This residential stair connects three floors in a San Francisco home we designed the architecture and interiors for. The entry area of this house was wide but shallow, and hence the stair shape was designed as a flattened ellipse but not mathematically pure – hence “ellipsoidal”. Moreover, at each floor the radii of the railing walls are different to create a subtle spatial dynamic. It’s complicated in actuality, but the result is elegant simplicity and lightness, which was the intention. These sculpted shapes were common during the Baroque era in Rome – especially churches designed by Italian architect Borromini, himself known for creating unexpected radicality.

Aside from the practicality of connecting three floors, it's the vertical and essential counterpoint to the stacked horizontality of the house; further accentuated with natural light from the skylight that is also ellipsoidal, which brings in that special California light. Carefully proportioned treads with open risers allow light to cast down while creating shadow play. They also allow for views from the library to the living room, to the Golden Gate Bridge beyond. From an artistic perception, it's designed to be the poetic sculpture of the house - to be admired from above, below, and laterally in three directions. It constantly changes based on the viewers position, lighting, and level of intoxication.

The steps are bleached walnut, the walls and railings are curved plaster, and the handrail is ebonized walnut with blackened-steel pegs for a floating effect. The design was created in a series of stages that started with pencil sketches to computer models and resulted in carefully-dimensioned plans, sections, and details to build from. A concentrated collaboration with the craftsmen occurred during construction. Mock-ups were created for the tightest curves and the handrail. The rest of the house now pays homage to the stair.

Credits:
- Cass Calder Smith Architecture + Interiors - Interior Designer - Barbara Turpin-Vickroy
- Cass Calder Smith Architecture + Interiors - Principal Architect - Cass Smith