Why so few residential rooftop decks in Washington DC 20002?

I'm buying a house close to the H street corridor in NE DC zip code 20002. It falls in the zone R-5-B, I want to know if I would be allowed to put in a rooftop deck. The neighborhood is not clified as a historic one although all/most of the houses were built in the 30's and 40's. The DC building code doesn't seem to forbid it, the zoning laws don't seem to forbid it. It seems like a great idea and I'm curious why nobody seems to do it in this area, or at least not that I know of. In Baltimore MD many similar style row houses have them. I've done as much research on the web as I'm going to but if anyone has anything to add it would be appreciated.