The project is sited atop a historic Manhattan brownstone townhouse. The built structure acts as an envelope for a new exercise swimming pool and helps integrate swimming as a new program in the client’s daily domestic life. The rooftop addition is designed to provide its occupants with complete visual privacy from neighbors during rooftop swimming, while itself remaining invisible from the street level. Exterior sightlines, interior occupation, daylighting and egress all influenced the overall massing volume, while the concept of an envelope constructed from a single continuous surface determined the material resolution and detailing of the project. Wedged between two existing party walls, the surface is gently folded to accommodate drainage and negotiate its transformation from the roof to the wall. On the exterior, the composite surface is clad in black industrial rubber, while its interior face is, in contrast, lined in glossy, finely detailed, aqua-tinted porcelain tile. A section is carved out of the overall volume to allow for access and daylighting, and is highlighted by a mill-finished aluminum fascia that traces the profile of the cut and further reveals the curved geometry of the surface. A horizontal window orients the swimmer toward a tightly framed outside view, while translucent cellular polycarbonate panels provide diffused light on the inside.