Earthen floor over concrete slab?

Hi, I'm wondering if anyone here has experience putting down an earthen floor over a concrete slab. I've heard from some people that it's impossible, from others that it's possible but difficult, and from others that it's quite doable.

Anyone here actually done it?

If you have tips or recipe recommendations, I'd love to hear them!

Thanks,
Rosemary

Hi Rosemary.  It can be done - no problem, but do you want a floor that is not real durable?  A group of people did it here east of town.  My floor is on the ground but the same as they used.

By far, of all of the earthen floors we have, the cob with four coats of boiled linseed oil is the most durable.  Rolling heavy things over it will damage the surface - carry and set them.

I suggest at least an inch to an inch and a half of cob using the proper ratio of clay and sand with straw.  Thinner than that may work -you may have to experiment.  Doors may hit it or you may have to block out areas around opening doors.   30% clay 70% sand including all aggregates that may be in the natural clay - ours works out to about a 50/50 natural clay/sand mix.  Use enough short cut straw to have plenty in each handful to keep the cob together.  I suggest cutting the straw bales along the strings = both sides , with a chainsaw before cutting the strings on the bale.  This will give you straw that is a lot easier to work with than long straw.

You can use flat laid boards on top your floor to gauge the thickness of your cob.

As it dries completely  - maybe a couple weeks-  it will shrink and crack.  After it does, come back with a grout mix and fill the cracks with fine sand/clay grout out of a grout bag.  Add some acrylic cement bonder or Elmer's glue to make the grout more durable.  I added crushed charcoal to darken the grout and accent the cracks more.

Four coats of linseed oil thinned about 50% with mineral spirits if you do that sort of thing will make the oil soak in for a deeper thicker surface coating.  Recoat over the years as necessary.  We use acrylic to re-coat the surface.  I think water based Polyurethane would make a more durable finish so you don't have to redo it as often as the linseed oil or acrylic.  Note that I am not a total purist.

A swimming pool trowel and rubber grout float is great for finishing the dampened surface of the cob.  It will have a nice slightly uneven but smooth hand made looke when finished.  Add water on the surface as needed to finish it but rework it until a bit more dry for a smooth finish.

Here is our cob floor on the front porch

Thanks a lot for the detailed reply! Sounds more hopeful than I'd feared.

The deal is that my partner and I arrived a couple of weeks ago at his family's old farmhouse in rural France, where we intend to spend several months doing various permaculture-inspired projects, one of which is the renovation of what was once an indoor cow barn and pig sty. It's a big open room with fairly low ceilings, and it had a dirt floor, which we were thinking would be perfect for making an earthen floor, using gravel in between. However, his parents insisted on putting down a cement floor right before we arrived, against our advice... so that's what we're stuck with right now. We'd really love to do the earthen floor if we still can, so I'm glad to hear that it may not be that difficult!

The only other problem that we can see is that the cement was poured only a few days before Christmas, so it's not quite dry all the way. We're wondering how that will affect our plans. Any ideas?

[the attached photo is the room as it is now]

Thanks again,
Rosemary

not a problem you will want to isolate the floor with an inch or so of perlite clay mix. it keeps the floor from being so hard on your joints.

the layers are perlite, base, finish, the finish coat will need to be steel floated for a very smooth surface. takes a bit of strength and time. you can save a little time if you put a little linseed oil in the finish mix as you are mixing it then do the three coats of oil and wax after its been steel floated. makes for a very durable floor. one warning is that your chairs need to have wide feet and not be those folding chairs. we had a larg guy drill a hole in ours by rocking his chair back forth an entire day. fixing it was easy but it sure made me mad at the time.

Concrete gains most of it's strength i the first 7 days then most of the rest within 28 days.  After thsat it kind of cures for a lifetime. Dampness shouldn't bother much except to slow the cob drying  a bit.

You could possibly just stain it brown to give an earthen look with out the maintenance of an earthen floor?  It really depends on your reasons for wanting an earthen floor. 

Nice little place.

Awesome floor, Troglodyte. And, yes, cool looking place, rosemary. I'd love to see and hear your reasons for the earthen floor and how it goes if you do it.

From my verry limited experience, an earthen floor has more warmth (maybe only figuratively, but maybe literally, too) and a nicer feel than concrete.

I'm in a bottom floor condo with a concrete slab under carpet. I'm looking ahead to removing the carpet some day, and am wondering if putting down an earthen floor in its place would provide a level of insulation, both for warmth and sound. I imagine it could be a decent (better even?) alternative to a cork, bamboo or wood floor. Especially if it helps with sound a bit. Perhaps it depends on the finish - how brittle (such as the polyurethane) or how soft (wax and oil) it is. Anyone have thoughts about this?

Thanks, Jocelyn.

The brown floor at the bottom is the cob floor.  The upper part with the slate and above it is cement stabilized soil cement similar to the mix used in the cob less the straw.

The Portland cement stops the shrinkage and keeps it too soft so the cob is better.

4 coats of linseed makes it a bit like linoleum on top - pretty good wearing but still susceptible to damage by heavy stuff.

I have heard of several people around here (New Mexico) putting earth on top of concrete.  Here is a good how-to article that mentions it as well:

http://www.landerland.com/efloor/efloor_inst.html

an exerpt

"...So, in my opinion, the best floor would be a 4 in./10cm thick, 3000 psi concrete slab with added fiber and PEX radiant heat pipe with a final 1/2 in./13mm earth top...."

aesthetics and idealism aside, you already have a perfectly functioning solid floor in the concrete itself, why add a labor/maintenance intensive surface

Hey Ernie,
Did you put a vapor barrier above the perlite?  I'm curious if you put the base floor directly onto the perlite or did you put it onto the vapor barrier.   Wanting to ensure that doesn't cause cracking.
Thanks,
Dimitri