap my keystone in place, so I didn't want to risk it. However, I pulled it out today, and indeed quite a bit of mortar leakage. However, one day later it was still easily cleanable so it turned out quite nice. The side supports are a lot worse, I've probably gotta clean them with a power tool... or try to sand them or sth... any tips on cleaning dried up mortar?

Wire brush and white vinegar worked pretty well for me to clean up excess mortar; for bigger squeezed out bits, just aggressively scraping with a trowel can often do the trick.

[Toiletman]

Thanks for your comment UtahBeehiver !

Latest status update: The halfbrick courses are all in and I've been hard at work cutting the quarter bricks. In doing that I've run into a little issue. Actually I'm not sure if it's really a problem, but I'd at least like to share it such that other people don't make the same mistake.
As probably everyone has, I've been using the great calculation sheet by deejayoh. While cutting the bricks I assumed that I had to spread the cuts equally across the full brick. However, that's not really the case: the sheet by default calculates the longest side of the cut brick to be a half or quarterbrick size. As a cutting jig I'm using Chipster's design, flipping the brick after each cut. This results in a long and a short side sharing the full length of the brick --> instead of 'two long sides' as the calculator assumes by default.
This difference is quite small at small angles (lower in the dome), but gets larger and larger the higher up you come. For the half bricks it did not cause any realizable problems, but when cutting the final quarter brick course I was struggling to get the spacing correct, hence I dove in. It now seems that the bricks for course 10 I cut are basically 5mm / 1/8" too long. I've still got to put course 9 in, so I haven't been able to test yet to see if it would really bother me. In the end it would result in more mortar usage, which shouldn't really be a problem. Luckily course 11 hasn't been cut yet, so for that one I can correct.

Lessons learned:

• Check the calculator thread more carefully, deejayoh mentions the possibility to adjust the brick width there
• 'Quarter bricks' are really only 'quarter bricks' for course 9, afterwards they become smaller.

Find below some pictures of the current status and also illustration of how course 10 and 11 bricks fit inside a full brick.

Did you guys run into the same issue? Or did you correct for it beforehand in the calculator? I might've just missed this in other posts as the focus while looking through these threads was on understanding the generic concept and creating the cutting jig.

[UtahBeehiver]

Nice use of laser level. It is common to see the dreaded droop after clearing the inner arch with your dome courses. Looks like you are right on par.

[Toiletman]

Another small update: I got the first 'full round' layer in... it's also the penultimate halfbrick layer. One more to go, which is already cut so hopefully I can put it in tomorrow. After weeks of rain, flooding and cold, now we have a heatwave.. so probably working in very small batches and getting things very wet again. After that it's time to modify the IT to fit the small bricks and see how that goes.

After the large tolerances on the firebricks I was worried about the build going out of whack. Luckily the laser proved that it's worked itself out correctly. there's max 5mm / 1/8" difference in the extremes.

[Toiletman]

Hey Toiletman the dome arch I was not very worried about it falling so I think I mat have only waited 45 minutes to maybe 1.5 hrs. It was not very long at all. I was worried I had a bunch of mortar to clean up so once it was solid enough that it was no longer squishier I pulled the form and cleaned up. I had to pound my key stone into place with a deadblow mallet so I knew everything was tight so just made sure nothing was wiggle and went for it. The front arch I waited a bit longer but a big part of that was waiting for a friend to get there to help me lift the vent into place . I had just done the vent arch and then vent like 2hr later so was probably 3 to 4 hours after I had set the arch in place I pulled the form. It was also a tight fit so I was not worried about it moving. I was more worried I would hit it trying to lift the vent into place and knock something loose. This was my experience your mileage may very. I would say do what is comfortable for you. But is should hold just fine in a few hours. The dome braces the walls for you.

Randy

Wow, that's pretty quick. I had to gently tap my keystone in place, so I didn't want to risk it. However, I pulled it out today, and indeed quite a bit of mortar leakage. However, one day later it was still easily cleanable so it turned out quite nice. The side supports are a lot worse, I've probably gotta clean them with a power tool... or try to sand them or sth... any tips on cleaning dried up mortar?