Alternative to suspended slab

[Tikibrewer]

I'll be pouring my foundation slab this week and am in the planning stages for the rest of the oven. I'm not too keen to deal with forming, setting the steel and pouring a suspended concrete slab for the hearth so I'm looking for alternatives. My precast oven will weigh about 1600# finished and the mfg states this model can be placed on a welded steel stand with 3/4" cement board as the base for the hearth. I'm building an 8" CMU stand and was hoping to use two layers of cement board (3/4" total) instead of a suspended slab. The cement board would be supported along the edges by the CMU stand and in the middle with either angle iron or steel framing. The hearth is a 2" CaSil board layer under the fire brick floor. Any thoughts?

[MarkJerling]

I'd pour a reinforced slab, but that's me. The cement board will have negligible strength so it all depends on the steel supports under.

[fox]

I would also make a form and fill with vibrated concrete, it is the best way.

[david s]

The strength of a concrete slab is proportional to the square of its thickness. This means a 2” thick slab will only be 1/4 the strength of a 4” one. The strength and therefore the thickness can be reduced if the span between the upright supports is reduced. This can be achieved by cantilevering.

[Tikibrewer]

Here's the progress so far on my Melbourne Fire Brick Co. P85 precast oven. Thanks for the suggestions above but I wasn't sure how I was going to be able to form the internal areas for a suspended slab so I ended up going with 2 layers of 1/2" Dura Roc. In retrospect I would have omitted the 4" CMU layer, used the Dura Roc as I did and poured a 3" slab on top of that. Either way this was easy, cheap and seems pretty bomb proof as is

[fox]

Than is an unusual way to go, it should be OK as you are not spanning any large spaces but you dont seem to be using much insulation under the floor bricks?

[Tikibrewer]

Than is an unusual way to go, it should be OK as you are not spanning any large spaces but you dont seem to be using much insulation under the floor bricks?

This is the design of the kit I purchased. They've been in business for over 10 years so I'm trusting the 2" cal sil insulation is enough.

[david s]

I use a thin precast supporting slab (two man lift) in my build and get away with it because it is cantilevered over the supporting piers. This reduces the span and eliminates the need to create formwork and the mixing and placement of a fair amount of concrete.

[fox]

This is the design of the kit I purchased. They've been in business for over 10 years so I'm trusting the 2" cal sil insulation is enough.

Yes it will be fine but is about the minimum recommended amount. Of course it is all relative to what you require from your oven.
Where will the dome sit with your design, on top of the floor or outside of the floor?

[Giovanni Rossi]

The 2" of CalSil board will be fine. These ovens are designed for only about 8 hours of residual heat. I'll be working with their D105 fire brick kit. It has more refractory and an underfloor heat bank and is said to have 72 hours residual heat though still having only 2" of CalSil. I am planning to and would suggest that you add 2" of Foamglas under the CalSil to address some of potential moisture infiltration from the slab that you read so much about on this forum. It may also give you a bit more heat retention.

[Tikibrewer]

Yes it will be fine but is about the minimum recommended amount. Of course it is all relative to what you require from your oven.
Where will the dome sit with your design, on top of the floor or outside of the floor?

The dome sits on top of the floor

[Tikibrewer]

The 2" of CalSil board will be fine. These ovens are designed for only about 8 hours of residual heat. I'll be working with their D105 fire brick kit. It has more refractory and an underfloor heat bank and is said to have 72 hours residual heat though still having only 2" of CalSil. I am planning to and would suggest that you add 2" of Foamglas under the CalSil to address some of potential moisture infiltration from the slab that you read so much about on this forum. It may also give you a bit more heat retention.

Prego Giovanni, this precast oven is supposed to retain heat for about 36 hrs so we'll see. Not sure how much I'll be using the oven for that purpose anyways so not too worried about that. As far as adding Foamglas, that would add too much height to the finished floor level and the floor tiles have been mortared in so that's not an option at this point, however; the kit does come with an aluminum flashing and a high temp appliance/oven sealant to address moisture infiltration.

[fox]

So the floor bricks are mortared together ?
That is an interesting dome, looks like some form of interlocking lugs on the side.

[Giovanni Rossi]

Ciao Tiki, Yeah, "residual" was a poor choice of words. You should have around 70^o at 36 hrs but you'll probably drop below 150 at around 20 hrs. I'm hoping to get > 48 hrs of good cooking temp out of the D105 with the extra refractory mass of the bricks vs casting, along with the extra 2" of floor insulation. Also looking at adding 4" more of dome insulation than supplied in the kit.

Sorry, I didn't blow up your pick to see you had already bedded the floor bricks. Certainly too late for additional layers below the floor! I am intrigued by your flashing and sealant comment. I've emailed the company to see what they offer for the D105. I have spent a lot of time on the Forum to read what others have done with the juncture at the hearth slab. I'm just trying to decide what's best for me from the myriad of options and opinions presented.

I really struggled with the decision between the P85 and D105. Looking forward to reading about your cooking experiences.

Giovanni

[Tikibrewer]

Per the mfg: the P85 will take approximately 36 hours to cool from 750°F to 150°F, with the door firmly shut. In the first 12 hours it will drop to around 320°F.