Cement Board under Fire Brick Floor

[avkroth]

My Floor has angle iron, cement with rebar and then 3 inches of calsil slabs for insulation. I am about 1.25" to deep and will be using 3"x24"x12" firbrick slabs for the floor of my oven. To raise the level the 1.25", is there any health or safety issues using cement board to place my firebrick on versus a cement/clay mix.

[ATK406]

Re: Cement Board under Fire Brick Floor

Cement breaks down at temperatures in excess of 600F. I am told it breaks down in a dramatic fashion too...though I have not tested this myself. I don't know about health risks - though most cement boards have some type of plastic/fiber lattice imbedded in it. If the CB is placed directly under your Firebrick it will certainly breakdown from the heat and may not provide the support you are hoping for and the fiber reinforcement may even produce some unpleasant fumes - which is what I think you were concerned about in the first place.

If it is not too late couldn't you put the CB under your Calsil insulation. I doubt the CB would be exposed to such high temperatures under your insulation - in any event I have not seen or heard anybody express problems with their cement slabs that were insulated from the hearth in this way.

Regards,
AT

[avkroth]

Re: Cement Board under Fire Brick Floor

So your thought is the fire brick sits right on the calsil block? So the calsil block is ok from a heat and health standpoint?

[Tscarborough]

Re: Cement Board under Fire Brick Floor

Concrete breaks down explosively because of the aggregate, cement used in board will break down by turning to powder eventually.

[david s]

Re: Cement Board under Fire Brick Floor

Concrete breaks down explosively because of the aggregate, cement used in board will break down by turning to powder eventually.

It will also break down explosively if water in it suddenly turns to steam. To fireproof concrete (to a certain extent) polypropylene fibres are sometimes added. If fire goes through a building, the fibres melt and provide a network of mini pipes that steam can escape through, reducing explosive steam spalling. Provided the concrete has not been too damaged it is often salvageable. The same principle is used for castable refractory.

[GianniFocaccia]

Re: Cement Board under Fire Brick Floor

Since you already have 3" of firebrick floor depth, you could always lift the calsil slabs (boards?) and either put another inch of board or pour 1.25" of vermicrete.

If baking bread is your focus, you could always add a layer of firebrick splits under your firebrick slabs.

[Dino_Pizza]

Re: Cement Board under Fire Brick Floor

I made my "temporary oven door" out of the left over cement board I used for my oven enclosure /housing. After 3 years of minor chipping and breaking, the heat has turned it brittle and cracked it quite a bit. I can no longer use it after last weeks relatively low-heat smoking.

The door is mostly used on low temp fires (way after I made pizza and I'm only interested in keeping the heat in for baking). So I've experienced how brittle and shattered it can be in 3 years of moderate heat. I know wfo floors are cooler than the wfo ceiling but it does break down and fail in even moderate heat after a few years so try the other solutions mentions.

[boerwarrior]

Re: Cement Board under Fire Brick Floor

avkroth

I'm not sure anyone answered your question about FireBrick directly on the CalSil Board. The answer is yes. CalSil has an extremely high heat resistance. Almost all Pompeii oven builds on this forum have the firebrick laid directly on the CalSil board.

If your concern is height - you could also put your floor firebricks on edge and raise the floor another 1.5-2 inches that way. You will have more heat mass in the floor - which you may not want - but cement board (not recommended) would also give you more heat mass.

Neil