No! Clearly cob can not tolerate being left out, unprotected in the rain, but very few building materials can. Wood homes for instance, would not last long if left out in the elements. Just like all other houses, cob structures are fitted with a roof and plastered in a protective coating. Cob homes in the UK (one of the rainiest places on Earth) have stood for hundreds of years! Cob, once dry, can tolerate shocking amounts of wetness. For instance, a cob wall we were constructing got left untarped (unknown to us) for over a week in heavy North West Coast rain without missing a beat. We now leave out dry, unfinished walls in the rain deliberately a few days before we begin work on them again to re-activate the clay. When it comes time to plaster a cob wall, we soak the bananas out of it with a full blast hose prior to applying the first coat. You have to be committed to the task to destroy an established cob wall with water.
Earthen building, sustainable living, and occasional blogging