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Is stone necessary under a basement slab if there is sand?

GibsonGuy | Posted in General Questions on

With my new construction, I asked my excavator to bring me a load of stone to place under my concrete basement slab.  He said it wasn’t necessary because there was at least a foot of sand below where the footings will be placed. Is that acceptable? Thank you in advance.

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  1. Expert Member


    Compacted sand is a good structural substrate, but the stone top-dressing directly under the slab (or insulation) has a different function. It is there to act as a capillary break so that moisture can not wick upwards. I'm not sure I'd call it necessary, but it is certainly best practice - although not used in many areas of North America, so your contractor may not be familiar with it.

    1. GibsonGuy | | #2

      Thank you. I too was thinking sand might be acceptable, but not best practice.

  2. Expert Member
    Josh Salinger | | #3

    I agree with Malcom and will add that the other big reason for the crushed stone (with no fines) is to create a soil gas depressurization field. Radon, for instance could be an issue and the stone allows for this to be evacuated.


    1. Expert Member
      MALCOLM TAYLOR | | #4


      Good p0int. I forget about radon, being fortunate to build in one of the few areas where it isn't a problem.

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