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