Natural Slab Floor Over a Concrete Slab

I was recently asked if it is possible to do an earthen or cement skim coat over a slab foundation. The answer is yes, you can do a skim coat of either clay/straw (earthen floor) or cement over the top of a concrete slab; however, there are minimum thicknesses that need to be maintained. At least 3″ of concrete are required unless a specialty skim coat is used (this takes extra skill).

There are specialty coating systems available that can be used if you have the skills, but in general, they are much more difficult to finish well. In fact, it may be easier to simply finish the concrete in such a way that the foundation slab can be used as the finish floor of the home. Be sure to protect the floors during construction. If you plan to acid stain the floor, make it a point to know the details of that art. For example, you would want to know that a stack of lumber left on the concrete during construction will cause the stain to fail in that area leaving a blank or discolored straight line that will not look very good at all in the finished floor. You will also have to be very clear with any subs that the slab is the finished floor and inform them that they cannot write notes on the floor with a construction crayon or snap lines with red chalk. There are many things to keep in mind here and some basic knowledge is a must. Knowing what to watch out for ahead of time will be a life saver in the end!

When applying a skim coat of stray and clay, it is a good idea to use about 2″ of material to get proper adhesion and compaction. Be sure to apply the earthen floor in lifts. In other words, apply one inch of material and then let it cure. Moisten the surface of the cured floor and apply another 1/2″ for proper adhesion to the base coat and then apply the final 1/2″ while the previous 1/2″ is still moist. This will ensure a tight bond between all the coats. Compact and finish the floor as you would any earthen floor. In most cases if an earthen floor is used, you can eliminate the concrete slab altogether and build the earthen floor over compacted gravel lifts. A foundation can be poured to carry the loads of the home and then the gravel and earthen floor can be built up inside the foundation.

If an existing concrete slab is to be covered, many options are available and the ones listed above are just two of many.

18 Responses to Natural Slab Floor Over a Concrete Slab

  1. andrew
    andrew Thu, September 13, 2007 at 1:29 pm #

    Can an Adobe/Cob floor just be poured in a similar fashion to a concrete slab floor?

    Would troweling in a 2×4 baseplate for interior non-loadbearing walls give adequate support? Would composite lumber perform better to avoid expansion and shrinking?

  2. Andrew
    Andrew Thu, September 13, 2007 at 1:29 pm #

    I do not have a lot of experience with earthen floors; however, I can speak from what experience I do have. The process for an earthen floor is different from that of a concrete floor. For an earthen floor, you need to build in lifts. IN other words, a few inches at a time and with different materials. The first lift is drain rock that creates the base for the floor. Drain rock does not have to be compacted, because of the nature of the stone and how it nests with itself. On top of the drain rock you need to add about 4″ of 3/4″ gravel that should be well compacted. On top of that goes the earthen floor material which also needs to be compacted in place. It is crucial that the moisture content of the floor be perfect as too much water will cause the earth to squish and too little water means the earthen material will be too dry to compact. Either way you will not be able to get adequate compaction. As you install these lifts, be sure to use a screeds board to ensure the material is installed level and evenly. The last 1/2″ – 3/4″ is the skim coat of earthen material which acts as the finish for the floor. It smooths everything out like a final coat of plaster and yields an even texture. After that has all dried, you want to put several coats of linseed oil on the floor and burnish it in. As you can see, it is a different and labor intensive process. You will also have to perform annual maintenance on the floor to keep it in good working order. One major advantage of an earthen floor (other than the natural materials of course) is that it is softer under foot than concrete. If you spend a lot of time standing on the floor, the earthen floor is much gentler on your body.

  3. Rudy Wed, December 26, 2007 at 2:26 pm #

    Andrew –
    I am replacing carpet on a concrete slab and would like to do an earthen floor. I want to limit the earthen floor applied on top of the slab to 1 1/2 inches at most and 1 inch is preferred.

    I am not sure from the article what the minimum amount of earthen skim I would need. Could you point me to any resources you may have?

  4. Andrew
    Andrew Thu, December 27, 2007 at 10:13 pm #

    Rudy,
    I do not see it being a problem to stay with a one to one and a half inch earthen floor if the concrete is stable. If the slab is failing, you will need to fix it before you move forward with the earthen floor. In general, a structural concrete slab should enable you to finish the floors at the thickness you desire. I do not have a specific source for earthen floors that I would recommend. The truth is I don’t do that many of them in this area. You might try connecting with Bill and Athena Steen at the Canelo Project. I hope that helps.

  5. Emju, Detroit Zen Center Fri, April 17, 2009 at 5:40 am #

    Rudy, I am with a Zen Center in Detroit. We have a 200 s.f. floor, with 1 1/2″ rigid insulation over a solid wood deck. On top of the insulation are 1/2″ radiant tubes, tied down. We were going to pour 1″ of concrete, then cork on top. We are wondering if we can instead, put down only an earthen, strawbale style floor directly over the radiant tubing (red plastic). Any opinon??? Thank you for your time.

  6. Andrew
    Andrew Mon, April 27, 2009 at 10:23 am #

    Be careful pouring concrete under 3″ thick. It is not advised to pour it any thinner than 3″ as it will break apart very easily. You have to add special binders to do a thin pour. My biggest concern with an earthen floor over a wood deck would be the potential for cracking. That deck will move and if only an inch thick, the floor will not be able to resist cracking under that pressure. The same is true if you use concrete. You will need to be sure you have enough thickness to the slab (either concrete or earthen) to avoid cracks while at the same time, make sure your wood deck can handle the extra weight of the thicker slab without deflecting or failing.

  7. Bryan Horne Wed, September 9, 2009 at 3:34 pm #

    Andrew,

    I just recently bought a 1 story old house that had carpets on top of concrete, slab on grade. I want to put an earthen floor down on top of it. The ceilings are already under 8 ft. and my wife and I would prefer to not have them any shorter. I ripped out the carpet and glued down underlayment… How thin can I make my earthen floor? Would it help if I rented a grinder and grinded down the concrete (not to lose hight, just for adhesion purposes? I was originally hoping for a floor about 3/4 in. which would butt up to existing floors nicely.

    Also what kind of earthen mixture do you recommend? (sand, clay ratio) And do you use any straw or horse manure in your receipe?

    Thanks,

    Bryan

  8. Andrew
    Andrew Wed, September 16, 2009 at 8:54 am #

    By Bryan. I think that you will be fine with a 3/4″ layer of earthen floor. The grinding of the slab would certainly help with adhesion, but I don’t know if it is necessary or not. I guess it depends on how smooth and shiny the existing floor is.

    In terms of recipes, I don’t do enough earthen floors to have a specific recipe to suggest; however, I would suggest adding some horse hair or straw as a tensile strengthener. I would also suggest a small amount of lime as a stabilizer. Perhaps someone here has a recipe they like. There are also several earth plaster books that might help you with recipes.

  9. Kalani Mon, February 1, 2010 at 12:21 pm #

    can you do an earth floor directly on the earth?
    how much would a 200 s.f. floor cost??
    is there a good website for a earth floor recipe?

  10. Andrew Morrison Wed, February 3, 2010 at 11:14 am #

    Good questions.

    1. I would not recommend it. It’s important to have drainage and stable ground under the floor. Build up with drain rock and gravel to provide both requirements.

    2. That depends mostly on labor costs (if applicable) and how deep you have to dig and then fill to get below applicable frost lines and/or drainage issues as well as the cost of any materials you need to buy.

    3. Here are some good resources on the internet for earthen floors:
    http://www.ecoact.org/Programs/Green_Building/green_Materials/earthen_flooring.htm
    http://greenbuildingelements.com/2008/11/06/natural-building-101-how-to-make-an-earthen-or-adobe-floor/
    http://www.landerland.com/efloor/efloor_inst.html

  11. Mary McNutt Sun, February 7, 2010 at 6:17 pm #

    I would like to know if creating a 1” earthen floor on top of a plywood floor is a workable project. We have built our own straw bale house with earthen plasters and successfully laid our own cob slab earthen floor over gravel sand sand etc. I now have been asked to lay a floor in another home over a conventional ply wood floor. I have some concerns about weight so want to keep it thin but don’t want it break up either.
    Thank you for your time, Mary McNutt

  12. Andrew Morrison Sun, February 7, 2010 at 7:48 pm #

    1″ will not be thick enough. Plywood has a certain level of deflection in it and that deflection will cause the slab to break up. You’ll probably need to reinforce the floor so you can add additional thickness to the slab. You really need the substrate to be static with as little deflection as possible.

  13. Javier Dougherty Tue, June 14, 2011 at 1:32 am #

    We are a group of volunteers and starting a new scheme in our community. Your web site provided us with valuable info to work on. You have done a formidable job and our whole community will be grateful to you.

  14. Andrew Morrison Sun, June 26, 2011 at 4:51 pm #

    Thanks Javier!

  15. Patty Drexler Sat, February 8, 2014 at 1:35 pm #

    Hi Andrew,
    Looking good here. Have all but the interior Eco-Mortar coat left on the interior of my SB.

    I am wondering if, or other readers have much hands on experience with Soy-Crete concrete slab flooring stain. How it holds up to wear and tear, and any special pointers on application???

    Hope all is well with you and your family. Think Spring!!!!
    Patty

  16. Andrew Morrison Sat, February 8, 2014 at 3:56 pm #

    Very cool Patty. I don’t have any information on the Soy-Crete. Hopefully someone else can help you out with that. You might also try asking on Planet Straw Bale.

  17. Robert Parsons Fri, August 26, 2016 at 1:21 am #

    Hello Andrew

    My name is Robert Parsons, I live in the southern most location of Australia in the state of Tasmania in the city of Launceston with temperatures ranging from around -3 to 15 in winter and for most of the seasons except for summer.
    I would greatly value any help or advice you may be able to provide to me.
    Simply put Andrew I going to build a greenhouse around 500sq metre’s to start and want to be as cost affective as I can and having visited other hydroponic farm’s they spend high amount’s on warming using Biomass, Gas, Wind Turbines, Solar and electricity or use a combination of these method’s.
    Being a firm believer in natural method’s I know very little of the insulation factor’s regarding Straw.
    Before you answer my enquiry I have a couple of ideas or concept’s.
    The first is to dig the 2ft or 4ft depth of the entire area of the greenhouse, then lay a double side of black plastic surrounding the entire area so moisture doesn’t penetrate, then lay tightly packed straw bales as flooring with the outside being 2 bales high again with the black plastic on the outside.
    My 2nd is to have a mixture of shredded straw, paper and saw dust, the highest ratio being paper due to it’s fire retardancy, finally mixing all together with a slurry or weak pourous concrete to allow the warmth of each component to escape into the greenhouse.
    My enquiry is how much warmth does a straw bale provide and also Andrew what would you consider for warmth in a greenhouse?
    Thank you for your time and any guidance.

    Sincerely

    Robert Parsons

  18. Andrew Morrison
    Andrew Morrison Mon, September 5, 2016 at 4:59 pm #

    Hi Robert. In my experience, lowering the greenhouse into the ground a but will help dramatically reduce costs because of the constant temperature of the earth. I don’t know that you need to insulate against the earth. The biggest loss of insulation is to the air.

    A straw bale house is roughly three times as energy efficient as a conventional house (here in the US). Bale walls have extremely high R values (R40 rating in the US). They do an amazing job of regulating heat loss. The key is to plaster them with a natural and slightly permeable material (clay or lime) so that they can manage the heat loss while remaining protected from the elements and from moisture damage. Cement does not do a good job of that because it traps moisture in the walls.

    Hope that helps.

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