Oven floor thickness

[SamJ]

Hi folks , we're getting an oven built and I'm not cinvinced this looks right to me (have attached photos). The floor of the oven has been poured separately and is going to be placed on the base. Its max 5cm thick and no rebar. Is this adequate? Your expert input much appreciated!

[UtahBeehiver]

Depends on the size of the oven, with no rebar 2" is very thin, these oven can get very heavy. I suggest you down load the Forno Bravo eplans which are free or very low cost, step back and review. Very hard to correct issues once concrete is poured and bricks laid.

[SableSprings]

I agree with Russell that 5 cm & no rebar is not a good gamble here. Do the 3 cross support beams have rebar? If they do, that would help a lot. Concrete has good compression strength, but poor tensile strength ( only about 10% of its compression strength). Are you doing a cast or brick oven? Also helpful to let us know the intended size (diameter of cooking floor). Make sure this builder is planning on adequate insulation, weep holes, and a contact break between the concrete hearth and the base insulation. You don't want that concrete slab to be a heat sink from your cooking floor or allow water to seep into your base insulation from it (rainfall pooling on the exposed slab areas). We've seen a lot of these ovens built that are missing critical components and will never work as anything but eye candy in a back yard.

Spend some time looking at the well documented builds in the sticky topics for newbies. We're here to help you have a good experience using a well designed WFO!

[JRPizza]

I would not be half surprised if that slab cracked if it was picked up by the long ends!

[SamJ]

Thanks, Russell. That was my impression too, will check out the plans.

[SamJ]

JR, you were not wrong. It cracked, which it seems is a blessing in disguise

[SamJ]

Thanks Mike, the beams have rebar, but I'm now getting them to pour a 15cm slab directly onto the base with a central 50mm ceramic fibre board set into the concrete. On top of that will get a skim of oven cement. Then I thought a layer of fire bricks on top.
planning on a 90cm cooking floor, and a brick oven. That is the paln at least!
what is a contact break and where would weep holes occur?

[SableSprings]

The primary problem with most insulation is the wicking/absorption of moisture from various directions. Primary ingress of water to the bottom insulation is from seams between the slab and outer oven footprint. This can come from pooling of rainfall coming off the dome or from rain on the flat surface of the slab (upon which the oven sits). A follow-up problem is how to dry out insulation under the oven initially and if it gets wet (or just absorbs moisture from high humidity and non-use for long periods of time). Over the years, folks on the forum have come up with two simple design innovations to deal with these potential and "normal" moisture problems. The first is to drill or embed tubes 8-10 mm (1/2") diameter through the bottom hearth slab. Generally these weep holes allow trapped water an escape route, 3-5 of these weep holes under the oven are all that's needed. (Note: we also recommend using some insect screen attached with silicone seal to the underside of the holes to keep insects from using them as nests sites.)

In order for water to "find" the weep holes and escape, laying porcelain or glass tiles makes a contact break between the concrete hearth slab. No contact...no capillary action up into the insulation. I personally think it's easiest to find some squares of smaller mosaic tiles that are mounted on a mesh backing. In the US, our mosaic tile sheets are sold in a 12" square (~30-32 cm). By laying the squares with the mesh side up, you provide support for the next layer (insulation) and the spaces between the tiles create nice little channels to allow water to find the weep holes. Since you don't care about the actual pattern of the mosaic tiles, you often can find odd squares left over from jobs or in tile stores where they are cleaning out discontinued or excess products.

Here's a link to a build that does a very nice job of showing and explaining weep holes & use of tiles to keep the floor insulation dry. (There are many other examples like this in the forum...)

https://community.fornobravo.com/for...andpoint-idaho

Good luck, and I hope you consider the weep hole/tile system rather than putting the ceramic board directly on the concrete. No need to put a layer of oven cement on the fibre board...a thin layer of sand/clay mix is often used to level the floor but these floor bricks need to be able to move a little during firing cycles.

Mike

[UtahBeehiver]

Plus one on Mike's recommendations

[SamJ]

Gentlemen, this is incredible advice - thanks very much.
Mike, thanks for all the great detail - much appreciated! Will definitely get them to include the weep holes and and the tile layer.I had watched a few YT videos, but no one I'd seen had mentioned this important detail.
Looking forward to getting it done now.

[SableSprings]

Glad that my "wordy" explanation helped (carry-over from being a teacher ). If they set the weep hole tubing slightly below the target surface of the hearth slab, it will make it much easier to screed the pour. Cotton ball in the tubing top end will reduce/eliminate concrete filling the tube. After the slab cures, you pop out that tube from below with a piece of rebar or long screwdriver & a hammer. Normally, this will pop out the concrete a little bit as well, making an effective funnel down into the tubing.

Don't forget to put wedges under any weight bearing supports for the form...that concrete slab weight can really make it difficult to remove the supports!

[UtahBeehiver]

From our experience on the Forum, most You Tube vids are not the best practices and many times just after clicks on their projects. In fact there are some really bad ones out there.