Using Existing BBQ structure for foundation.

[Bryan1076]

Hello everyone, Please see the picture attached. When we bought this home 2 years ago the prior owner had build a "BBQ" area to slide grill into. Turns out it is Cinder block filled with concrete, rebar reinforced, and the slab appears to be almost 5" underneath. From the very beginning I have been thinking it the perfect location for my oven. My QUESTION: The dimensions are a little small. Not including the veneer rock, the cinder block portion appears to be about 57" wide and maybe 44" deep. (I removed one of the Cap veneer pieces in the photo so I could determine size. Can I build a platform that spans outside of the foundation walls, so I can accommodate a 36" oven, or maybe even 42"? I was thinking of using steal 2X4's in an "X" formation, resting upon the existing foundation. Then put hardy board on top of the frame and poor the hearth on that. Also, the other challenge is that the existing structure slopes about 1/4" with slab. I would need to put shims under the steel 2x4 structure to make sure all is level. See picture. I would love to hear the pro's and con's of this idea. Thank you! See picture attached.
Bryan

[UtahBeehiver]

Not sure you can even get a 32" ID oven in a 44" deep section when you add OD brick wall, vent chamber, outer arch, landing. You may have to cantilever out the front and back. I am not sure steel studs can handle the weight of an oven unless you are doing structural steel.

[Gulf]

+1 on Russell's post. Forget the steel. Lay more block in line with the existing blocks until you get up to the comfortable hearth height that you will need. Then, cantilever to fit the size oven that you want. Cantilevering above the block wall gives you two options. You can keep the 90 degree corners for a doggie house style or you can round the corners for a custom fit, if you are going with an igloo style.

[SableSprings]

As both Russell and Joe noted (and you thought and I agree ), the existing base stand will need to support a top slab with considerably larger depth and width dimensions for even a 32" oven. Just as an FYI, some basic ballpark dimensions for a small brick build are:

A 32" ID oven requires about 49-50" final width (32+(4.5x2)+(3x2)+(1x2) ID+walls+insulation+render) and depending on your landing/flue arch, somewhere in the 62-65" range total depth (IMHO).

And that's if you are using ceramic batting for the cover insulation rather than perlcrete or vermicrete. If you did a cast version with 2" walls, it would reduce those dimensions by about 5" or so. Look at the dimensions of the Forno Bravo assembled residential (cast, modular) ovens (or the offered base stands) to give you some good size information. You might even decide that getting something like a Casa2G90 modular kit might be a good option/value for you.

Once you've determined how big you need the top slab to be, I'd recommend giving yourself an additional 6"-12" on the sides and back. It's nice to have something to stand on while you're building and great to have a place for setting your adult beverage down while you're working the pizza . Your existing foundation's 5" depth is plenty good and the 1/4" slope (pretty standard here in Oregon) provides good water drainage off the slab. Easy to just make an adjustment to the top slab forms to make it level. However, if you can, giving the top slab a very slight doming will allow water to drain away from the oven base (and insulation) instead of pooling on top. David S on the forum advocates drilling a few small holes through the top slab under the oven to give any wayward water a drainage option.

Pouring a top slab that gives you the width and depth needed will require some solid cantilevered form work around the perimeter. I used plywood covered with plastic under the open and extended areas of my stand/top slab but wish I would have just used cement board so I could have left it in after the pour. Either plywood or cement board will sag with the weight of the wet cement so don't forget to put temporary supports under those areas...and put some shims in place so you can get the support props out (without a saw ) after the concrete sets. I totally agree with both Russell and Joe that the idea of steel underneath isn't really a viable option...a good rebar mesh in a 4" thick top slab will give you all the strength you need to support the oven.

Looking forward to your build! Keep us posted and take plenty of pictures.

[Bryan1076]

Thank you Mike, Russel and Joe. So with what Joe said, I am not sure I can lay more block because the current structure is basically cut into a raised yard. The pole in the background is important, and I am concerned with digging down to pour more slab and lay brick. I have a sail structure over head that I want to keep and that pole is part of it. Anyway, I know I will need to cantilever front and back. Max depth is likely shaping up to be about 65." This will allow 1" space between the pole you see in the picture and the back of the slab, and extend about 8" beyond the front of the structure. With curved front edge slab, it should be ok without sticking too far out into my patio. Leaving space on the sides is going to be tricky but I see the point about needing a place to stand and a great place for my beer! My thoughts are to go with igloo style to minimize overall size and use 3' of ceramic blanket covered with mesh, fireclay, stucco or smooth finish. I want to keep it fairly simple visually. Also...I have another idea which I am going to share in the next comment. Thanks!

[UtahBeehiver]

You mentioned a sail structure from the pole. Will the venting from the WFO be an issue. There will be smoke and heat above the chimney vent.

[Gulf]

Sorry Bryan,

I wasn't very clear. Just from the pic, I assume that you stand is a little low for a WFO. Looks can be deceiving. It may not be. I was meaning, that if you needed more height, lay more block directly on top of the blocks of the original construction.