Floating slab or not?

[Eastside3]

Well after a year of seeing that wooden crate of my Premio 2G 110 sitting in the garage, I had begun work, got the patio poured, and have a very robust structural slab that Im building everything on. I decided to build the base structure wider and longer than is typical, and had added additional support with another row of CMU blocks in the center. Not only will this provide structural support beneath the hearth slab, but it breaks up the space and allows me to have 2 separate compartments. The main compartment will be for wood storage and the back side will eventually have a door put on it where it will house the many tools, peels, ash bucket, etc... The other reason I am making the base wider is that I wanted to have ample space to allow for additional insulation yet still have an air gap between the oven and the walls that I will construct later in the construction phase.

Ive added a beam bond layer on the 3rd layer, and the cores are filled with concrete. I drilled into the slab below and hammered in the 1/2 rebar. Much of what I did was a combination of what Ive seen done by others, what makes sense, and what a couple of contractors suggested. Id rather overbuild than not!

I was all set to frame the base, lay wonderboard in the center openings, set my supports, add the rebar, and pour the hearth slab. My intention was to have a 4" slab poured so that it also fills in the voids of the top row of CMU blocks. It makes sense that this would anchor the hearth slab and that structurally everything would be much more sturdy. .... Then the head scratching began. A local Mason told me to be careful and suggested AGAINST having the hearth slab extend into the block voids. He said that despite the layer of insulation that the oven will sit on, that the concrete slab will get hot enough to cause the concrete to expand. He says that the expansion would more than likely crack the blocks. He recommended a floating slab, where concrete board could lay over the entire base (not just the interior), laying directly on top of the blocks. This would mean that the concrete wonderboard would not be anchored to anything, and rather it would freely lay on top of the CMU walls.

Has anyone had any issues with expansion and shrinkage in your hearth slab? How did you construct, frame, and pour your hearth slab, and what would you do differently if you had it to do over again?

I figured it made best sense to pause before going forward, and post this question.

[boerwarrior]

Re: Floating slab or not?

Your Mason is incorrect. If you use the right quality and thickness insulation you will be able to put your hand up against the outside of the insulation even when the firebrick is at 1,000 degrees.

So you do not need any special consideration for the hearth concrete. Just make sure it is strong enough to take the weight.

Looking good so far!

[NCMan]

Re: Floating slab or not?

Your mason is partly correct and partly incorrect and knows what he's talking about.....somewhat. As for it getting hot, and it causing problems, w/proper insulation, quite unlikely. But, a floating slab is the best way to go. It's called a bond break and allows for the slab to contract and expand more freely. Is it absolutely necessary? Probably not. But....he is technically correct as far as a floating slab goes. You should be looking into vapor/moisture barriers between each layer, also. My 2?.

[Eastside3]

Re: Floating slab or not?

Ive read books where those building ovens for bread baking will build in Ash chutes. Naturally any break up of contact from one object to another creates a break, so I can see where an ash dump helps. I plan on just creating a 1/4 inch gap between my hearth stone and the landing material (not sure what ill use yet). Ill let the ash that will accumulate fill in the gap. Its not a big gap, but it does break up the direct contact.

I see that the instructions given through FB seem to suggest filling the voids with scrap (they say to use your empty bags) and then fill the corners, or every other cell, with concrete. nowhere do they say to NOT fill this in at the time of pouring the hearth slab. I was close to doing just that. I framed in the openings of my two compartments and was going to go buy tome 1/2 inch concrete board. Then I was going to frame everything and pour concrete in, filling in the entire surface as well as filling in those empty cells. From a structural standpoint it seems like a very sound way to go, and it was exactly what I planned to do. Then again, I say that when Im taking into consideration structural integrity, shifting, and overall sturdyness. Thats to say, thats a logical approach if I didnt take into consideration the reaction that Concrete has when exposed to high heat. I do go back to FB again and wonder why this isnt stated as being a potential issue to be careful of. Does their insulation layers do that good of a job at preventing heat from transfering to the concrete slab?

When I look at the Alan Scott design, his floating slab is suspended simply by the rebar that sits atop the frame. Someone else brought up the fact that the rebar is subject to weakening and rusting over time. I thought that was a solid point as well.

So now im completely perplexed. The concrete hearth is the foundation that the entire upper wall and roof structure would be built on. If this slab is going to expand such that it would break things then I shouldnt build the walls on top of it. It would seem to me that I should do the following. Follow my logic and by ALL means chime in with your thoughts. If im going overboard, forgetting something, or just plane crazy, I value your contributions and thoughts.


What I WAS going to do:
Frame the inner voids that are going to be the ceiling of the wood storage compartments. Build a supported wood frame and make sure that the concrete backer board is sitting flush with the top of the surrounding cinder blocks. then frame the perimeter of the structure, pour concrete to fill in the entire surface, and I will be left with a structural hearth base that is also filling in the open cells of the top row of blocks.

My thought is to now do this:
Fill the blocks with concrete so that the perimeter CMU blocks are strong and solid. Then after creating a wooden support structure over the wood storage compartments, lay concrete backer board over EVERYTHING. This way the concrete filled CMU blocks will have backer board on top of them followed by the concrete hearth slab. The hearth slab can freely expand and contract as it sits atop concrete backer board. While this seems great for the slab to move, I have NO idea now how im going to build the upper walls when they were always going to be a gable style roof built directly off the concrete slab per the FB instructions.

My other thought, and by far the simplest, is to beef up the 3" of insulation board that comes with the kit, and add more ceramic insulation sheet(s). I could go with 4-5 inches total of insulation thus mitigating the heat transfer to the concrete slab beneath it. it makes sense that there has to be a certain degree of heat that is reached before concrete expands. It also makes sense that building a 5" inch slab instead of a 3 1/2" slab would create greater mass to absorb the heat before it gets to a point where it will expand. so in essence, add more floor insulation and make it a thicker slab as well. I DONT want to build walls and put expensive stone vaneer on walls that will shift and crack.

Im not hearing others report problems they are having with their bases being broken. Is that to say that the insulation panels provided actually do a fantastic job at preventing the concrete from actually getting hot to begin with? Somewhere in here there is a simple answer, I just need to find it.

[Tscarborough]

Re: Floating slab or not?

Don't think so hard.

[Eastside3]

Re: Floating slab or not?

NCman, vapor barriers between what exactly? Do tell.

TB, yeah I knew I was getting into the "Over thinking" phase.

To remedy my concerns im going to get more insulation beyond what the kit comes with. Any suggested material? I dont see any place on FBs site where one can just order another sheet of insulated board.

[NCMan]

Re: Floating slab or not?

NCman, vapor barriers between what exactly? Do tell.

TB, yeah I knew I was getting into the "Over thinking" phase.

To remedy my concerns im going to get more insulation beyond what the kit comes with. Any suggested material? I dont see any place on FBs site where one can just order another sheet of insulated board.

A vapor barrier under your hearth slab also acts as a bond break. But, if the slab is tied in w/rebar into the wall, it's only good for a partial vapor barrier. Then, after pouring your slab, you can place another one between the slab and your insulation layer, if you use a wet mix. This is all "best practice" and I recommend doing it. It's up to you to decide.

[UtahBeehiver]

Re: Floating slab or not?

Any refractory supplier carries CaSi board. Thermo Gold 12 is an example.

[Wozza]

Re: Floating slab or not?

So now imMy thought is to now do this:
Fill the blocks with concrete so that the perimeter CMU blocks are strong and solid. Then after creating a wooden support structure over the wood storage compartments, lay concrete backer board over EVERYTHING. This way the concrete filled CMU blocks will have backer board on top of them followed by the concrete hearth slab. The hearth slab can freely expand and contract as it sits atop concrete backer board.

Eastside, how did you get on with this? I was thinking of doing the same, but more to be able to include vapour barrier between the blockwork walls and the hearth. As NCMan says...

A vapor barrier under your hearth slab also acts as a bond break. But, if the slab is tied in w/rebar into the wall, it's only good for a partial vapor barrier. Then, after pouring your slab, you can place another one between the slab and your insulation layer, if you use a wet mix. This is all "best practice" and I recommend doing it. It's up to you to decide.

I hadn't realised until reading this thread that the concrete for the hearth was meant to also fill the holes in the blockwork. I'm building the oven against the back wall of my garden and local rules means I have to leave a 450mm gap between the wall and the oven. I was going to back fill this gap with soil up to just below the level of the hearth (plus a little around the sides).

Now I'm concerned about water wicking up to the oven itself, so I thought that Eastsides method of casting a 'floating' slab, plus the inclusion of a vapour barrier would solve my problem.

Anyone see a problem with it? And would the vapour barrier just need to sit on top of the blockwork wall and then lay the concrete sheet onto that and pour the concrete directly onto the sheet? Or would it be best to lay the concrete sheet directly onto the blockwork wall and put a vapour barrier between that and the poured concrete?

I'll still pour concrete in every other hole in the blocks (with a reo bar), but it will be physically separate from the hearth.

And as I have a hell of a lot of dirt to get rid of, I was going to fill the remainder of the block holes with soil. Less to manhandle to the front of the house into the skip. Is that a dumb thing to do?

[david s]

Re: Floating slab or not?

If the oven is adequately insulated then the supporting slab will not get hot. Even when the oven has been cranking away solidly all night the supporting slab should only feel warm to the touch from underneath. Concrete sitting in full sun gets way hotter than that. So I don't think you have anything to worry about.

[NCMan]

Re: Floating slab or not?

Eastside, how did you get on with this? I was thinking of doing the same, but more to be able to include vapour barrier between the blockwork walls and the hearth. As NCMan says...



I hadn't realised until reading this thread that the concrete for the hearth was meant to also fill the holes in the blockwork. I'm building the oven against the back wall of my garden and local rules means I have to leave a 450mm gap between the wall and the oven. I was going to back fill this gap with soil up to just below the level of the hearth (plus a little around the sides).

Now I'm concerned about water wicking up to the oven itself, so I thought that Eastsides method of casting a 'floating' slab, plus the inclusion of a vapour barrier would solve my problem.

Anyone see a problem with it? And would the vapour barrier just need to sit on top of the blockwork wall and then lay the concrete sheet onto that and pour the concrete directly onto the sheet? Or would it be best to lay the concrete sheet directly onto the blockwork wall and put a vapour barrier between that and the poured concrete?

I'll still pour concrete in every other hole in the blocks (with a reo bar), but it will be physically separate from the hearth.

And as I have a hell of a lot of dirt to get rid of, I was going to fill the remainder of the block holes with soil. Less to manhandle to the front of the house into the skip. Is that a dumb thing to do?

A floating slab is the best way to go. You can fill the holes in the block w/dirt, beer cans or whatever is handy. That's where all my empty concrete mix bags are, along w/a time capsule inside an otherwise empty Gatorade bottle. Perhaps a beer bottle or three, too. It makes no difference. If you dry stacked your block, it's best to fill the corners and a few others w/concrete for strength. The weight of the slab rests on the top of the block, not what's in it. Just remember to place a layer of plastic between each layer for both a vapor barrier and a bond break and be sure it extends all the way to all edges for a total separation of the slab and wall.

[stonecutter]

Re: Floating slab or not?

"......You can fill the holes in the block w/dirt, beer cans or whatever is handy. That's where all my empty concrete mix bags are. Perhaps a beer bottle or three, too.

You really are a mason aren't you!

[NCMan]

Re: Floating slab or not?

You really are a mason aren't you!

Hey....I'm just thinking it keeps the jobsite clean and safe. Safety first!!

[Wozza]

Re: Floating slab or not?

A floating slab is the best way to go. You can fill the holes in the block w/dirt, beer cans or whatever is handy.

Floating slab it is, then.

From your description it sounds like you've put a plastic vapour barrier between each course of blocks. Maybe I've read that wrong as I would assume that the concrete fill would break that barrier. And on the back of my blockwork support walls I'll be backfilling with soil just about up to the hearth, so it's a vapour barrier between the top of the support wall and the hearth I was concerned about.

Would you put the vapour barrier between the top of the support walls and the concrete sheet, then pour the hearth slab on top of that, or would you sit the concrete sheet directly onto the support walls, put a vapour barrier over that and then pour the hearth?

And I like the idea of the time capsule... 'Dear great great great grandson. Sorry about the global warming but I hope the pizzas are still tasting great'.

[NCMan]

Re: Floating slab or not?

I placed a vapor barrier between the hearth slab and the insulation layer. If you check out my build, it may be easier to understand. I also placed one under my hearth slab pour and under the base slab. Since you are backfilling the back that high, you should address that, I agree. The more you do to keep moisture away from your actual oven, the better, whether it's groundwater or rain. And water can wick up into concrete and block quite easily, as well as through layers w/out vapor barriers.