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Frenchcanuck 07-18-2007 11:31 AM

Starting dome, have question...
Hello :)
I am finally ready to start the dome (50" diameter). I would like to make sure I do this properly, as it is for our Café. So here come the questions:

1) Does anyone have any pictures (silly question, I know:o) that show clearly how they joined the opening to the dome?? I have seen some pics, and although the plans do describe it, I am still uncertain (and a little nervous) about how to do it.

2) Do I need to make the mortar a little stiff? or will this compromise the strength of the mortar? (3 part sand, 1 part lime, 1 part fireclay, 1 part type "S" concrete??) What would be the best consistency of this mix?

3) Is an arch opening better than an angle iron supported square (rectangular?) opening better? easier? (opening: 20" width x 12" height?)

4) 50" dia. floor , 21"? or 20" dome height?

5) Finally, I will be using styrofoam for the form, what is the easiest way to get the correct curve of the dome? Since I can't just use a compass would I be better to just eyeball it and draw by hand the form pieces?

I think that is all the questions I have right now. Any help would be greatly appreciated.


dmun 07-18-2007 12:19 PM

Re: Starting dome, have question...
I'd suggest building your entry arch and vent first, on the flat, then build your dome up to where it meets the entry walls.

I didn't use that recipe, but I think it calls for portland cement - don't mistake it for type "s" mortar mix.

You always want concrete products mixed with the smallest amount of water you can - wet mortar is weak mortar. Don't forget to moisten your bricks.

Big compass? Use string. Tie a loop at both ends, put one around a fixed nail, and the other around your pencil.

JoeT62 07-18-2007 01:16 PM

Re: Starting dome, have question...
If you're doing a 50" dome with a 20" height, a compass (or dmun's string trick) won't work - both methods generate an arch based on the arc of a circle (you would end up with a 25" height). I'm not sure how you would outline that arch, other than actually taking some bricks and doing a dry run of them on their sides - start with the soldier course and the top of the arch, and connect the dots with bricks. The advantage is you'll be able to plan out exactly how many courses it will take as well.

As for connecting arch & dome, I am struggling with this right now. My angle grinder is getting plenty of use!

DrakeRemoray 07-18-2007 01:20 PM

Re: Starting dome, have question...
As to Arch opening vs Angle support...I think this depends on your plan for venting. I personally think a cast vent/hood has the best chance to capture smoke while an arch with pipe attached may allow more smoke to exit the front of the oven...

Dome height I believe has more to do with door height than anything...what dimension door/entry are you planning?

I do think that Dmuns idea of a pencil and string will work, you just need to use a larger than 50" circle and use an arc of the circle, not a half circle. I used a similar method and mocked it up like this, then I transferred that arc to the styrofoam. (note that I did this mockup before I cut the bricks down to 1/2s)


dmun 07-18-2007 01:49 PM

Re: Starting dome, have question...
Oops, I forgot it's not a semicircle. You can draw an ellipse with a loop of string and two nails, called foci. It took some looking, but here is an elipse calculator. The third example lets you determine the string length and focii spread for any given elipse.

Ellipse Calculator

dave d pizza lover 07-18-2007 06:32 PM

Re: Starting dome, have question...
also having trouble with dome height when i laid out my dome a 42 inch oven with a 20 inch dome it looks so muc lower than the ove in the f b plan at the oven dome page . wondering if im doing something wrong the radius that worked for me was 25.5 inches thanks dave d :)

Dutchoven 07-18-2007 07:27 PM

Re: Starting dome, have question...
Drake is quite on the spot about the dome/door height question. That relationship is extremely important and by all accounts I found during my research the ratio of door height to dome should be about 60 to 63 percent or about 14 to 15 inch high door to the 20 or 21 inch high dome. You could also layout your arch using a circle template for a larger circle than 5o inches, all you would have to do is then measure from your center to the 50 inch diameter and cut "skew" bricks for your soldier course to match the remaing space and "spring" the dome.
Just an extra thought.
All the best!

Frenchcanuck 07-19-2007 05:23 AM

Re: Starting dome, have question...
Thx for all the suggestions...... Wouldn't 60% of 20" be: 12"? and 63% of 20" be 12.6"? Wondering how 14" or 15" came about......
Rick :)

DrakeRemoray 07-19-2007 07:24 AM

Re: Starting dome, have question...
Rick, that seems correct to me. I have a 38" oven, 20" high dome, door is 12" high by 19" wide.


Dutchoven 07-19-2007 04:59 PM

Re: Starting dome, have question...
You are right...bad math...guess the 25 inch number was still sticking in my head and I didn't think twice.
Good catch!

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