Zen Spa & Observatory — Sonoma, CA

Matching Vision Without Compromise

Zen Spa Cor-ten detail
6,000 SF
Studios Architecture
Cello & Maudru Construction
Merit Award, Structural Engineers Association of California Excellence in Engineering Awards, 2007
Tim Griffith

The integration of architectural vision with intricate and innovative structural design realized – without compromise – the owner’s vision of a unique retreat at Zen Spa. Holmes’ design of this private space, with its own observatory tower, pushed the boundaries of structural engineering with a tight integration of architecture and structure.


Hover State: Each structural component articulated architectural purpose. The floating butterfly roof, a thin-plate covering the 70’ x 36’ column-free astronomical projection room, was achieved through a warped concrete post-tensioned slab with a woven mesh of strands that allowed tapered at the exposed edge. A sparse and irregular grid of columns allowed the roof to hover over the volume while laterally anchored back to a concrete core at the adjoining wet spa and service areas.

Zen Spa facade
Zen Spa in construction
Zen Spa in constrcution
Zen Spa at Night

Custom Engineering: The facade mullions were custom-engineered and cantilevered from the foundation to enhance the floating of the concrete roof, and to allow architectural freedom above the transom.


Zen Spa interior mullions
Zen Spa & Observatory

Sky’s the Limit: The observatory building, which supports a 20” telescope, is a truncated pyramid structural formwith exposed cantilevered stairs. Laterally stiff precast concrete panels ensured compliance with the minute deflection tolerances for the telescope. Corten structural steel plate stringers formed the feature stairs matching the aesthetic of the plate steel cladding.

Sonoma Observatory

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