Fireplace footings

[czechsonofagun]

I will be adding a fireplace to our back porch. The floor is 20" above the ground where the fireplace is going to be. I want real fireplace with masonry chimney.

Now the question of footings. In our area the freeze depth is 24", I was going to dig out 30" deep and pour 12" thick footings with rebar - 12" is from Rumford book. Now what?

- I could build it up using concrete blocks, fill it with gravel and cast concrete platform on top. As of now, this is my plan.
- I could build it up and leave it hollow and cast platform on top. Question is how to access it, how to support it and why in the first place. It would hold a pot of gold maybe and not much more. In my country the gold is hidden under the stove in many fairy tales
- I could fill the hole with 22a (rock with stone dust) and compact it and cast concrete platform on top of it - level with the ground and build from that up. I am not sure how much I can compact it with hand tools.
- I could just make it a concrete block from 30" under to 20" above - quite crude, IMHO.

Suggestions, experience?

 

[mbohuntr]

Just a thought... If you fill it, and water gets in there and freezes? Whichever way you go, I would make sure that water cannot get trapped in there.. Even a sloping bottom with a weep hole and backfilled with #2 so it can't heave..

 

[Einthewoods]

Here is an interesting article that you might help. Probaly not the answer you are looking for, but...

Scratch-built Masonry Fireplaces | brick.com

"All void areas within the body of the fireplace from the foundation through the chimney should be solidly filled with masonry mortared in place."

If it were mine, I probably would have built it with block and filled it with sand, compacted it and then concrete (with rebar) over it to form the base on which I would have built the hearth.

 

[jfh28]

typically if you can access the void below the fireplace you leave it hollow as an ash dump and put a cast iron door to remove the build up of ashes......that doesn't sound like it will be possible in your case so filling it or leaving it hollow will probably be ok......the key is you will need to build a reinforced concrete pad as your fireplace hearth.....you can refer to details in the book "architectural graphic standards".......do not rely on compacted fill to build your fireplace hearth......and if you are in a n area subject to freezing then check with your local building official or local masons to see what they normally do..........Jack

 

[PILOON]

I'd be very tempted to simply use 3 or 4 sonotubes (reinforced with rebar) mounted on a suitable base.
Then for the fireplace create a reinforced base cast over a form.

Another option could be a 3 sided block wall again with a reinforced 'table top' to build the fireplace on.*
Think along the lines that you could have a door and use that area as storage for garden tools or whatever.

*This is exactly what I did only inside my house. My 'wall' protruded the living room floor onto which I built a fieldstone conventional fireplace. (read real heavy)
In the basement (family room, actually) is a slow combustion wood stove encased in the cavity.
LOL when the insurance inspector visited he was critical of the clearance between the stove and the walls. LOL once I rapped on the solid cement enclosure he passed me OK. That was 23 years ago and still works just great.
In my case there was a huge rock cap onto which to pour the base.
(actually my whole house is on solid rock, It'll never move!)