 In modern times, most earthen floors are often laid over the top of a subfloor of tamped gravel or cob or adobe, and then a mixture of clay, sand and fiber are mixed and leveled onto the subfloor.The finished layer can be 1/2 to 2 inches thick, and once dry is sealed with a drying oil (like linseed oil).Earthen floors can be laid over the top of previously installed wood floors but weight can become an issue.