In most flat roof buildings, dormer windows are not common; however, in historic Capitol Hill row homes with mansard roofs, there are often dormer windows. Essentially, dormer windows are used to bring light in directly through the roof. Except in this case there is no need for a skylight. The original architecture was built in a way that framed a dormer out, directly through the roof.
Mansard Roofs were first used prolifically in Paris, France
The old French property taxation system was applied based on the number of floor levels in a building. A French architect named Francois Mansard, born in the 16th century, popularized the historic tax hack.
The picture below was taken of a building here in Northwest Washington DC. This building has five dormer windows, a few more than typical DC buildings, but here you can see a perfect example of dormer windows built through a mansard roof.
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