D.C. Mun. Regs. tit. 14, r. 14-405
In any room that is otherwise a habitable room, only that portion of the room area that has a clear ceiling height of not less than seven feet (7 ft.) shall be counted as habitable room area, except as otherwise provided in this section.
In any dwelling unit or rooming unit, not more than fifty percent (50%) of the total habitable space may be in a room or rooms having a sloping ceiling.
In those rooms with a sloping ceiling, at least seventy-five percent (75%) of the habitable room area shall have a clear ceiling height of not less than seven feet (7 ft.), and any part of the room where the ceiling height is less than five feet (5 ft.) shall not be considered in computing the habitable room area.
All habitable room area shall have a minimum clear head room of six feet eight inches (6 ft. 8 in.) under beams, pipes, ducts, or other construction projections from the ceiling.
Beams spaced closer than four feet (4 ft.), face-to-face, shall be considered as joists and shall have a minimum clear head room of not less than seven feet (7 ft.).
D.C. Mun. Regs. tit. 14, r. 14-405