Some architectural scholars believe it to have originated along the coast of the Carolinas, hence the colloquial name.
This protects the porch deck from exposure to the elements and also leaves it well shaded from the sun most of the time.
[1][2] Most commonly seen on historic folk houses, the rain porch also came to be adapted to the monumental porticoes of some Greek Revival mansions, such as Rosemount and Kirkwood.
[1] The overhang became especially exaggerated in some areas with copious amounts of rainfall, such as the Eastern Shore of Mobile Bay in Alabama.
This article related to a type of room in a building is a stub.