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Best Gravel Under Slab-on-grade Foundation

idahobuild | Posted in General Questions on

Hello all,
Slab-on-grade, 2100 sqft residential build question.

What would be the recommended gravel for 1) easy to level/screed after plumbing install and 2) facilitate passive radon mitigation?


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  1. plumb_bob | | #1

    Washed drain rock with no fines is your best bet for the material under the slab as it is free draining and requires little or no compacting (depending who you ask). It also complies with soil gas/radon venting requirements. On top of the leveled drain rock goes your poly sheet, which is sealed at all edges, joints and penetrations.

  2. dfvellone | | #2

    3/4" crushed and washed works great. Very easy to work with and will compact well.

  3. Expert Member
    Michael Maines | | #3

    Another vote for 3/4" washed crushed stone. Some people make a point that it doesn't compact much, you still want to run a plate compactor over it to consolidate the aggregate.

    After using Glavel under the slab on a recent project, but above a layer of crushed stone where we located the radon system, I want to try using Glavel for all fill. It supposedly remains gas-permeable so it would work for radon mitigation.

    If you're in a place with more sand than stone, sand also works, as will other sizes of aggregate--the key is that whatever size the aggregate is, the pieces need to be a similar size and shape, with no "fines" (silt or clay) which can clog the drain pipes.

  4. idahobuild | | #4

    Thanks for the input all!

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