# What kind of stone under concrete slab?



## SubSailor (Feb 25, 2008)

I am getting ready to have a foundation poured for a garage this summer, I am new to foundations so I asked my building inspector what kind of stone I needed under the slab and the only response he would give me was "You figure it out" It would have taken him less effort to list the stone, but thats for another day.

Anyways, help is appreciated, I have come across a dozen different reccomendations on the internet and I don't know which is best.


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## concretemasonry (Oct 10, 2006)

You do not want "stone", whatever that means in your area. Clean stone does not compact at all unless it is dirty or contaminated with finer materials.

You want a compacted base that is similar what is used under roads and driveways. He was correct in not giving you a specific material, but could have given you a specification to use when you buy or contract it.

Is it a "floating foundation" with thickened edges or is it a foundation of a stem wall on a strip footing with a true floating slab between the stem walls. Those are two distict types with plusses and minuses depending on what you climate and needs are.

You asked him a specific question for a material that is acceptable without any regard for how it performs or is installed. There is no one answer since you obviously do not have a real design/specification.

Dick


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## vsheetz (Sep 28, 2008)

I have built a couple threee workshops and garages - all being in southern and non-freeze areas I put down a monolithic slab - with packed in sand for a base on undisturbed ground after the grass or any other organic matter that would decompose was removed.


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## jomama45 (Nov 13, 2008)

What you want is what we would refer to here as gravel, a mix of gradients of aggregate, from 1" stones down to fine material such as non-washed sand. A few viable options depending on what's it's called in your area: "3/4" road gravel, recycled/ground concrete, 3/4" traffic bond, etc..."


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## SubSailor (Feb 25, 2008)

concretemasonry said:


> You do not want "stone", whatever that means in your area. Clean stone does not compact at all unless it is dirty or contaminated with finer materials.
> 
> You want a compacted base that is similar what is used under roads and driveways. He was correct in not giving you a specific material, but could have given you a specification to use when you buy or contract it.
> 
> ...


I asked him a specific question with him knowing the specific type and size of foundation, location and drainage attributes. He chose not to answer it.

it's going to be a pier slab system, 6" thick slab poured together with 12" dia. footings about every 6 feet.


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