For Those of You That Have Built a Masonry Chimney ..

This forum is for common products and questions such as chimney installations, CO detectors, coal bin designs and a variety of other general topics that do not fit into the other forums.
bsteckel
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Post Sat. Oct. 19, 2013 4:21 pm

thinking of bulding a 8x8 block chimney on top of my poured slab in my pole barn. slab is 5" thick, 4000 psi fiber reinforced concrete. Is is possible to build a larger pad with rebar reinforcement on top of my slab or is it absolutely necessary to saw cut a piece out and dig down for a footer. My intention is to put the chimney inside the building. I am guessing total height to be 15-20 feet.


Den034071
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Post Sat. Oct. 19, 2013 4:26 pm

B mason 40 plus years .Id say the floor will act like a spread fooy footing .Only danger can frost enter bld.If so must cut floor an go 3 feet 4 footre jack
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waldo lemieux
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Post Sat. Oct. 19, 2013 4:38 pm

Psi is psi. Doesnt matter where you fight the battle......unless, the soil under the slab is incapable of sustaining the weight of the chimney(which is unlikely).If that were to be the case your floor could sag with the footer on top. were you to cut a hole in the floor at the perimeter of the above slab footing then if it settled the footing would merely settle through the floor causing no real damage. Cutting a hole and digging out under is ridiculous unless you don't want to look at the footer on top of the slab. My guess is that you don't really need to do anything and you certainly don't need to dig through the floor. FWIW, Im a builder of 30 yrs and do all my own concrete work as I don't trust anybody with my foundations and we guarantee all our erections :oops:
When faced with a seemingly impossible task, my grandfather always said "can't never can, untill try comes along"

bsteckel
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Post Sat. Oct. 19, 2013 4:43 pm

thanks for the quick replies .. I have no idea of the weight of a 15+ foot single flue chimney ... was hoping to not have to saw cut a piece of my new floor out ... I have no problem pouring another "slab" on top of my floor .. but it sounds like that may be overkill anyway...

Rigar
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Post Sat. Oct. 19, 2013 7:16 pm

bsteckel
just my two cents:
masonry chimneys when built correctly are extremely heavy.
they typically require their own footer...and for good reason..
your floor is very strong.... but has no where the bearing capacity of a foundation.
my biggest concern to you would be ( as previously mentioned) ... frost protection.
... another reason to keep it independent from your floor.
....would it work built on your floor??- probably...this is just someting to consider.
...I am certainly not gonna go preaching building codes .... :lol:
...but there is a reason for footings...

...good luck on this project!
....'Rigar

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Lightning
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Post Sun. Oct. 20, 2013 3:37 am

I'm no mason, but I would also be concerned about the weight of the chimney if built on the floor. Do a rough estimate of how much it would weigh. You can probably get this information online. Add up the weight of the flue tiles, blocks and mortar needed. This might help you decide on how to handle the foundation.. You definitely wouldn't want your chimney to crack the floor and start unevenly sinking though it. There would be no fix other than tearing it down and starting over. That's my opinion :D

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Freddy
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Post Sun. Oct. 20, 2013 6:25 am

If the chimney is going on a corner or outside wall I might have consideration, but if it's inside the slab I'd build that bad boy right on the floor. Ya, chimneys are heavy, but a single flue only 20 feet tall, it's not THAT heavy. The most it's going to weigh is 200 lbs a foot....so 4000 lbs for a 20 foot chimney. You say the concrete is 4000 PSI, so mathematically, one sq inch will hold the weight. In real life it might take 6 or 9 sq inches! I think you are plenty strong enough. As for frost, in my opinion, if it manages to move at all, it will go up, it will come down. No problem. There's no way you'd talk me into cutting the floor....not unless you are building a full size double flue fireplace 35 or 40 feet tall.

Just my opinion! I'm not an engineer, but I have built a few things, including a 16 foot chimney on a slab. ( on an outside wall, but that one does remain heated)
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Rigar
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Post Sun. Oct. 20, 2013 8:05 am

ok.....psi is psi - BUT:

.....concrete floors and concrete footings are designed for different stresses.
most footers are designed to withstand tension stress (not snapping)
are typically poured with concrete of a lower PSI then most floors.
your floor may have. a COMPRESSIVE strength of 4000 psi....but at 4-5 inches thick will fail to tension at. 10% of that.

AGAIN.....building on ur slab may be fine...but just wanted to share what the difference is....basically this is why footers are THICK....with re-inforcement. in the lower half of the pour....to resist the forces of tension (snapping)....not compression
....'Rigar


waldo lemieux
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Post Sun. Oct. 20, 2013 8:20 am

a vertical load on any horizontal concrete will cause compression on the top surface and tension on the bottom ,footer and or slab period.
When faced with a seemingly impossible task, my grandfather always said "can't never can, untill try comes along"

Rigar
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Post Sun. Oct. 20, 2013 8:40 am

waldo lemieux wrote:a vertical load on any horizontal concrete will cause compression on the top surface and tension on the bottom ,footer and or slab period.
...true

...so do you typically pour footers 4 inches thick?
im sure you dont...
the reason is they are designed to withstand different stresses (and are usually LOWER psi and larger aggregate than floors)
...like I said earlier- this will probably work in his situation...but wanted to shed some light on different stresses.....and the reasons behind designs
....'Rigar

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Wiz
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Post Sun. Oct. 20, 2013 9:02 am

I had masonry chimney placed in work shop without pad or footer. Note location in corner does have footer for both blocked walls. Installer did say if it wasn't for placing in corner, he would've place pad on top of floor. Gentleman has 45 yrs of experience who installed mine. The crack that you do see in picture was there for years, after adding chimney crack hasn't gotten bigger.
20131020_084716.jpg
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Freddy
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Post Sun. Oct. 20, 2013 9:10 am

waldo lemieux wrote:a vertical load on any horizontal concrete will cause compression on the top surface and tension on the bottom ,footer and or slab period.
Very true... and that's why rebar is often added to concrete floors. When you put weight on top & try to "fold" the floor, the rebar gives tension strength and makes the concrete above the rebar want to compress, and it can't. I like to put the rebar in the middle, then if frost pushes up, it works in reverse. The writer mentioned it was fiber reinforced concrete. Maybe not as strong as rebar, but the fibers try to do the same thing... make it difficult to "fold" the concrete.
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bsteckel
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Post Sun. Oct. 20, 2013 9:16 am

thanks to all for the replies ... alot of good info going on here. Wiz, I was intending to use the mason you used, but they are so busy that the calendar is full until next year .. Most likely because they have very good pricing.

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LoschStoker
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Post Thu. Nov. 14, 2013 10:09 pm

A: 2-post truck lifts (under 12,000 LB. capacity):
Four (4) inches of concrete at 3000 PSI

And you won't be working under your chimney :D
If it makes you feel better pour a 3 1/2" slab on top of your floor for the chimney.
A chimney isn't hard to build, one block at a time. get chimney blocks 7"X24" round liners, Portland cement, perlite or vermiculite, 9:1 ratio
mortar. Chalk line your wall for the corners of your blocks. Mortar the liner ends just enough to level them.

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Carbon12
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Post Thu. Nov. 14, 2013 10:37 pm

Not sure what you plan to burn but have you considered a metal chimney?
No matter where you go,......there you are.


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