Monticello Historic District, National Register of Historic Places
Powell-Jordan-Key House, Monticello
Vanishing Georgia: Photographs by Brian Brown
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Vanishing Georgia: Photographs by Brian Brown 
Thanks for the info.
No, it was not a later addition. I owned that house from 1978 till 1995, and while taking an architectural history course at Georgia College (at that time), I did deep research on that house for my final thesis in the course. Mr. Jordan wanted a flat roof because he’d had problems with leaking in a past house; of course, the mansard roof did not fix that problem. At one point, the leaking was so bad over what was their dining room but was, when I lived there, the family room that the entire inside of the room had to be re-done by the family from whom my husband and I purchased the house. There is a “sister house” on an adjacent street; the two houses were owned by brothers. That house, too, has some mansard elements.
Wow. That roof looks like a later edition. Maybe to utilize the attic?