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.