Some of the oldest buildings and structures on the planet are made out of earth. Parts of the Great Wall of China were made out of adobe brick, as were some of the pyramids in Egypt. Devon county in England is known for having entire towns, hundreds of years old, built out of cob. And in the United States, New Mexico, we find the Taos Pueblo, a cluster of multi-storied buildings that have been continuously inhabited for over 1,000 years. It was probably built between 1000 and 1450 A.D., and as of 2006 it had 150 inhabitants.
Generally speaking, if you want your earthen house to last a very long time, anywhere between 100 and 1000 years, the most important thing to consider in your building process is a solid foundation and a good roof. A good foundation makes sure that the building does not settle unevenly due to its weight and protects the earthen walls from possible flooding and ground moisture. A roof with plenty of overhang will protect the walls from getting soaked and softened in the rain. This is not to suggest that earthen walls disintegrate easily; in many arid climates, people leave earthen buildings exposed to the rain and just fix up the wear and tear every now and then.
In this context, it is important to stress another factor, sometimes even more important than a foundation and a roof, that determines the longevity of a building: a loving relationship between the building and the people who use it. Earthen buildings tend to be more “lovable” for many reasons: they were made out of earth, rather than industrially produced materials, they were built by hand, often with help of the greater community, they provide great comfort and beauty, repairs are simple and inexpensive, and earthen houses have a good possibility of relieving people of the questionable practices of housing speculation and mortgages. The average house in the United States has a life expectancy of less than 50 years. And often it gets demolished, not because the house/roof/foundation is failing, but rather because the house was never really loved. Tired looking drywall and once fashionable looking building materials become too expensive to replace and housing speculation makes it worthwhile to tear the building down and ship it off to the dump. How sad for the people who built it and lived in it, as well as for our environment.
I believe it is reasonable to consider that houses last longer not necessarily due to the strength of the building materials, but rather because of the quality of the material. Buildings made out of glass, steel and concrete are much stronger than earthen buildings. However, a little damage looks really bad right away and is expensive to repair. We also physically and emotionally resist the hard, cold and industrial qualities of these materials.