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Old 09-09-2009, 09:40 AM   #1
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Piers under pad for outdoor fireplace?


A friend is preparing to pour a pad for an outdoor fireplace, with two flanking walls at right angles (forming an "L"). He plans to keep the pads under the two walls separate from the pad supporting the fireplace. The wall pads will "float", i.e. be set on 4" compacted crushed mixed stone, 4" concrete with mesh. They will adjoin, but not be connected to, the fireplace.

For the 50"x70" pad under the 3000lb fireplace + several ton built-up hearth, we were thinking 6" or 8" rebar-reinforced (1/2" on 18" o.c.). The frost depth is ~40" (N. Ohio). Given the high and heavy fireplace sitting on top, letting it "float" above the frost line seems like a bad idea.

One suggestion we heard was to dig piers underneath the pad to below the frost line. Reasonable? How many/what size? Can they be poured at the same time as the pad, or do they need to be poured separately? Would "open holes" suffice, or better to go with a back-filled pier form like the big-foot?

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Old 09-10-2009, 06:25 PM   #2
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Piers under pad for outdoor fireplace?


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Originally Posted by jdtsmith View Post
A friend is preparing to pour a pad for an outdoor fireplace, with two flanking walls at right angles (forming an "L"). He plans to keep the pads under the two walls separate from the pad supporting the fireplace. The wall pads will "float", i.e. be set on 4" compacted crushed mixed stone, 4" concrete with mesh. They will adjoin, but not be connected to, the fireplace.

For the 50"x70" pad under the 3000lb fireplace + several ton built-up hearth, we were thinking 6" or 8" rebar-reinforced (1/2" on 18" o.c.). The frost depth is ~40" (N. Ohio). Given the high and heavy fireplace sitting on top, letting it "float" above the frost line seems like a bad idea.

I agree with your fears as well. Way too much weight, height, & most importantly, effort involved to not look for some kind of frost protection & additional support. I would reccomend the 8"+ of concrete & at least that much rebar. Your talking a few dollars now vs. complete disaster later.

One suggestion we heard was to dig piers underneath the pad to below the frost line. Reasonable? How many/what size? Can they be poured at the same time as the pad, or do they need to be poured separately? Would "open holes" suffice, or better to go with a back-filled pier form like the big-foot?
This would be the minimum foundation IMO. It may work fine with simple post holes drilled & poured below the frost line, but keep in mind that if your in heavy, expansive soils, frost could still have a chance at moving the pad slightly.

I think setting tubes in the hole is the better method, plus it allows you to grade the gravel directly to the top of the tubes to get a better grade. I would reccomend as big of holes as you are willing to drill/dig & as many as possible. I wouldn't be concerned about the "Bigfoots" is the bottom of the holes are stable ground.

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Old 09-10-2009, 09:11 PM   #3
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Piers under pad for outdoor fireplace?


Thanks. As for using tubes, did you mean two separate pours, or just back fill the tubes and pour in one go? 8" tubes x 4 might be a reasonable approach. Even if you removed all the dirt under the pad, you'd have about 7000lbs worth of bearing capacity (assuming 5000psf soil bearing). The pad itself will weigh about 2500lbs, so that leaves 4500 for the hearth + fireplace.
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Old 09-10-2009, 09:40 PM   #4
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Piers under pad for outdoor fireplace?


I would pour it all at once. Probaly wouldn't be a bad idea to stick a rod into each post that extends into the slab, also.
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