Straps around the hearth form

[james]

This is a question for recent builders. Do you think we need to mention putting straps around the hearth forms to hold them straight? The original theory was to keep the concrete from bowing out the form, but is that really a problem?

In line with my new "keep it simple" approach (we've deleted the aluminum flashing and pour the same day references), should we eliminates the straps?

Worst case, is that you can use a 2"x4" on the diagonal to hold the form rigid.

Let me know what you all think.
James

[maver]

Doesn't a 2x2 on the diagonal get in the way of leveling the poured concrete? I think straps may be the simplest way to manage this when you are pouring an elevated pad and cannot just stake the forms in place.

I don't think the straps actually prevent bowing, by the way. If anything, they may contribute to it (think bow and arrow, the strap is a bow string). What they do is reinforce the corners in a very simple way. I used 2 straps to compensate for wood that had some twist in it to pull all the corners in tight.

[dmun]

Right now we're talking about a three and a half inch thick slab. I braced my forms, what I had, diagonally from the floor, but it didn't seem to exert much lateral force. Even if you were pouring another four inches of insulating concrete on top of the wet slab (which I don't think is necessary, you can let the slab set, where's it going?) I think putting the forms together at the corners with three inch drywall screws is enough.

[Chris]

I'm going to be pouring my hearth slab in the next week. I'll volunteer to go strapless if someone will agree to help me clean up the mess if my forms fail!

I'm betting that the straps aren't necessary. I plan to reinforce the corners, but that's about it. Those 2x8 seem plenty sturdy for a 3.5" pour. The perlcrete layer doesn't count.

I lined the forms with plastic in hopes of keeping the wood pretty for another use. As long as I'm not too sloppy, it should work well. Nobody's going to see the slab anyway.

-Chris

[james]

I always remind myself that no one is ever going to see your hearth. I laugh when I think about how much time I put into the stand and hearth the first time I did this -- realizing later they are hidden in the end.

I would venture that the straps are not necessary and the 2"x material is rigid enough to hold.
James

[maver]

I used pretty cheap, somewhat twisted 2x8 for my forms, and while I agree screws should be enough, and the straps were not helpful in preventing bowing, they are pretty convenient for racking the 2x8 into square to close the gaps at the corners from imperfect wood. Did anyone use the diagonal bracing technique? I'd suggest the straps are a good idea, the diagonal bracing may be dropped from the directions, and screws alone are just fine also. Ratcheting straps cost a lot more than screws, although since I bought them for the form I've used them a lot for other things.

[Chris]

Strapless

Rather than using straps to hold the form together, I reinforced the corners with some scrap to keep the nails from pulling out. It worked.
I tried to keep the concrete from getting too soupy, so it really didn?t exert much sideways force at all.

If I had to do it over again, I would have used 2?4 forms, instead of 2?8. Pour the concrete, screed, float. Then either piggyback another set of 2?4 on top for the perlcrete, or wait for the slab to set and then move the forms up. Trying to level a four inch slab in eight inch forms is no picnic!

I decided to rest the forms on top of the concrete blocks to avoid building supports for them. Hopefully the slab still has enough purchase on the blocks to keep from caving in. One thing I noticed from this photo is that I moved the form when hammering in the reinforcements, and forgot to nudge it back.

[Marcel]

2" x 10" forms

Chris wrote: "If I had to do it over again, I would have used 2?4 forms, instead of 2?8. Pour the concrete, screed, float. Then either piggyback another set of 2?4 on top for the perlcrete, or wait for the slab to set and then move the forms up. Trying to level a four inch slab in eight inch forms is no picnic!"

I decided to rest the forms on top of the concrete blocks to avoid building supports for them."
================================================== ===

(M) I have no trouble using 2 x 8 forms but would not again have rested them on top of the blocks, but secure the forms around that outside block perimeter. If your hearth stand is level, then screeding the concrete is no problem.

(M) If the option to slope the slab toward the baker is used, then any water would have the tendency to exit. I feel that the emphasis on level is not a big issue. Your pizza will not slide out the door opening, and a slight variance in height above the dome should not noticably affect a uniform temperature.

(M) I followed one of the options in the original FornoBravo instructions and also rested my 2"x6" wooden forms just barely (perhaps 1/4") on the concrete block stand. It was not a choice I'd make again as it left a 1/4" lip that I later had to meticulously fill.

(M) Instead of 2"x 6" studs on their 2" edge, I would now favor 2" x 8" studs. This way, I could use 2 of the 8" height to set *around* the outside edge of the concrete block hearth stand. The remaining actual 5" would be sufficient for both the refractory and insulating layers, irrespective of their order. I would then have no lip to later fill.

(M) I think I would use James' quaisi vertical supports under the 2 x 8 frame to support it since even a ratcheting strap might not support the form's weight.

Ciao,

Marcel

[Chris]

I'm not too worried about filling the lip that will be revealed by the forms. I'm going to have plenty of room for insulation. The walls of the gable-house enclosure will be lower half stone, upper half stucco.

I said "level", but what I meant was that I was worried about making the concrete a uniform thickness. When i thought I was finished, I checked the middle of the slab, and it was only 2 1/2 inches thick. Nothing a few more bags of concrete didn't fix. :-)

[james]

Nice Blog

Hey Chris,

Nice blog. Could you put a link in the blog back to Forno Bravo.com to that your readers know where the plans came from? That would be helpful.

On pouring the two layer hearth, one option is to draw a line down 4" from the top on all four sides and then using a 2"x4" to screde the concrete layer to be somewhat level. It's hiddened by the vermiculite layer, so all that really matters is that your top layer is level.

Keep going!

James

[Chris]

I added a bookmark on the main page, and a link on the resources page.

-Chris