Mortar between fire brick on oven floor

[Susan Egglaine]

Hi, new here. My talented mason put mortar between the fire brick on the pizza floor. Is this going to cause any problems? Anything I should ask him to do? Anyone else have a floor like this? I did a cross between the Tuscan and Naples style. I have not started the curing process yet, the outside stone just came Friday and he will be back out this week to finish up. Thanks for any help! Tried to upload a pic but failed.

[SableSprings]

Welcome Susan! When you go through the curing process, that bond between the cooking floor firebricks will crack at several places as the bricks expand and contract with the heating (all weak joints and stress points). It's no problem, as the cracks will fill with ash. I would inspect the cooking floor joints after the curing process and remove any obvious mortar pieces that may "pop" out. After using the oven for a while, the normal floor movement will be "set" in a specific pattern that probably won't cause any more joint cracking. By the way, how much and what kind of insulation are you using under the cooking floor? I'm assuming your mason will be placing good insulation over the dome (between) the dome and the outer decorative rock. (A little concerned with this being his second oven and having made the cooking floor "mortar mistake".) We usually like to see the curing done with the insulation on the dome and before any outside render/enclosure is completed. As long as moisture driven off by the curing fires has a place to exit...you'll be fine.

Picture uploads have a size limit of about 1.2 MB, use a free utility or choose to reduce the size of photo (most phones include that option). I crop and resize to 800 x 600 pixels, which will reduce a photo from 3-5 MB (often normal for phone photos) to 100-200 KB. That size limitation has been the most common issue with picture uploads to the site. The 800 x 600 size is plenty to allow good picture detail for us to view. Hope that helps...looking forward to seeing this cross of oven styles.

Also, it's helpful to us to know what location (state or country) you're in, to better understand weather and supply issues that may arise. (It's an option in your profile settings/input.)

[Susan Egglaine]

Thank you!!! I bought 2 rolls of the 1x24x25 ceramic insulation. They did not sell the boards and was told I could use the roll on the bottom, the mason and I made the call not to do that. We both thought it would settle over time and not be good. Used almost both rolls on the dome. The guys came today and started putting the stone on and I am very pleased! I was holding off building a fire because I thought the longer you waited the better but sounds like I am good to go.

[SableSprings]

So, I am understanding that you laid the firebricks for the cooking floor directly on the concrete top slab? You are correct that the ceramic batting would not support the weight of the cooking floor bricks, but you really do need something between the firebricks and concrete. Most folks that don't have access to the ceramic insulating board use a mix of either perlite or vermiculite with cement. The concrete will act as a giant heat sink and getting your oven to hold heat (or possibly even coming up to pizza temps (for any length of time) will be an issue. The other thing you need to be aware of, concrete will degrade at the temperatures targeted for pizza. If you have rebar in the slab, the thermal expansion differences between the metal and the concrete may cause cracking of the slab. Probably will not be a big problem initially, but depending on the top slab (thickness & composition) and oven's use...it may not be a good thing.

The top covering insulation will act as a flexible layer to absorb the expansion of the dome when heated, so the outer stone shell you're putting on should not be affected (as long as they are not compressing the batting too much). Again, as long as moisture can escape the dome (and you have it insulated to reduce the curing/firing shock), things should be fine...I'm just a little bummed and worried about the lack of insulation underneath.

[Susan Egglaine]

I just light my first fire today about 4:00, I was running around 300 (that is what I am shooting for) popped up to 380 dome, floor was at 170 after 3 hours. Just checked again 310 dome, 160 floor.
We will see, hoping for the best. Thanks!

[Susan Egglaine]

Think I have at least 24 hours curing time. I can still hear sizzling if I have higher heat. Using all dry hardwood. Will you still hear a sizzle if your oven is cured? Also any suggestions about what I can do to insulate the floor? Another layer of fire brick on top of the one I have? Would some type of insulation under the concrete slab be better? Thanks for any advise!

[JRPizza]

Adding another layer of firebrick would just be adding more thermal mass, which I believe you have an excess of with your floor bricks sitting on the slab. Perhaps some insulation under the slab would help hold heat, but as Mike said above the slab would have to be able to accommodate the higher temperatures without being degraded. You should maybe do a forum search to see if any other builders had to deal with no insulation under the floor and what if anything they did to remedy it. Copy and paste the words below into your browser search bar - it is better than doing a forum search:

no insulation under floor fornobravo site:fornobravo.com


At the very least you will probably go through a bit of wood getting the floor up to temperature and may have some reduced cooking times (sorry to say).

[SableSprings]

Susan, remember that the curing is raising the temperature slowly and consistently over time to +600F. It won't cure by simply keeping it at around 400 for a day or two. One trick suggested by David S is to put a piece of clear plastic over the dome while firing it. If there is still moisture being driven out, you'll see condensation on the underside of the plastic. (And no, your oven should NOT sizzle...the wood yes, but not the oven!)

Without having some pictures (during and after construction) or better details on your oven's floor construction, it's hard to come up with a solution to a floor without insulation. How thick is the concrete supporting hearth? Does it have lots of rebar, some or any? Is your cooking floor laid inside the dome or is the dome sitting on top of it? As JR suggested above, a search might turn up some options.

The best hope would be if you could lift up the cooking floor and lay down at least an inch or two of ceramic board, then re-seat the floor...the problem is that by raising your cooking floor that much, it might make it a problem to "work the oven". Any insulation you can put between the cooking floor and the concrete is going to help a lot. I suspect that when you are firing your oven, the concrete below will get pretty warm over time. I bet if you put an IR gun on the underside of your supporting slab, you'll see where a lot of your heat is being pulled out.

Even without insulation, you should be able to cook and use this oven...it just will not heat up as quickly on the cooking floor and you'll be constantly moving your fire around to heat up a spot to place the next pizza...but that pizza will still most likely be far better than anything you can buy at a chain restaurant.

[Susan Egglaine]

The crew is working here this morning on the floor. Spoke to the mason the floor is 4 and and a half to 5 inches of concrete with lot's rebar and the dome is sitting on top. Thinking maybe put the insulation batting underneath the concrete slab with Hardie board holding it in place. Hardie board was used in the form. What do you all think?
I will check temps underneath and floor temps tomorrow and use the plastic trick. Time to turn up the heat!! I did go to 600 dome on the last burn.
Thanks so much for the help!

[SableSprings]

There is no point putting insulation under the concrete...you want to keep the heat from being sucked out of your cooking floor. Insulating the concrete underneath is not going to help bring your cooking floor up to temp faster in any significant time (the entire concrete slab would have to be heated up to more than comfortable temps ). When you talked to your mason, did they at least put some sand underneath the cooking bricks? If they did, at least you have a little heat break between the concrete and bricks. Sand is not a good insulator, but it is better than nothing (especially if they put down a couple inches of it).

My three concerns are that; 1) you are going to find it difficult (or at least it will take a lot more wood and time) to bring the cooking floor up to temp for anything more than 1-2 pizzas without needing to recharge/reheat/move the cooking area, 2) the rebar in the concrete slab is going to expand/contract more than the concrete causing cracks that will provide water access to speed corrosion & rusting of the rebar and 3) the concrete immediately under the cooking floor will degrade over time from the high temps. Again, if you are doing an occasional pizza party, are not planning on doing roasting/baking over long time spans, have lots of wood and time available, and you have a pretty water tight enclosure for the oven, any WFO will provide you with some good eats.

I don't mean to be so negative, I just think you need to be aware that your oven's construction is going to down grade its performance & longevity compared to ovens built with insulation underneath the cooking floor. I have a friend who had an oven built without cooking floor insulation and he certainly has enjoyed using it...I just feel bad that this fairly large $ investment is made without a fairly important part (IMHO )

p.s. The plastic trick is really meant to see if moisture has been driven out of the oven shell and outer insulation...putting it over your final enclosure is not going to be a good indicator of your internal components "moisture status".

[Susan Egglaine]

Thanks! Going to fire the oven up here in a bit, going for my first really hot fire and see how she does. Having some storms moving through. I will update after I do.

[Susan Egglaine]

The best I could get was 500 degrees floor, 700 wall, a bit over 100 underneath. Took a bit over 2 hours to get there, don't think I can get it any hotter. It was really windy here yesterday. Do you think it will improve after more cooks?

[Susan Egglaine]

New game plan! Mason came out this morning, we are going to put a layer of insulation with fire brick on top of the floor. It will still give me 15 inches of height inside and if I don't like it I can always take it out.