42" Pompeii - Florida

[TomYork]

I'm more of a woodworker, but love pizza and thought I'd try my hand at building an oven. My concrete work has not been what I would consider pretty, more along the lines of good enough. I have the materials for for the insulation layer between the hearth and the oven floor and will work on creating the form for that next. Here's my progress so far. Feel free to offer suggestions as I move forward.

[UtahBeehiver]

What is you floor insulation going to be and how thick so we can guide you. This is an important stage of the build, once laid down, no going back. BTW, good start, be aware it will be a little difficult to work on the back side with the wood and brick fence there.

[TomYork]

Thanks for your reply, I plan to make a form and then pour 4" of a 5 : 1 perlite : portland cement mixture before laying down the fire brick floor. I've also included some additional pictures which includes my latest progress.

[david s]

Drilling a few holes through the concrete slab from the base up will provide a drain for moisture to escape. When the hole reaches the top surface it will probably blow out a bit which is actually an advantage, leaving a slope towards the drain hole. Even better, although not essential is to lay some tiles over the concrete supporting slab (but not covering the holes). This raised the insulating slab so it won’t get wet feet and provides channels for steam to find its way out more efficiently.

[UtahBeehiver]

I am pretty sure David S will advise you to make sure the pcrete is "really" dry before laying the fire brick down on it. Removing water from floor pcrete is difficult to remove once the fire brick is laid on top. Also to be sure if you only pour pcrete in the shape of the oven, make sure all parts of the oven, ie floor, dome, wall, etc ) have insulation under it.

[david s]

I am pretty sure David S will advise you to make sure the pcrete is "really" dry before laying the fire brick down on it. Removing water from floor pcrete is difficult to remove once the fire brick is laid on top. Also to be sure if you only pour pcrete in the shape of the oven, make sure all parts of the oven, ie floor, dome, wall, etc ) have insulation under it.

Thanks Russell,
If you go the tile addition below the vermicrete insulation you can also go a step further and hold the vermicrete up from the gaps between the tiles by placing some weed mat over the tiles.
See attached my experiment for drying a vermicrete slab.

Vermicrete insulating slab PDF.pdf

[TomYork]

Thanks for the comments. For the 42" interior oven build i calculated a 51" diameter circle to account for the 4.5" fire brick around the perimeter. I've also taken into account an 8" deep vent landing that is 26" wide including the thickness of the brick. I then would plan to cover the entirety in a 3" blank and then stucco/tile over the top of that. The 4" layer of perlcrete would be visible under the oven wall and landing until covered by the insulating blanket.

I'm not sure what this means though:

If you go the tile addition below the vermicrete insulation you can also go a step further and hold the vermicrete up from the gaps between the tiles by placing some weed mat over the tiles.

[UtahBeehiver]

What Dave means, that if you place tiles down on hearth with spacing so water can channel out to the weep holes then any pooling water will not absorb up through the pcrete from the hearth. The weed fabric is so the channels will not get blocked when you do the pcrete pour. Wet insulation is one of the key factors in poor thermal performance of an oven.

[david s]

Thanks Russel,
An explanation of poor performance of an oven with wet insulation, is in the modification of two ovens that I made work successfully. The particular design had a steel frame over which was placed 3/4” marine ply, then calcium silicate board, then the firebrick floor. Although marine ply is weatherproof it will not survive constant moisture, so water getting into the oven either through the mouth or by cracks in the outer shell, will fall by gravity and accumulate under the floor. In the case of both of the ovens I repaired, they had cracks around the dome to flue joins, which I repaired with render and high temperature silicon that allowed the flue to expand without cracking the outer shell.
underneath I drilled a number of 1/2” holes through the marine ply which was (in both ovens) rotten in a number of places. This allowed an exit for under floor moisture and after a number of long low and slow fires the ovens dried out and initial performance was restored.
The lesson I learned here was that water needs to be kept out, but if it enters an exit should be provided. Also don’t trust marine ply to be totally waterproof it’s not and it rots.

[WaterDog]

UtahBeehiver I assume by David's reference to 'weed mat' is probably what I think of as landscape cloth here in the US - used under mulch in flower beds to reduce weeds. Is that correct? Understand the purpose just never heard it mentioned in this context before. Makes sense.

[UtahBeehiver]

Yes, same stuff. Something that is permeable to transmit any water that gets in there but to keep the pcrete mix from blocking the channels to the weep holes. Sometimes, some smaller tiles have a fine mesh backing that some builders have placed face up but you can't tell for sure if this works or not after the pour.

[david s]

Aluminium insect screen is another option. Cheap and easy to cut, but stiff enough to hold up the vermicrete over a substantial gap.

[WaterDog]

Thanks for confirming. Will incorporate into the plan.

[TomYork]

I drilled a few weep holes and have to add some more tile, but want to make sure this is what you guys are talking about below the 5:1 perlite / portland cement mixture. The hearth is slightly sloped back to front so I have 3 weep holes near the oven landing. Once the rest of the tile is added I plan on covering the entirety with landscape fabric. Thanks.

[SableSprings]

Yes Tom, the porcelain tile is impermeable to moisture and the gaps between tiles provide channels for moisture to "find" and escape out the weep holes. Using some sort of mesh overlay keeps the 5:1 insulated concrete from shutting/filling those escape channels. The reason I like to promote the mesh backed mosaic tile squares is the gaps between tiles is well protected when the sheets are laid upside down. Using the tiles as you have works great as long as you are "protecting" the gaps from being filled. Sorry I didn't get you this answer earlier, but it definitely looks like you already had it figured out.

Looking good and excellent work on sloping the hearth to encourage moisture away from your build.