couple of detail question before I pour the hearth

[firepie]

I'm planning on pouring the hearth this weekend and I have a couple of detail questions.

First, the local garden center can provide 8 cu ft of perlite(they only have 2 bags left) or 10+ cu ft of vermiculite (medium). So should I go with the vermiculite or perlite?

Second, I am thinking about putting a slot in the hearth (I'm thinking 1.5 inches by 12 inches) so I can rake/brush the ashes into a metal container underneath the hearth. My concerns are: what to use to block out the opening when I pour the hearth that I can easily remove once the concrete is cured? - would a piece of 2 X wood work or would that become a permanment part of the hearth. Also, is an ash slot a good idea? would a different size be better?

Tom

[Robert Musa]

congrats... you've reached the most physically demanding part of the project. after the hearth, its all downhill.

i like the perlite because its white and its easy to see if you have mixed in the cement evenly. the vermiculite, on the other hand, is the same color as the cement and its tougher to see how well the cement has been distributed.

forget the ash slot. i have one in mine and i have never used it. its (it's?) much easier (and faster) to remove the ashes with a peel.

[firepie]

How important is it to have the metal flashing as a slip plate between hearth and block stand? It's hard for me to believe the thermal expansion is going to be an issue. I would have thought the expansion would occur at the firebrick layer.

I have rebar that sticks out above the blocks (where I filled the block cores with concrete and rebar) about 2-3 inches, I was going to grid the rebar off to make it flush but then I was thinking why not leave them as is. It would make a solid connection to the hearth but then again maybe I don't want that solid of a connection.

I'm starting to think I'm over thinking these little details.

Best Regards

Tom

[Robert Musa]

i would err on the side of caution and cut the rebar off. this thread goes into this issue in a little more detail as well.

[PizzaMan]

Projecting rebar may not be a bad idea...

When I reached this stage, my big-time architect buddy suggested I leave my three pieces of rebar protruding; earthquake damage with un-connected and very heavy pieces could be substantial in our neck of the works (California's central coast) if not somehow tied together.

At the same time, my protruding pieces only act as pegs, and the whole hearth and dome could be lifted in one piece with a forlift, if I ever chose to move it.

I'm no expert, but I trust this guy with his earthquake obersations.

Michael
aka PizzaMan

[Marcel]

Consider loose fitting caps on your re-bar to allow shifting

(M) I have not done this but if you decide to have re-bar protruding above your hearth stand, perhaps you can have your earthquake protection and still allow for limited shifting.

(M) The idea would be to make some PVC caps of, perhpas 3/4" tubing. Cap these on one end with either a plug or a hat type cap which is cheaper.

(M) Invert this cap so it covers the re-bar in your already dry slab.

(M) The caps should be able to move from side to side as their internal diameter will be larger than that of your re-bar. Now, with the rebar covered with caps that are somewhat taller than the re-bar extensions, pour your hearth slab.

(M) Unless I've missed something you should have pegs that can shift inside their PVC hats if there is thermal pressure but not so much that the whole slab could slide off the stand in an earthquake.

(M) If I'm wrong, after the earth quake, don't come looking for me.

Ciao,

Marcel

[jengineer]

Cap it

I concur with Marcel - this is one of the ideas that will be put into the new revision of the plans that we are working on. (Marcel make sure you put this in when it comes to the revisions). I would also to tend to still include the flashing. It could be a pain, but that means either punching a hole for each of the vertical rebar pieces sticking up through the block stand or having flashing in overlapping sections with notches cut out for the rebar. Weather you decide that the ? inch of possible slippage or in R. Musa?s case the crunching of vermiculite is worth the $10 spent on flashing and ? hour of cutting and placing is worth the effort or not is up to you. The original plan includes it and the revised plan will still include it. The purpose of the flashing is to reduce the chance of cracking the hearth due to thermal expansion. Using the flashing and the PVC Tubing and caps gives you:
a slip plane,
a floating (use that term loosely since the weight of the hearth and dome is not light in weight) hearth that will not readily come off the block stand and
the the capability to forklift the oven off the stand if you decide to take it on the road to a new location.

[Les]

When is the expected completion date for the new plans? We are headed up to James' place mid May and will decide to build or buy. If I decide to build, the updated plans would be a Godsend. Iv'e seen a ton of improvements / mod's over the last six months.

Les...

[jengineer]

progress report

Authoring and editing has been a rather slow process. We have completed the backbone which is the table of contents and are now working on the "Before You Begin" sections. That means we have not yet gotten to breaking ground for the foundation.

If you have kept up with the forum then you know that
1. drainage is important to consider, CanukJim posts

2. Clay soil, freezing snow, try pouring deep footing ala CanukJim Sonotubes but then pour your stand instead of using the block wall method, Drake Posts
Read number 3 and stop the pour at a heaight that is at least 9-10 inches below where you want your landing to be.

3. flip the hearth layers, pour the reinforced concrete 3.5 inches first and then pour the vermiculite 4 inches. If you want more thermal mass under your oven add a floating hearth on top of the vermiculite layer (2 inches).

4. You can use conrete board (various names for it) instead of plywood as a former for your hearth. It will require more support as it is structurally weak. However unlike the plywood, which we recommend you remove after the concrete has set, you can leave the cocrete (wonderboard) in place.

5. Many builders are probably not paying attention to US universal building codes. It is up to you to make sure you have the proper permits and you build according to code. These instructions may or may not be within code for your country, state, county, parish, city... and you may need to adjust accordingly. Such as the recent warning that Building code says that chimneys have to be 24 inches above any structure within 10 feet.