Thinking on using a HardieBacker to pour the concrete top. Should I build support?

[Bethencourt]

Hi Everyone!

New to the forum and writing my first post here. I live in Acton Massachusetts and I have the intention to build a 42" this next spring / summer
My intention is to build a corner version and probably I'll connect with a few other elements to form an outdoor kitchen.

I've been reading many posts, I've read the PDF with the instructions, and I've been doing research for a while.
The time to get more serious with it has come, and as many of you, I have a ton of questions.

My first question: I think I want to pour the concrete top using a HardieBacker board, instead of wood. I've seen that people leave a few inches to the edge for the concrete to fill the wholes on the cinderblocks and reinforce the strength of the top. As the board will rest on the cinder blocks on the sides, do I still need to build support with 2x4's towards the center? or would the board be able to hold the weight of the concrete?
I haven't been able to find anything related to it.

Thanks in advance!

[UtahBeehiver]

You will need to support thecenter bottom side of the HardieBacker Board during the pour, why even chance it, if it blows out then the effort is wasted. Just some 2x4 bracing will work. I suggest you look at the Treasure Archive sticky in the Newbie Section, these are some of the more documented builds on the forum.

[Sampatch]

We I built my 42” corner install last fall, it is tied into an outdoor kitchen. A cinderblock base built according to standard FB instruction. Only use cement backer board in outside installation. Fiber Hardie backer board has wood fiber and may not perform well over years. Be sure your first course of cement block is level. I filled every other block column channel with concrete and rebar leaving 12 exposed above the block top; bend them down later to tie into oven floor rebar. Other block columns filled with scrap like empty concrete bags, but leave a couple inches to be filled during over floor pour. I built a deep wood storage area reaching deep toward the outside corner. A lintel was built out of three1/2 cement block with rebar using old school methods. After cure it was installed above wood storage opening to support oven floor. Harder cement board was set in place bringing edges to lap 1” inside perimeter block edge of top row of block. Two simple T shaped braces were installed in area with wide span between main block wall and wood storage area. These two braces were sized and friction fit into space with top of T at height and length to nicely support backer board resting on T tops. These 2 T brace stayed in place and remain there today. T brace top was 3/4 scrap about 4 x 20 attached to a vertical 2x4 with a single screw. Another T brace also placed To support backer board above wood storage area; this one removed after pour plus 7 days. Use alkaline resistant tape on cement board, mortar it down with any mortar you have. Rebar placed on backer board using about 2” elevation above cement board. Tie floor rebar to cement block rebar. Forming and pouring oven support floor to size per plan.

[Chach]

Bottom Line the Hardi Backer is not strong enough on its own to withstand the weight of the concrete during the initial pour. I myself am a bigger fan of Duroc but thats just my preferance. The main reason people use this to support the slab is because they dont have to take it out after the pour its mold resistant and its also non combustible. I say just go with osb and frame it like you would and when your done you can still use the osb for other framing or to make your arch forms. Then your using tje material a few times instead of buying the hardi backer and leaving it in the there. There is no value to having the backer under the slab other than being able to leave it in after the pour. My 2 cents.

Ricky

[Bethencourt]

Thanks everyone for your tips. I will definitely look deeper at the "Treasure Archive"
Getting more excited as I keep learning more.

Has anyone ever used the Brazilian (and other South american countries) method of the broken glass bottles and salt as an extra insulation layer under the oven floor?

Thanks again

[UtahBeehiver]

You see this in old school builds and not thermally effective. You would be better off with newer tech insulations, ie ceramic insulation board and if out of your budget, then use perlite or vermiculite/portland mix for under the oven.

[Bethencourt]

Thanks UtahBeehiver I thought so, but wanted to check.

I was thinking more on an extra layer with it, not to replace the Vermiculite. I definitely want to keep it low and the cost of the ceramic board it's important.

[JRPizza]

I used 1/2" hardibacker and cut 2x4s to run horizontally along the underside of the butted joints, then cut 2x4s to run from the ground to the bottom of those support boards. Had to whack those vertical boards pretty hard to remove them after the slab had cured - that's quite a bit of weight and I'm sure the whole thing would have collapsed if I had not supported it. I went with the cement board as I wanted a "set and forget" approach and didn't want to deal with sealing the joints along the internal periphery if I had used plywood that had to be removed.

https://community.fornobravo.com/forum/pizza-oven-design-and-installation/pompeii-oven-construction/380308-39-inch-corner-build?p=380324#post380324

[Bethencourt]

JRPizza Thanks a lot for your message. that's exactly what I have in mind. Your built it pretty similar to what I was thinking of. I enjoyed very much looking at all the info in your post.
I'm also a Sourdough baker

[SableSprings]

Welcome Bethencourt! As JR noted, the under supports are definitely needed (that concrete is incredibly heavy ) and if you can, make the ground support 2x4s a little short and then put in some easy knock-out wedges/shims under them to bring 'em up to contact with the board. It really helps to remove the 2x4s later cause as JR noted, you'll be amazed at how difficult it is to remove the supports with that solidified concrete slab pressing down from above. I used plywood scraps instead of the concrete board and cut pieces to fit my "gaps" and then lined the form with plastic. The plywood released quite easily from the cured concrete with the plastic liner (although as with JR, I had to use a sledge hammer to get some of the plywood supports out...and beating on support boards while under the supported slab is not my idea of fun ). The plastic left a pretty smooth finish on the concrete and I could easily cut and remove any of it that showed after the forms were removed. I know that often the forms are coated with oil to help the release, but the plastic seemed an easier way to go for me.

You'll enjoy the difference in baking bread in one of these ovens...it's really amazing the oven-spring you get on the loaves. (My sourdough/Levain is named Chef Bill and currently he is our only pet...don't have to walk him at all ).

[Bethencourt]

Thanks SableSprings As I get closer to the actual building, I will decide if to use Cement or plywood. Both seem fairly straight forward.

It's still definitely winter here and I'm only in the preparation / research phase. Snow yesterday, more snow tonight.....
Taking all kinds of notes and ideas.

My initial idea is to build a Gazebo and connect the oven to a brick BBQ / outdoor chimney. Add some concrete countertops and probably build a bar on one of the sides facing the outside.
The I want to finish it all with Slate. Home depot has pretty decent tiles 12x12 for $1.5. I sed them for installing a floor in the mudroom and looks fantastic. My idea is to cut each in three pieces and use them as a veneer. It's starting to look like a huge project for a one man show, but I'll see how it goes.

With the snow, I can't even measure realistically outside.

Would it be a good idea to start with the Slab and the Gazebo first?

[SableSprings]

The slab is going to be mandatory as your first task. I had my slab done by professionals, then built my oven and a temporary winter cover. After winter, we put up the covered den around the oven and "entertainment area". If I had to do it all over again, I'd seriously consider doing the den on/around the slab first and make sure I planned my chimney location and orientation carefully. The main problems I had were 1) accessing and removing the temporary oven enclosure after the final enclosure was built, 2) my chimney stack roof opening was difficult to create because of the joist positions, and 3) I used metal roofing and because my chimney wasn't "square" to the structure, the waterproofing boot around the opening was a real hack job. The other advantage of advanced cover for your build is the ability to work in the rain on the oven.

Definitely think about the size of your gazebo and how many folks you might want to entertain under cover (as well as countertops, bar area, etc.). My slab space seemed huge until the oven and den got put in place, then the added counter space, then the Camp Chef propane cook-top, then the cooling rack, then the table and chairs, ...oh, my ! (and don't forget, you will need to have a pretty good working radius outside the oven opening to swing your loaded peel around without knocking someone's head off... )

[JRPizza]

Like Mike says, knowing exactly where the chimney will poke through your roof is one of the difficult parts of planning an oven inside an enclosure. We built the frame of our structure (not really a enclosure with only 2 sides) and put up temporary cross members to hold the tarp and keep the area relatively dry. Once I got the chimney built and the adapter plate installed I was able to space the cross members to provide the clearance I wanted. I used a plumb bob dropped from above to get the proper spacing for the beams. When it was time to cut the opening in the roof we used a laser pointer held against the inside of the adapter plate and slid it around the periphery while someone marked the projected circle on the tarp above. This gave a surprisingly accurate estimation of where the opening needed to be.

[david s]

Turning attention to forming the actual slab, it is important to try to limit water penetration between the supporting slab and the insulation, particularly if you intend on an igloo design rather than a dog kennel enclosure. This is an important consideration for ovens exposed to the weather. Because most underfloor insulation is so absorbent any water finding its way in there is easily taken up. A layer of foamglass insulation is good here as it is the least absorbent. One way to limit this is to slope the slab slightly away from the centre.so water will not pool in the middle and water running off the dome will travel outwards rather than towards the centre. In addition to assist moisture removal, som holes right through the supporting slabnear the centre will allow moisture a path to exit. This is most easily achieved by inserting some foam plugs during or after the pour which can be easily removed after the slab has set. Alternatively holes can be drilled after the slab has set, but you need then to know where the steel reo is located so you can miss it when drilling.

[Chach]

I used corrugated steel decking I found on a jobsite. It was scrap left over and it is structual as well so I Just put some angle iron around the opening and dropped this in and it will hold a ton of weight and it can be left there in place as it also is a structual part of the slab along with the rebar. Im sure that what I done is way overkill but i was able to do it so I did. Also many dont have access to these types of materials for free but if you see construction sites around you can always ask as there is always cutoffs goimg into the trash. I Also didn't like how deep the wood storage was so i built a wall 32-34" back from the opening as i didn't want to crawl in to get wood and that also helps support the slab but whatever you build to support the slab in that part will be permanent because you will not be able to access the void once poured. Million ways to skin a cat.

Ricky