Concrete slab for an outdoor fireplace

[gvanwage]

Hey y'all. I am going to be pouring a concrete slab to build an outdoor fireplace on top of and need some help from any concrete guys out there. The dimensions for the slab will be 6'x4' and 12inch deep to support the weight.

Here is my question... I have heard that you should put down a sub-layer (sand, gravel, drainage stone) for the slab to rest/float on however, I have also heard that with something this thick the sub-layer is not necessary. I was told by one guy that you can pour a couple of piers under the slab but the sub-layer is not necessary and you can pour strain onto the dirt. Is this true or should I put down some subgrade?

What does the green screen suggest for this question?

[glen]

Mine is on a 4 inch slab and then elevated and use different concrete mortar and bricks for fireplace. Sorry no help but know you have to use fire materials for the actual fireplace

[manwitaplan]

What PSI concrete are you pouring 12” thick. Even 3000 psi at 5 1/2 “ is kinda overkill for a fire pit. You will want your sub grade as compacted as possible.

If you wanted piers in the corners you could take post hole diggers and add some piers.


Sent from my iPhone using Tapatalk

[Jnorth]

What kind of dirt is your sub grade?

[gvanwage]

What kind of dirt is your sub grade?

The sub grade is clay.

[gvanwage]

What PSI concrete are you pouring 12” thick. Even 3000 psi at 5 1/2 “ is kinda overkill for a fire pit. You will want your sub grade as compacted as possible.

If you wanted piers in the corners you could take post hole diggers and add some piers.


Sent from my iPhone using Tapatalk

It won’t be for a fire pit. I’m talking a full on outdoor masonry fireplace with 12’+ chimney.

[ClayW]

It won’t be for a fire pit. I’m talking a full on outdoor masonry fireplace with 12’+ chimney.

With that size of structure and your clay soil, I'd recommend putting some engineering into it

[Blank]

Yeah, at an average of about 40# psf for your brick, you could be looking at a couple tons of weight. It seems to me your base might be a little small, and I would consider enlarging the square footage. 4'x8' at 5 1/2" with re-mesh or re-bar would easily support all of that .

[mjhaverkamp]

Make that slab about a foot wider and deep than what your fire place dimensions are, I would pour an 8 inch slab with two piers at least 6 foot deep and 18 inch diameter, or you could get it engineered, lol. Last thing you want is a leaning fireplace,

[iamntxhunter]

First you need to figure out how much weight your fireplace will be. The footprint of your slab that it will be built upon is most important in order to spread the load.Most clay type soils can support 2000 psf but it needs to be compacted with the right moisture level for long term stability. Having the right moisture level will prevent excessive swelling of the soil when it rains.
There are resources online that can help you since you know the specific size and can figure out the weight. A 6" thick slab with rebar and just 3000psi concrete can support a lot of weight.

[BlackoutRam2500]

Are you having settlement issues around your house slab? If it were me I would pour it like a regular slab. I would do 12" grade beam around the perimeter where the base of the fireplace will line up and another beam down the middle with a #5 rebar tied 8" O.C. and pour the slab 8-10" thick.

[Mike D]

When they poured mine it was slightly larger than the footprint and 24” deep. The back corner was sitting on a pier.


Sent from my iPhone using Tapatalk Pro

[IAFFHunter18]

I just built one about a year ago, full size all masonry and my stone/brick layer who has built many of these had me pour a 2’ deep slab and tied it in with rebar. May be over kill, I don't know but better to be safe than sorry.

[ON THE EDGE]

We drive fire trucks on 12” thick double rebar matte concrete driveways. We always recommended 6” thick 1/2” rebar for commercial driveways. Have a hard time believing you have to go that thick, but obviously you can see there is a significant amount of opinion on the subject. The one thing I recommend no matter what is to call before you dig unless you know 100% where every gas, water, electric, cable, sewer, and other lines are on your property. Better to be safe than sorry! Post up pics when you’re done!!

To answer the subgrade question.......clay will always swell and contract. The advice on what you home does now will help if you know if the builder put a sub grade. Soil testing and engineering is the only accurate answer to what you need as a subgrade. But if you want to keep it simple, dig it deep, drop in some piers, put some cushion sand, use an overkill amount of rebar, and pour it solid. Hard to mess that up

[ClayW]

We drive fire trucks on 12” thick double rebar matte concrete driveways. We always recommended 6” thick 1/2” rebar for commercial driveways. Have a hard time believing you have to go that thick, but obviously you can see there is a significant amount of opinion on the subject. The one thing I recommend no matter what is to call before you dig unless you know 100% where every gas, water, electric, cable, sewer, and other lines are on your property. Better to be safe than sorry! Post up pics when you’re done!!

To answer the subgrade question.......clay will always swell and contract. The advice on what you home does now will help if you know if the builder put a sub grade. Soil testing and engineering is the only accurate answer to what you need as a subgrade. But if you want to keep it simple, dig it deep, drop in some piers, put some cushion sand, use an overkill amount of rebar, and pour it solid. Hard to mess that up

This^^

My main concern would be the subgrade.