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Community and Q&A

Sub-slab drainage stone size

JTyler | Posted in GBA Pro Help on

I know the typical recommendation is 3/4″ no fines. Foundation contractor and stone supplier are recommending I use 1.5″. Foundation contractor is trusted and has used product a great deal including under some very dry basements that I’ve been in. Stone guy says he has 3/4 no fines – but it’s more money for no gain. I have seen the product they are calling for; it’s a mix of ragged and round stone that would surely drain very well. I can tractor stone in, so will be able to minimize trying to stick a shovel in it. SDR 35 interior footing drains. Any reason not to follow their recommendation?


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  1. GBA Editor
    Martin Holladay | | #1

    The main reason I prefer 3/4" stone is that it can be moved with a shovel or rake. If you are very good at leveling stone with your tractor, bless you. Not everyone is that good with a tractor.

    I always end up filling low spots and raking high spots by hand.

  2. Expert Member

    Has your contractor used it under foam? I can see it working well directly under a slab, but levelling 1 1/2" fill so that foam lays without voids underneath sound like a difficult job. 3/4" clear crushed interlocks into an almost flat surface. larger rock, especially if it includes round stones, might make this difficult.

  3. JTyler | | #3

    Thanks for the input guys.

  4. davidmeiland | | #4

    It would be incredibly tedious to try to level 1-1/2" with a small machine or by hand. You could get it flat using a road roller, but that seems unlikely for a residential slab. We use 7/8" clean and it can be done fairly easily by hand raking and a plate compactor.

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