Real carriage house doors were also historically built without a jamb around the doors because the structure was nothing more than a barn or a large shed with a non-square opening. The structure provided roof and wall protection only. The doors were also trimmed to the size of the non-square opening so you ended up with cock-eyed doors! The result – wind blew in from all edges of the door, and, there was nothing to stop water from entering also even if the installer could create a uniform gap all around the door; very unlikely.
