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Community and Q&A

concrete-free basement slab gravel size

maine_dorothy_2 | Posted in Green Building Techniques on

Our new Maine home (climate zone 6, ~1375 sq ft footprint) is being built with a concrete-free basement slab. (8″ layer gravel, 2 layers of 3″ EPS foam board with vapor barrier between foam layers, then 2 layers 3/4 ” Advantek plywood).  Due to a misunderstanding, a 6″ bottom layer of size 2″ crushed gravel has been put down instead of 3/4″ crushed gravel. The top 2″ of planned smaller gravel has not yet been put down. Excavator wants to put 2″ sand on top of textile on top of the size 2″ gravel. Does anyone have recommendations re best choice for the top 2″ layer yet to be added, especially in light of having 2″ crushed gravel as base instead of 3/4″?
Any significant concerns re the already-laid 2″ size gravel layer?
Thank you.

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  1. maine_dorothy_2 | | #1

    bump. any thoughts? thanks

  2. onslow | | #2


    I will venture that a key concern is how secure the drainage is. My own basement foam sits on 3/4 washed rock set over bedrock that carries spring melt water under my slab. Fortunately at a fairly fast pitch. The word gravel to me has always meant very coarse sand-like material as I grew up near Maine in an actual gravel pit. Not active after my parents built our house, but the hillside was a lovely three layer gravel bed left by the glaciers retreat. Given you said 2" gravel I will first assume it is what I called washed rock with no fines.

    If the crushed material IS interspersed with fines like road bed material, it will hold more water than I would be comfortable with unless the ground conditions are very dry. Adding a two inch layer of sand especially over fabric will enhance the risk of excessive moisture building up under the foam layers if any ground water infiltrates the gravel. That would not spell a happy long term condition.

    Simple way to assess the water holding capacity of whatever material is currently there would be to put some in a large colander and weigh it, then pour water through it, wait a short bit and weigh again. You can do the same with the sand if placed in a cloth bag first. Starting with dry sand first of course. A luggage scale could be a handy thing to have.

    I would tend to put washed pea stone over fabric if topping the relatively rough 2" layer is the only choice left. If it is meant to provide a flatter and gentler surface for the EPS to rest upon, pea stone will be fine just a bit harder to rake level than sand. I would also question how one compacts 2" stone, which would have been one of the steps if it had been 3/4".

    The pea stone will not be easily able to infiltrate through the fabric if small stress openings are created by the lumpy first layer. A sand layer over fabric would be a higher risk scenario. Settling in either layer would be bad. A bit of settling under a concrete slab over foam would not telegraph upward. With foam and plywood as the only "slab" I would be concerned that overtime you could see a dip develop in some areas. Hard to predict. Will the fabric hold tight over the very lumpy first layer? Time will tell.

  3. maine_dorothy_2 | | #3

    Thank you Onslow for your thoughts. Yes, It is 2" washed crushed gravel without smaller sizes and without fines. The excavator did use a compactor on it. But even though compacted, it remains very rough. I like your idea of washed pea stone (~1/4" ?) over fabric. If it is easier to get crushed 1/4" minus gravel, I am thinking that would also be okay. Do you agree? Dorothy
    We have many sand and gravel pits nearby; our new building site has many surface glacial erratics (not huge ones) but current nearby home has only sand and is next to a long glacial esker.

  4. maine_dorothy_2 | | #4

    also, it is not an unusually wet site

  5. steve41 | | #5

    Hi maine_dorothy-

    I'm also in Maine and currently building a concrete-free raised slab. I'm not a pro, so take my feedback as such:

    I've been very impressed with the strength/integrity of the heavy grade non-woven geotextiles. If it were me I would:

    1. Put a small layer of 3/4"(?) washed stone on top of the 2" stone, a minimal layer to fill in some of the voids in the 2" stone near the top. Rake it in and compact without raising the current level. The goal of this would be to fill in some of the larger voids so that the top layer would be better supported by the geotextile and less apt to settle.
    2. Add heavy grade non woven geotextile
    3. Add fine stone layer, level, compact.
    4. If needed due to compaction, add more fine stone and recompact

    With that said I do think you'd be fine without the first step, it's just added security.

    Best of luck with your project!

  6. onslow | | #6


    I think Steve41 has the ticket for your situation. Glad to hear that the base fill was compacted to whatever it could be. At least the risk of settling into the soil may have been accounted for. I would have wetted the soil under first and then compacted to ensure maximum stability. But then I tend to over do.

    Evening up the surface with some 3/4 washed rock will help reduce the stress on the fabric. Do use something tough enough for the job. The fabrics for covering septic field infiltrators would be a good place to start. You will only get one shot at this. Add the pea stone and grade out. I would add a layer of heavy plastic film over the layer of pea stone before doing the EPS. Tape it off at the foundation perimeter. I would add a skirting of equally durable plastic film around the interior perimeter that extends above the final floor height. Even with capillary breaks from the footing there can be a certain amount of moisture rise in the foundation walls. Best to keep the foam and floor as isolated as possible.

    Remember a "not unusually wet site" may briefly become on later. Belt and suspenders can often prevent a loss of pants.

  7. Expert Member
    Michael Maines | | #7

    Ditto what Steve said. The top layer could be finely crushed stone or angular sand.

  8. andyfrog | | #8

    Would glavel have any application here?

  9. Expert Member
    Joshua Salinger | | #9


    The first layer of gravel laid down over the undisturbed soils is the bulk water control layer and it is critical that it not have any fines in it. It sounds like this is the case. This layer is also the soil gas depressurization field for radon piping. 2" rock will work fine as this will stop capillary action from wicking liquid water up towards the building. When we have built this, we used a layer of landscape fabric over the gravel with no fines and then used 1/4- crushed gravel over the top of that. The fabric keeps the fines from filtering into the larger stone below. This layer is screeded level, but not compacted as we use 2" which just won't stay compacted. I suppose one could compact this if one had enough grade to allow for a thicker layer. Suffice it to say we haven't experienced any settling with just the 2" layer.
    You will want to set all the foam over this 1/4- gravel, ideally staggering the seams of the foam as it is prone to shrink and the gaps could reflect through and could also lead to minor thermal bridges.
    I would put the plywood/Advantech directly on top of the poly sheet. If there was a leak, tub overflow or flood, you wouldn't want the foam to sit in the water as it would take some on and take a long time to dry out leading to potential mold or smells. If the wood was directly on top, the hygric redistribution of the sheet good will wick moisture to the surface and allow for better drying.

  10. maine_dorothy_2 | | #10

    thank you everyone for your input. this is all very helpful and shows us the way forward

  11. onslow | | #11


    I would still recommend a primary vapor blocking plastic over the final gravel/stone layer AND another directly under the Advantech. The EPS may shrink a bit, but being "indoors" it won't experience the temperature swings of an exterior wall or under shingle location. The chances of keeping the plastic sheeting under the Advantech perfectly intact are low. Moisture rising from below should be your primary concern. I does not take bulk water for the base layer to be relatively humid.

    You do have the advantage of negative pressure in the lowest layer if the radon system is meant to be on 24/7. Between the plastic under the Advantech and under the EPS there should be a minimal risk of pulling air down from the living area in large enough volume to create a condensation problem. The gaps created by foam shrinkage would be insignificant thermally.

    1. Expert Member
      Michael Maines | | #12

      Why do you say the chances of keeping the vapor retarder intact are low? If you're using 10-mil or higher, it's pretty hard to make holes in it, and with it being carpentry instead of a concrete pour, you can see when you've made a puncture and you can fix it.

      The reasons to go concrete-free are mainly to reduce cost and carbon emissions. Adding a second layer of plastic works against both of those goals.

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