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Crush and Run Instead of Gravel Under Slab

mikeysp | Posted in General Questions on


I am in Zone 4a.

Can I use Crush and Run under concrete instead of 3/4 gravel?

I am able to get Crush and Run for half the price of 3/4 unwashed gravel. The Crush and Run is on discount right now.

Thank you!

– Mike

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  1. VA_Engineer1 | | #1

    The problem is the crush and run - or "crusher run" as I've heard it called - is compact-able - and 57 stone - gravel - naturally is compacted when it's dumped/spread. so you have the chance of settlement under/of the concrete and or cracking of a slab with a compact-able sub-base - unless you are able to fully compact it in lifts - layer by layer.

    what is the concrete for? floor slab? driveway? garage?

    1. mikeysp | | #2

      Thank you! It is for a slab for my house, so I guess I can't use it.


    2. Jon_R | | #8

      AFAIK, the only gravel considered self-compacting is pea gravel.

  2. mikeysp | | #3

    If I use unwashed 3/4" is that ok? It is $50 less than washed, but I do not want any structural issues with my slab. I am doing all the prep work and hiring a concrete guy for the pour/finish.

  3. Expert Member


    Filling y0ur whole foundation with clear-crsushed is a waste. No one i know here does it. Crusher run compacts easily with a bit if diligence, and is much easier on your buried plumbing. The reason to use rock is to provide a capillary break, so it's a good idea to top dress the 6" below the slab with a drainable fill.

    1. mikeysp | | #5


      I had assumed 4" was sufficient. If it needs 6", I will comply, but at $675 per 22ton load, it hurts. :)

      Also, when you say - [top dress the 6" below the slab with a drainable fill] - you are talking about the 3/4" gravel between the ground and the concrete, or in my case between the ground and Fomular 250 25 psi foam and then concrete. Of course, plastic between rock and foam.

      I spoke with a contractor and he told me don't waste the $50 extra for washed rock.... but, I like to confirm with guys who are committed to best practices, so I do not make a mistake.

      Thank you.


    2. Expert Member
      BILL WICHERS | | #6

      Malcolm, would you advise the use of a layer of geotextile fabric between the layer of crusher run and the layer of gravel to prevent mixing over time? Just a thought...


  4. onslow | | #7


    IMHO you would do better to use the washed 3/4 as your topping over other fill. The fines left with unwashed 3/4 might help hold any moisture collecting below. Not sure how you are embedding utilities below the slab, but extra moisture is seldom helpful long term. Local water table conditions should probably dictate your choice. If you have a radon situation that requires venting, then washed 3/4 would also be a better choice for best flow and reduced infiltration of the piping should you ever have a flooding event that raises the ground water under the slab.

    I suspect the actual per ton cost of 3/4 washed is not all that much greater than unwashed. The trucking and placement costs likely exceed the net cost of the stone. In my particular situation, I DID fill the entire foundation with washed 3/4 rather than have a base with 4" topping. Wanting to pre-install radon vent pipes was one reason, the other was net cost of all stone vs. soil that needed to be compacted and leveled before topping with stone. If I recall correctly, the difference was going to be only $3-400 due to the much higher labor costs of placing, compacting and leveling in lifts thanks to the very sloping bedrock I am built upon.

    The ground water here runs along the top of the bedrock if present, so the chances of having water infiltrate under the slab via rock fissures is quite high. Having 100% 3/4 washed rock ensured that all the water entering under my floor slab would be able to migrate to the lowest corner where the exit drain is placed. (In addition to the full perimeter drain system.) A truth pipe I set in the corner above the exit drain has proven my suspicions about water getting inside the foundation. If I had gone with compacted fill I rather suspect the water would create a giant wet sponge under the slab/gravel. Perhaps not harm done, but I am glad to see the drain working to keep things as dry as possible down below.

    Last note, I put down recycled 25 psi XPS under the slab and on top of the radon pipes in the rock bed. The plastic went down next, not under the foam. I think that is the more common placement. I found the net re-inforced plastic from HD to be quite tough and able to stand up to the many trips across it during pex and mesh placement. Local habits may differ.

    I used polyester fiber additive in my garage slab, which is still perfect. It may not be compatible with certain floor staining treatments, but I do not have direct experience. Proper welded mesh or rebar, seated for height will go a long way to secure a stable slab. Everyone will note that concrete will crack, so plan on where it will crack with relief cuts or grooving the pour. Even in the milder parts of the country, drying too fast will affect the final product, so be ready to mist or cover the slab with fabric to hold moisture steady as it cures. Do note that the fabric will leave traces of its presence, so might nix that if you are staining the slab for flooring.

    If the slab surface will become your flooring, then plan for rain to pay a visit the moment it is all floated out. Mother nature loves to be a pain. How to manage that is above my pay grade.

  5. maine_tyler | | #9

    >"Of course, plastic between rock and foam."
    I believe the recommended order of operations is, from bottom up: crush, foam, plastic, concrete.

    Hopefully no-one here is confusing washed stone with 'clean' crush stone. Clean, can of course mean different things, but sometimes simply means it's been screened/sorted. It merely has the fines that sticks to the stone after the screening process. Washing is another step entirely that I believe uses water to (no surprise) wash the stone.
    I too am curious what is standard/best practice, but I suspect most people don't use the fully washed stuff. Not around here anyway; it's hard to come by.

    I don't see the fines in unwashed crush presenting problems with moisture retention so much as migration and clogging of filters, if those are in play.

  6. CMObuilds | | #10

    Is this for slab on grade or a basement?

    On grade absolutely can use a compatible fill, you slope away from the pad anyways so you dont need the function of the pea stone.

    If its a basement you could, but you could get yourself into trouble down the road. We have put basement slabs over sand when we are in the river valley and we know the water table is much lower than our slab. Most often we use pea stone in a 8” fill inside the basement and around the exterior drain tile at a 45 deg angle up against the basement wall with filter fabric over the stone, then a sand backfill.

    I had a client who had water wicking up the center support wall in a basement so we dug the exterior perimeter, all the drain tile was full of fines and when the water table rose the water sat under the slab and wicked up through a control joint. You could hear water gurgling in the radon pipe. All those fines will clog everything up in a hurry in certain situations.

  7. mikeysp | | #11

    I am building a slab on grade house slab. The crush n run is about $325 a load right now and the 3/4 washed is about $700.

    1. CMObuilds | | #12

      I'd save yourself the money you're going to have to compact it either way, you can put sand down if you wanted to save even more money.

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