A one-piece cob floor is the longest lasting type of earth floor. The floor surface sits on top of whatever you've decided to use for the base layer.
You can make a floor with one 3/4 inch layer or two 1/2 to 3/4 inch layers of your cob floor mix (with optional additives) thinned down to a consistency that can be troweled on. Make it as dry as possible but wet enough to trowel. The less moisture there is when you lay it down, the quicker it will dry and the less it will crack. Keeping your trowel wet while working on the floor will make it a lot easier to smooth the surface.
tools you'll need
The floor is basically the same stuff as the walls, sometimes with extra sand added
depending on your original mix. (See page 72 for proportions for making cob.) Sift all the ingredients quite finely for the final layer of floor.
If you use straw in your floor mix, chop or grate it finely. Use shorter pieces of straw and less if you don't want it to show in the floor surface. The straw can be grated quite small. (See page 153 for information on how to break up the straw.) If you have a clay rich soil and a shortage of sand, use more straw instead of the sand. Some natural builders don't use any straw in their floors. Make tests until you find a recipe that works.
Go over the section on plaster additions (pages 154-156) and apply that info to the floor mix.
glue. Adding a little Elmers glue helps harden the floor.
ground psyllium seed husk. This is a wonderful addition to a cob floor! Have you ever noticed tennis courts or tracks that feel like "soft" concrete? Psyllium gives them that soft, rubbery feel. This makes the floor easier on your legs and on your dishes. A little bit of this stuff goes a long way. Too much will make it hard to trowel. Make up some test batches with different amounts of this additive in each until you find the percentage that works best. For a 12 foot diameter round room, you'll need about a pound of psyllium.
manure. A very common ingredient for earthen floors in many parts of the world. This can be used as the fiber in your recipe.
blood. Ox blood is a common ingredient in old floor recipes. It's supposed to make the floor a lot harder. These days you can buy blood meal at plant nurseries. Use this on the top layer.
wood ash. This is an additive that I have come across in old literature. I haven't tried it yet.
oil. A little bit of oil can be added to the last layer of floor. You'll probably want to use an oil coating on the finished floor, in which case adding oil to the wet floor mix would be unnecessary.
flour. Like with plaster, the addition of flour paste will harden the surface.
Try out different floor recipes before you decide which one(s) to use. Do lots of tests. Outdoors, make little samples of the base material and trowel the test mixes onto it. A two foot round sample will be plenty. Let the samples dry, protecting them from direct sun and from rain. How do they look? See how they hold up when you walk on them. Once you've narrowed down what works, make some larger test batches of the same recipe to make sure it's the one you want. You can try out your homemade sealants (see page 66) on the samples too.
Make test batches varying the ratio of sand to clay, until you come up with a recipe that doesn't crack and holds up when you walk on it. Cracking means too much clay, and too much sand will cause the floor to slough off when walked on. If you are doing more than one layer, you can make the bottom layer 75-85% sand and small gravel.