question about hearth slab framing

[johnnybrewmeister]

I'm pretty much following the FB plans for my stand and hearth, and am getting ready to pour the slab for the hearth floor. My side forms are 2x4s for a 3.5" slab, and they are resting on & screwed to 2x4 legs to get the right height/level. But my forms are reallly really wobbly with this setup! I would love to nail/screw the form "legs" to the concrete blocks to stabilize things, but that's probably not too practical. Am I the only one who has a wobbly hearth form?
Should I worry? How can I fix it?

Thanks a bunch,
John.

[stonecutter]

Re: question about hearth slab framing

Get a couple packs of composite shims and wedge them under the form to stabilize it. Just snap them off if the won't release after the form comes off.

[SableSprings]

Re: question about hearth slab framing

I'm pretty much following the FB plans for my stand and hearth, and am getting ready to pour the slab for the hearth floor. My side forms are 2x4s for a 3.5" slab, and they are resting on & screwed to 2x4 legs to get the right height/level. But my forms are reallly really wobbly with this setup! I would love to nail/screw the form "legs" to the concrete blocks to stabilize things, but that's probably not too practical. Am I the only one who has a wobbly hearth form?
Should I worry? How can I fix it?

Thanks a bunch,
John.

John, it's really important that your slab form is solid. I did a similar thing (wobbly form) for my first slab pour using bits & pieces of wood that were laying around--and it was a nightmare. Never having worked with that amount of concrete, I did not realize how heavy and how much outward pressure that wet mass of a +yard would exert. Make sure you shore up the form...you should not be able to wiggle it when you're ready to pour. And you don't want anything secured to the top of the forms as it will interfere with the pour and the scree work (leveling)...also a mistake I made with my first attempt

Here's a thought if you're having trouble getting the form to "firm up". I did a hearth pour a while back where we had restricted space and couldn't get a solid form setup. We did the best we could, then drilled several small holes along the middle line of the 2x4s forming the perimeter. Next we ran stiff wire back & forth through the holes side to side in the 2x4s to tighten & reinforce the form's vertical strength & integrity. It was important to be fairly close to the middle so the sides of the form didn't get pulled out of vertical. (Of course the wire was separate from the slab rebar.. ... although I'm sure the soon to be embedded wire added something to the slab strength.). After the slab hardened, we cut the wire on the outside and pulled off the forms. The small stubs of wire remaining were easily cut off the concrete slab edges.

[NCMan]

Re: question about hearth slab framing

I'm not sure if what I did will work in your case, but here's what I did. Keep in mind, I used solid cap blocks on top and they were already nice and level. I simply added some Tapcon screws in a few mortar joints, along w/a shim or two where needed. I just placed the concrete in fairly easily and spread it around as I went, to stabilize the pressure. The forms never budged, even when I tapped on them to get the honeycombs out. It is quite important that you get your slab nice and level at this point. Best of luck to you.

[Les]

Re: question about hearth slab framing

May be too late for you but I placed some plywood inside to block 3 1/2 below the top. Was able to brace the hell out of it from below and simply knocked the braces out when done.

[johnnybrewmeister]

Re: question about hearth slab framing

Thanks for all the tips!

Stonecutter, the "legs" are already shimmed to level things out and stabilize, but still it is quite wobbly.

SableSprings/Mike, I've already learned from your past experience, because I was gonna screw some braces to the top to see if that would help - now I won't try this.
I like your idea of the wire...might give that a shot.

NCMan, are you saying you just screwed some of your "legs" to the block stand? I could probably get some tapcon screws (if they aren't too pricey).

Les, I can't honestly picture what you're describing. Got any pics? I know I've been thru your build thread, maybe it is time for a re-visit.

Thanks,
John.

[SableSprings]

Re: question about hearth slab framing

John, here's some pics I found that show how we did the wire reinforcement through our wobbly form's sides. We were up at Hihium Lake in British Columbia and everything had to be hauled in (last 1/2 mile from the clearing to the house is a barely passable old logging road...Kubota RTV & foot traffic only). That's why you'll note scrap metal mesh pieces used to make up for the lack of more "real" rebar. You can see the wire wrapped around the nails on the form perimeter to keep it tight and also get a feel for why we had a form that was a more little wobbly than most

FYI, the piece of foam in the photo was pushed into a rectangular cut in the cement board and stayed there through the pour. The foam (void form) was removed when the concrete hardened and thereby left an opening through the slab that ultimately became the ash dump slot.

[sharptailhunter]

Re: question about hearth slab framing

I used 2x6 boards so I'd have about 1.5" of board skirting around the CMU stand. From your description, I'm picturing that the bottom edge of your 2x4 frame/form is level with the top of your stand? If so, you could easily add 2x4 skirt braces to the bottom of your frame by screwing pieces of 2x4 under the frame by using another scrap piece of wood to scab over the joint, if that makes sense? Just make sure your screws aren't so long that they'd interfere with either your stand or protrude through into where the concrete is going to be poured. You already have the vertical supports in place, so it sounds like you just need to keep the whole box frame from shifting on top of your stand? The skirting can help with that as well as some buttress braces that extend diagonally down and out from at least the middle of each of the four pieces forming your box frame. Personally, I would do both. You're welcome to look at my build thread (42" Idaho Spud) as I just poured my hearth. I didn't use the buttress braces because I had some different forms that gave me stability against the sides of my box frame from bowing out. Not sure if any of that helps...

[johnnybrewmeister]

Re: question about hearth slab framing

Thanks SableSprings...you've given me some other ideas on how I could possibly use wire to tie things to areas of my (dry-stacked) block stand that might have gaps I can take advantage of. The pics of how you used wires helps a bunch.

Sharptailhunter - I'll search a bit for your build thread, 'cuz I kinda think I know what you're talking about, but not sure I 100% getit.

Thanks,
John.

[NCMan]

Re: question about hearth slab framing

Thanks for all the tips!

NCMan, are you saying you just screwed some of your "legs" to the block stand? I could probably get some tapcon screws (if they aren't too pricey).

Thanks,
John.

John.....No, my forms are not attached to the foundation in any way and there is no need to do so. It's simply sitting on the block and the Tapcons are just there to keep it from shifting. I'm not sure I understand what you mean by "legs". I'm guessing that they are pieces of wood you've added in an attempt to level up your formwork(?) If so, normally they would not be needed, but I will guess again and figure your blockwork is not level? Pictures help a lot on here. Then people can better understand what you are asking about. Anyway, yes, all I used are a handful of Tapcon screws and a few shims, as shown in my photos. That is all that is really needed. I simply screwed my 2x4 forms together, installed my vapor barrier, set the forms on my blockwork, laid my rebar mat in place, added a few Tapcons to hold it in place while pouring and I was ready to pour in about 10-15 minutes. Anything more than that is overkill. Also, it's good that you are planning to pour only 3.5 inches of concrete. 3.5-4" of concrete is all that is needed. No need to do anymore than that. People get way too carried away sometimes. Can you add some photos?

[almondsurf]

Re: question about hearth slab framing

Why not use 2x6 boards like I did? It worked great. I had my wife help me hold the boards up. I had them about an inch or so below the "slab" that I was laying. This way I was able to suck it tight to my concrete block to hold it into place. I put 2 screws in each end to hold the boards. I just worked my way around. Put one board up, measured 3.5", made sure it was level as I screwed in my next board. The last joint was a little tough because I had to make sure it was very tight, but I really liked how it turned out. I dont even know if I 'needed' the support boards, but put them there just in case.

I poured my slab just a couple weeks ago. Make sure you have enough concrete, I didn't. Hope it helps.

[NCMan]

Re: question about hearth slab framing

Nothing wrong w/that method at all. The way I described is much easier, faster, less people needed, saves concrete, money and labor. I thought that might interest him.

[johnnybrewmeister]

Re: question about hearth slab framing

NCMan, my slab is going to be the same size as the block stand, so the form for it cannot rest on top of the stand. So the form is just outsize the top outer perimeter of the blocks, and the "legs" are the vertical supports that hold the form up. I'm pretty sure this is the same as the FB plans call out.

Anyhow, I ended up with a modified version of what SableSprings/Mike suggested - I have rebar extended up from the block cores, that will be in the slab thus tying the two together (block stand and slab). So I drilled small holes in the wood form on all 6 sides and used wire to tie the forms to these secured rebar pieces. Now the form is rock-solid in all directions.

Wonderful source of knowledge this forum is!

John.

[NCMan]

Re: question about hearth slab framing

Glad you found a way that works for you. Best of luck!!

[almondsurf]

Re: question about hearth slab framing

Nothing wrong w/that method at all. The way I described is much easier, faster, less people needed, saves concrete, money and labor. I thought that might interest him.

I guess I can see the utility in that