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Community and Q&A

Slab on grade – Sand backfill

sethery05 | Posted in General Questions on

I’ve read all or most of the article and posts talking about how sand is bad to have under a concrete slab for various reasons. I am looking at a slightly different detail where the sand is the lowest layer rather than directly under the slab as discussed in the other articles.

I am building an elevated slab on grade house. The finish floor is about 30″ above the existing grade and I need to backfill the foundation with roughly 18-24″ of material. I will be pouring a 4″ concrete slab over 6mil vapor barrier which is on top of 4″ EPS foam. I was planning on 4″ of washed stone under the EPS foam (for radon purposes). I was planning on using sand for the lowest layer and compact it in 6″ lifts. The only reason for using sand in this case is I can get it very cheap compared to crushed stone with fines or other washed stone materials. I’m concerned that the washed stone may migrate down into the sand or vise versa have the sand migrate up into the washed stone and cause settling of the slab. Am I right to be worried about this or is the sand ok to use?

As an alternative detail I do have a lot of extra woven road fabric I could use between the layer of sand and washed stone to prevent these from mixing?

Thank you,

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  1. GBA Editor
    Martin Holladay | | #1

    As far as I know, sand can be used as a base. Using a layer of road fabric or landscaping fabric between the sand and the crushed stone layer makes sense to me.

  2. sethery05 | | #2

    Thank you Martin!

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