How much foundation?

[chipster96]

I'm putting in a fireplace and brick chimney against an inside wall in an already existing house. The floor is 5" slab, that's all I know for sure.

I want a sense of whether the existing slab will be enough foundation for the chimney.

I believe the completed system will be under 2 tons.

Will the current slab:

A. easily be sufficient
B. require another concrete pad of some sort on top
C. can't do it. Gotta cut a huge whole in the slab, dig down and deposit multiple feet of concrete.

I was thinking of adding another layer of concrete where the chimney will go Say three inches, with horizontally laid rebar, to distribute the weight a foot or so further out.

 

[Highbeam]

The official and proper foundations are far more substantial than a 5" slab. So not A.

 

[John_M]

Chipster, If I had to make this decision I would select a modified version of (C). Here's why: The current slab might have been poured so it is resting on the footer of the foundation. If so, there is a chance the ground under that part of the slab where the fireplace foundation will be located has settled over the years. That portion of the slab might be supported by the footer only. Placing a heavy chimney on the slab over that voided area would likely eventually collapse the slab at that point. Even if the slab is not poured on top of the footer and is poured only within the area defined by the footer there could still be a void under the slab at that point. The only practial way to determine if the slab is still resting on compacted or undisturbed soil is to cut a hole and look around down there.

About five years ago I had an architect design a new house for my return to NY. We have fairly strict code requirements in this central part of NY and I wanted his design to exceed these engineering requirements. You might ask why these strict code requirements should be exceeded. Here's why: While living in Alaska and Idaho I saw too many new houses with collapsed roofs after a sequence of snow, melting temperatures, freezing temperatures and more rain and snow. The loads on these roofs were too much for even the most current code requirements.

My new house has a major support beam running the full length of the house. This beam removes any obvious vertical movement from the floor. The two end walls and four evenly spaced steel posts support this beam. Each of the posts required a poured concrete pad 3' x 3' x 12" deep for proper support. Six pieces of 5/8" rebar were placed within each of these forms. Three pieces of rebar were evenly spaced, installed about 4" above the (bottom) of the pad and pointed north to south. The other three pieces of rebar were evenly spaced, placed about 4" below the (top) of the pad and faced east to west. The four inch floor/slab was poured on top of these support pads. So, there is 16" of concrete supporting each of these posts. You might want to use a modified version of this design as your minum starting point.

Be aware that I am not a structural engineer or a codes enforcement officer. My purpose in the post is to give you a starting point in building a proper support pad for your new fireplace. It is usually more practical to take the extra effort and expense to do a project like this correctly at the beginning. Having to redo this job after the chimney is in place might prove to be inconvenient, at best.

Best Wishes and good luck with the project.

John_M

 

[Highbeam]

Maybe the first question is why on earth would you want to build a masonry fireplace and chimney? On the outside wall that setup is about the worst case scenario for efficient heating. Is it to be a decorative fireplace? Do you intend to install an insert into it to try and make it a heater? The only really good masonry chimneys are in the middle of the house and are built as a "masonry heater" and not the traditional american fireplace which actually sucks more heat out of the home than it makes. They do look pretty if that's your intention.

I ripped out a masonry fireplace and chimney to install my freestanding stove and have never regretted it.

 

[TreePapa]

The correct answer is (D) consult your local buidling codes. If no local building codes, consult IBC / UBC (International Building Code / Uniform Building Code). Also check with your homeowner's insurance. Likely answer is you will need a more substantial foundation UNDER the current slab. However, Highbeam's point gets to the heart of the matter. You can install a nice freestanding, EPA II woodstove easier, faster, and less expensive than a masonry FP. As for looking purty, check out the Jotul woodstoves; they're very purty. If you insist on a traditional FP, there are several "zero-clearance" EPA II fireplaces that can be installed without an additional foundation, and with wood or steel framing (less $$) rather than masonry. You can "face" the chimney with brick or stone or what have you if you want a traditional look. You'll get more heat, less pollution (smoke) and less weight, and still have what appears to be a traditional FP.

Yes, my sig says I have a traditional FP. It was in the house when I bought it.

Peace,
- Sequoia

 

[Bubbavh]

+3 get a free standing wood stove. Save yourself a lot of time money and trouble, and you'll be able to heat your house with it too.

 

[billb3]

Every masonry chimney I've seen around here had its own foundation, size relative to load.
Fireplace or no. Inside or outside.

Having had an outside chimney tilt because the foundation was not deep enough (below frost level) and on percolation troubled fill besides (150 year old chimney) a proper load base is kinda important.



I heated a beach house with wood with an 'typical outside wall American fireplace chimney' (not even a rumford) for three Winters . I don't know where this "net-loss' crap comes from. The Declaration of Independance wasn't writtten in a cave.

 

[Cutter]

Chipster I with you on your masonry fireplace. You might want to check out the New Aire systems. They are great and built close to you. Carl Junction MO. We heat a 4500 sq/ft house with tall ceilings with a madel 44. And the answere to your question is C. Four inches larger in each direction and 12" thick with plenty of 1/2' rod layed out in a mat. The New Aire allows you to direct your heat through ductwork it that is a possibility in your situation.
Brad

 

[Cutter]

One more item I forgot to include is that in todays fireplaces you always ducted in combustion air from the outside. No net loss
Brad

 

[EatenByLimestone]

When I put my chimney in I had to dig down below the frost line and pour a 1 foot thick footer. I can't remember how far out to each side it had to go before I could start piling up blocks.

Given the choice again I'd go the metal route. It would be more expensive but go up faster.

Matt

 

[billb3]

I'll have to keep that net loss theory in mind when I want air-conditioning this Summer