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Cheesehead 08-12-2010 02:32 PM

Newbie dome question - 2.5" thick instead of 4.5?
OK, I'm in the planning stage for building my dome.

Firebricks are typically 9 x 4.5 x 2.5. The practice seems to be to cut them in half and stack them so that the dome is 4.5" thick.

I'm thinking of putting the bricks in "sideways" so that the dome is 2.5" thick instead. I'm planning on cutting the bricks trapezoidally (is that a word?) so that they fit together with a minimum of mortar (and hopefully a little more strength).

Why 2.5 instead of 4.5? Well,
1) I did some heat transfer calcs and discovered that there's no way the whole brick is getting to the target 850 F. They're actually pretty well insulating if you look up the thermal conductivity.
2) I've got limited space and a significant other who has a dread of large edifices. Especially those put together by her husband. So any space savings is important, if for no other reason than I can tell her that I'm doing my best to keep the size down. :p
3) 2.5" seems to be OK on the floor.
4) From what I can see, the cast refractory ovens aren't 4.5" thick.
5) $$$ - fewer firebricks.
6) I have a crazy compulsion to do things differently.

Some disadvantages that I can see:
1) Lower thermal mass. So it might cool down a little faster. Counter - the heat transfer calcs I did (very rough!) tell me that I should still have a whole lot of heat.
2) Less structural integrity. Counter - I'm hoping that keeping the bricks cut to fit with small mortar joints will help here. Also, the external layer of perlcrete ought to add a little strength - not much, but enough to hold things in place assuming I do a good job with the mortar joints.
3) I have a crazy compulsion to do things differently that gets me in trouble sometimes.

Thoughts anyone? Thanks. Wonderful site BTW.

Nic The Landscaper 08-12-2010 03:14 PM

Re: Newbie dome question - 2.5" thick instead of 4.5?
If you have skill with laying up brick this is completely feasible in my mind also. See dmun's geodesic dome builds for some helpful thoughts on laying up brick in complicated manners.

I cant remember if dmun added much mass to the outside of his dome or not, but one recommendation I have for you is that you parge the outside of your dome with a refractory mortar to add strength and adhesion between bricks. Pearlcrete is not equivalent for this function and has little to no bonding strength.

Also a thought, why dont you cut the bricks into quarters rather than try and calculate and replicate complex angled cuts? The smaller brick segments should keep your mortar joints tighter and theoretically stronger.

Cheesehead 08-12-2010 05:15 PM

Re: Newbie dome question - 2.5" thick instead of 4.5?
I've seen dmun's thread. Pretty impressive.

As for calculating angle cuts... I'm a geek (well, a scientist, same thing :D ) It's part of the fun, I've already got the spreadsheet set up.

Tscarborough 08-12-2010 05:17 PM

Re: Newbie dome question - 2.5" thick instead of 4.5?
I don't think it will be a problem. As for the trapezoids, all that really matters is the joint at the face. Optimize for that.

KraemerBAC 08-18-2010 10:09 PM

Re: Newbie dome question - 2.5" thick instead of 4.5?
Where in Wisconsin and get us some pictures when you get started


Cheesehead 08-19-2010 07:05 AM

Re: Newbie dome question - 2.5" thick instead of 4.5?
I'm in River Falls, really close to Minneapolis/St. Paul. I'd noticed that you were in Beloit. You'll have to tell me how the Packers are doing this year, 'cuz all the newspapers & TV talk about here is the Second Coming of # 4. :rolleyes:

It will be awhile before I get started, this is part of a big patio redo. We had an old stone patio, about 200 square feet, but there wasn't a flagstone that had mortar still attached to it. There was also an old BBQ and outdoor oven, which gave me the inspiration for searching out how to build a new one, and led me here.

We're going to expand the patio to about 350 square feet with about 50 feet of retaining wall AND the oven base. The oven is going to be in the corner, recessed into the slope of the hill.

I'm hoping to get together with the concrete contractor in the next couple of days, we're going dyed & stamped surface. I need to figure out all these little dome details (like the size) now because we need to decide how to lay out the slab - he's doing 12" deep with rebar under the retaining and oven walls for support. The wall & oven base is going to be done by my brother-in-law and me this fall once the slab is done, dry-stacking CMU with the cores poured then veneered with stone pulled up from the old patio and BBQ.

Yesterday, I installed about 3/4 ton of railroad ties as a 2' tall by 30' long retaining wall on the downhill side of the patio. I'm hoping to get the slab poured by mid-September and have the walls done (including the oven base, probably not the hearth) before the snow flies.

The oven will be next year's project, with an absolutely uncompromising finish date of June, 2012 - my daughter's graduation. Not that I need to have the oven for that, but the SO has declared that the backyard needs to be neat and trim, with nice grass and without piles of dirt, stacked rock or CMU. :p

Cheesehead 08-19-2010 11:13 AM

Re: Newbie dome question - 2.5" thick instead of 4.5?
1 Attachment(s)
OK, let's try an attachment with a photo. This is the site where it's all gonna happen. The patio's going back about 15 feet past the current hole, following along the line of the gash on the left to about 5 feet to the right of the railroad tie retaining wall by the garage. The back corner (right) has lilacs planted and we're going to plant hedge roses between the garage and the patio.

The oven will be in the back of the flower pot, seen in the far left corner of the pit.

Edit - Yay it worked! My compliments to Forno Bravo and their software. This was the most painless photo post I've ever had in a forum.

dmun 08-19-2010 02:31 PM

Re: Newbie dome question - 2.5" thick instead of 4.5?

2) Less structural integrity. Counter - I'm hoping that keeping the bricks cut to fit with small mortar joints will help here.
This is absolutely correct. A thinner dome is a weaker dome, and mine is full of cracks despite using heatstop mortar, and having closely fitted joints.

If for space reasons you must have a thin dome, you should spring for a modular oven. I don't find any real advantage in heating time in my oven.

If I were doing it again, I'd use the 4.5 inch thickness, and the homebrew mortar, which seems much less prone to cracking.

luca 08-21-2010 11:01 AM

Re: Newbie dome question - 2.5" thick instead of 4.5?
I have a similar question but this is about the base. I'm in VT and can get granite dirt cheap so I'm planning to use a 2.5 thick inch thick granite slab for my base. I will then put a 2 inch layer of Vermiculite/concrete before I start laying the firebricks. The question is, is this enough thermal mass for the base? (I've noticed that nearly all threads discuss a 4-6" poured concrete slab for the base, but then they go on to insulate with Vermicrete before laying the fire bricks, so what is the point of such a massive slab?!) Any suggestions/comments would be greatly appreciated!

Tscarborough 08-21-2010 12:01 PM

Re: Newbie dome question - 2.5" thick instead of 4.5?
The 4" concrete slab is not a heating mass, it is simply there for support. 4" is the minimum thickness for normal reinforced concrete. You can go much thinner with engineered forms of concrete, but for poured in place 3500 PSI concrete, 4" is what it needs to be. You will be fine with your granite support slab, provided it can handle the loads (which it most likely can), but you probably need more insulation under your firebricks, in the 4" range for vermicrete or perlcrete.

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