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How to get slab details right for clay soil build

mikeysp | Posted in General Questions on

Hi everyone.

I live west of Nashville (Zone 4a) and will be breaking ground very soon on a 1300 sq ft ranch for my family.

I want to use a slab,and wanted to know what are some safe slab details but I have “heard” that the soil conditions are not slab friendly in this area: clay soil mix with small rocks. It is called chirt locally.

I thought a stiffened slab might be a good fit or perhaps an overdig slab and fill it up with creek gravel and rocks?

Hiring a guy to run a rod into the ground to determine compaction is not to spendy, but getting an engineer to design a slab is not something I want to add to my world record attempt at low budget, poor man’s building science techniques as my pockets are shallow and I am not going to take on any debt.

Local inspector does not require engineer for slab. He just built his own house on a slab. About same size house, but he followed typical slab on grade with just the edges and centerline dug to frost line for load bearing purposes and 4″ everywhere else. However, I do not want a nightmare later. Spending a  more on the slab that is overkill is better to my mind than paying a few thousand for an expert. The only cost in my mind is the materials as a week extra of my time is ok with me and I charge myself zero $$  🙂

I plan to have a few loads of chirt braought in also in order to elevate the site for excellent drainage as well, if I do the stiffened slab (waffle)

I thought a slab would serve well for a few reasons: Easily eliminate nasty crawlspace without need to make crawlspace part of conditioned space. Finish floor does not need any covering as I plan to stain concrete and it will save money.

I am doing 98% or more of all work myself on this house and have even scrounged a lot of materials already (roofing, siding, windows, cabinets, polyiso, light fixtures, etc..etc…).  I have built 70% of my last two homes, but was completely ignorant of building science for those houses.  I will follow a VERY budget minded approach to this build while working for a tight, well insulated, and properly ventilated build.

Here is the floor plan and roof will be built with rafters, no trusses.

I very much appreciate your advice or clarifying questions for a budget approach to getting the details right.



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  1. user-2310254 | | #1


    Have you talked to any foundation contractors? They should know what works in your area.

    Consider installing a couple of inches of rigid foam under the slab (see You might be able to source reclaimed foam for cheap, and your home will be more comfortable and efficient.

  2. Expert Member


    You may find this discussion helpful:

    Just out of curiosity: Does your house have cathedral ceilings and a structural ridge?

  3. joenorm | | #3

    Side question: are all clays considered expansive? Or is it a specific type that causes problems?

    1. Expert Member
      Michael Maines | | #5

      "Soils with smectite clay minerals, including montmorillonite and bentonite, have the most dramatic shrink-swell capacity"


  4. user-6184358 | | #4

    Spend a few thousand $ on a soils report for the site. They will come out and trench or bore and take soil samples with them to the lab and test them. Then give you recommendations of how to do a slab on grade foundation. It will save you the guess work and unnecessary cost. Then if it all goes wrong you will have some one to sue.

  5. JC72 | | #6

    This is a big issue in Texas. Try a Google Search, "Slab design in Texas".

    You'll get a lot of hits.

  6. Jon_R | | #7

    Keep in mind that soil doesn't expand/contract when you keep it at the same moisture level. And that non-differential movement typically isn't a problem.

    Edit: so I'd use a vapor impermeable underground roof around the house to a) deflect moisture well away from the foundation and b) reduce the amount of drying near the foundation. Put gravel + a vapor barrier under all of it and be careful with tree roots.
    Hopefully this sufficiently reduces changes (over time) and differentials (location) of soil moisture content.

  7. mikeysp | | #8

    Steve Knapp, I have not spoken with contractors because I do not expect they know "Best Practices" or "Building Science". However, if they are doing work after structural engineers draw it up, that may be a good idea. I will see if I can contact someone. I do indeed plan to use foam inder the slab. Even considing a stem wall to slab break with foam also.

    Malcolm, I wiulll have a flat ceiling at 8 or 9 feet tall. I realized that I will likely need to use trusses after all because of the 29ft 1in. span. My previous houses were 24fyt wide, so they only had 12ft span to centerline . My thought was to have a header in the open area (living room/kitchen); however, 15 ft span may make rafters a bit stretched. I will look at span tables soon enough and make a decision. Hate to have trusses fabbed as that increases my cost in dollar bills. Of course, if I stick frame rafters, I will have a ridge board. Good thing about trusses would be the ability to specify knee trusses so I get sufficient insulation at the eaves. I will look at an idea I saw on here of addeding a band on the ceiling joists and set rafters atop that in order to gain eave depth for more insulation.

    Tim R, I am hoping to avoid the thousands on this, but if that is needed. I will spend the money. Also, if the engineer is going to get sued for a failure, he would be foolish not to over build it himself and drive up costs more. Of course, if my pockets were deep, it would be a no brainer, but my set of facts are unique. Again, I will if this is where the compass points me.

    Jon R, that is very interesting data, but I am not sure how to interpret it. Can you help me here? Does it mean that if I have very good drainage and overhangs that keep the water away, I will go a long way in avoiding this problem? I certainly plan to have excellent drainage. The first house I bought was a brick slab ranch and water would pool near the ground level brick and it would wick into house at Master bedroom and carpet would get wet. No good.

    Thanks for the links gents. I will be reading them this evening when I come inside for the day.


  8. user-2310254 | | #9


    This Matt Risinger might be helpful:

  9. user-6184358 | | #10

    The soils engineer will tell you the soil type and give a written description of the recommended foundation design. They don't draw it or design it, this is what a structural engineer would use to design the foundation.
    The soils engineer will alert you to the issues with the soils. That way if you don't have problematic clay then build as normal. Don't just guess at the soil issues.

  10. mikeysp | | #11

    Steve, thanks for the Matt Risinger link. He is the building science evangelist that I first heard.

    Tim, that is a very good idea. I will see if I can find a local soils engineer.

  11. user-2310254 | | #12


    This might be more information than you want or need:

  12. Jon_R | | #13

    Note that rebar and well placed expansion joints are critical for exposed slabs.

    You can use "void forms" to provide a space for soil to expand into without forcing the concrete to move.

  13. walta100 | | #14

    A low cost low risk plan is to find the old man that has been digging foundation in your part of the world for the last 20 years.

    He has made almost all of the mistakes there are to be made and learned not to repeat them. He will know what good and bad dirt looks and feels like around there. Ask for his opinion, yes he may have green teeth holes in his jeans and drive a rusty old truck try to remember he is an expert. In small towns one or two bad jobs and the word of mouth can kill a business.


    1. Expert Member
      Michael Maines | | #16

      Walter, that's not always effective advice, in my experience. On one recent project, a new home with a walk-out basement, I had designed a well-insulated, water-managed system to keep the basement warm and dry, and spec'd European triple-glazed windows, an HRV and mini-split for heating, cooling and dehumidification. The old-timer concrete guy ignored all of my details and ridiculed me for not including standard basement windows that could be left open all summer "so it wouldn't get moldy down there." Some examples: no sub-slab insulation or vapor barrier; no capillary break, standard damp-proofing without the dimple mat spec'd. Structurally, he replaced my engineered 36" square footings with rebar with a wheelbarrow full of concrete just dumped in a pile and pushed into a rough circle maybe 30" across.

      I could go on, but the point is, just because someone has been doing something successfully (in their mind) for 20 or more years doesn't mean it's right.

      1. walta100 | | #17

        To my me ear it did not sound like Mike the original poster was not asking for a new fangled insulated foundation with lots of stuff the old guy had not seen before. My suggestion was to ask the old guys advice and not tell him what you want.

        I agree you will get in trouble if you try to tell the old guy, how to do his job and ask for a bunch of stuff he thinks is weird.


  14. mikeysp | | #15

    I looked at all those links and also spoke with local contractor and he recommends I use a footer, foundation wall, and piers. My questions:

    Can I
    1. cut out a flat area on my building site as depicted by the cut line (shown in green in attached pic) and....
    2. then form up my footer on top of the ground and then backfill around outside to make proper drainage?

    The ground surface will be undisturbed soil.

    I would have plenty of space under the floor.

    After back filling the footer will be below the frost line (zone 4a).

    The only alarm in my head is that the side of the footer under the house would not have soil against it. However, if I go this route vs the slab, I will make the crawl space unvented and part of the conditioned space.

    Pic is only to answer the question of forming footer on top of a cut. I will have foam board, radon venting, rebar, plastic, etc.. I have never sen anyone cut out a flat surface and form footer on top of it like this.

    Reason to do this would give me max clearance in crawl space without higher foundation wall.

  15. walta100 | | #18

    That will be a big design change.

    A wood floor is a big upgrade most people think it is worth the extra cost. Just remember everything you saved in foundation costs will quickly be spent on joists plywood and floor finishes.


    1. mikeysp | | #19

      Walter, I see the added cost on those materials, but on the other side, for a slab, I will need a concrete man to handle the pour/finish, plus any geo guys I would hire on for testing and or consulting/design.. Whereas, I can self perform 100% with the foundation, framing, and finish floor. Even the form boards I can make with one of the pine trees I felled by sawing it up on my sawmill.

      Of course option brings me to another area of little familiarity... how to economically make a conditioned crawl space? What should the details look like? I will do some reading this morning and talk with the building commissioner for our county.


  16. mikeysp | | #20

    If I do a conditioned crawl space and pour a 2" non-structural slab, would it not be just as problematic with the expansive soil? I know it would not effect the finish floor, but it seems the heaving would still be applying hydraulic pressure to the thin slab and cause it to break up. Of course it would be have thermal breaks at the ground and foundation wall, so it would be floating.

  17. user-2310254 | | #21


    I think you are getting ahead of yourself. You don't know if your site has expansive soil or any other issues. I Google geotechnical services in your area, and it looks like an engineer would charge you $150 for an evaluation. If it were me, I would spend a few hundreds dollars a this point to better understand my site conditions.


  18. mikeysp | | #22

    Steve, thank you for the sanity check and that is exactly why I am posting my steps. . I thought the red clay I see mixed in with the dirt meant "Expansive Soil" unquestionably. I know it is that way at least 5ft deep because I installed my septic system and when I was excavating for the field lines and tank, it was that way every ounce of the dirt I disturbed. Also when I was clearing the property I would dig around the trunk and push the trees over and all that dirt was also redish/orangish clay soil mixture. However, your point is very well made.

    I just called three of the googled numbers and will get a call back from two. I spoke with an engineer on the third call and he made the following points:

    The clayish soil I am seeing is very likely a 1-3% clay content. He said we do not have much problem with expansive soil in this area. He also said it would be about $1500 for him to come out with me digging some test holes with my skidsteer backhoe attachment. He doesn't think I have anything to worry about.

    I have an appointment this afternoon with my county building commissioner and he has 34 years experience building plus the last several years as the building inspector for the county. So, I will ask him. I know he just finished his own ranch house on a slab, on top of the bunch of the local chirt (Soil/clay mix) that all the local dirt roads are made of. He is on the side of a fairly steep hill and he had a dozer cut out and compress several feet of the chirt and poured his slab on that.

    Anyways, I will post what I find out.

    I certainly would pay a few hundred dollars, but $1500 is a bit much if it is a nice to have vs foolish not to have.

    After building my last few houses, I know what structural, insulation, and air sealing regrets look like, as well as hiring a sub based on bottom dollar without knowing his reputation for quality.


  19. jameshowison | | #23

    Consider Helical Piles for this issue. Definitely friendly to self-building as well. Jordan Smith has a new Youtube on this: These are extensively used for temporary buildings in the fracking industry.

    I just used these for a small outbuilding and they worked great. It was $2,400 for four piers (that was the minimum call out charge). Two minor issues: on one the "knuckle" of the shaft joins ended up right where I needed the top bracket, so I had to get a welder in to cut it off and weld an extension on. A bit higher crawlspace and it wouldn't have mattered.

    Then a weird one. It got very dry here this summer (duh) and the earth receded from the shafts. With only 4 piles for the whole building there was some sway in the building. Went away as soon as it rained, but annoying. A little higher piles and some cross-bracing to control the racking between them would help with that. Or mine only went in ~12ft, so deeper would avoid that, I'm sure.

    Then there's air-sealing under the wood floor. I just crawled around taping proclima roll stuff, but I did see this article in fine home building that I thought was neat. They built the floor section on the ground then used a few material lifts to raise it above head to work on the underside, then lowered it back down.

    Then there's the rolls royce system I love that it is, in theory, able to be re-leveled from above.

  20. walta100 | | #24

    All I am saying is to do the math.

    If the main goal is lowest cost floor and foundation a stained concrete slab is very hard to beat.
    It is hard to get into trouble with a slab on grade. Crawl spaces can turn into mold factories if you get it wrong.

    I tend to think that if expansive soil was common in your area the local pros would be well versed in how to deal with the problems. Since the engineer said expansive soil is not common, I think you are making a mountain out of a molehill.


    1. mikeysp | | #25

      Walt, after speaking with the engineer and the building commissioner, I have concluded the same, I have imagined that the soil is expansive. I also read one article that may be propaganda shy of the truth. Turns out the soil compacts very well here. I recall that when faced with unknowns people tend to operate on assumptions from past experiences or current expectations rather than facts.

      I picked up my permit while at commissioners office.

      So, looking around on here and BSC (Building Science Corp), I am trying to figure out insulation details for slabs. They show exterior insulation and then a layer under slab portion in zone 4a. I have seen other sites online with foam all the way around the footer also.

      Does anyone have a link to best practice?

      Monolithic seems to be a nice one pour approach, but I also see folks pouring footer and then pouring a stem wall, or laying block with L-block on top course and then pouring a slab. Thoughts, ideas, links are most welcome.

  21. mikeysp | | #27

    Ok, I have looked at all the links. I have concluded that a monolithic slab will work fine here. Next obstacle: I see warnings to be careful of termites and foam, but I do not know how to interpret such a statement. Does this mean termites will move into my sub slab foam and eliminate it, creating a massive collapsable void? Or does it just mean that they will make a trail north for dinner on the wood products? I know the latter is a concern, so I good break, termite shield, and pre treat area is important. Are there any other concerns such as the former? Again , I am in zone 4a (Nashville) and we get a very significant amount of moisture and there are termites a plenty.

  22. krom | | #28

    If you are using new foam, you should be able to order it borate treated, but right now there is snow outside my window, so my termite experience is zero.

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