Stone-Ideas.com 
In a separate report from our Stone Insight series, we describe the technology of post-tensioning stone
https://www.stone-ideas.com/122868/stone-insight-the-post-tensioning-of-the-staircases/
A one-of-a-kind monolithic stair, this post-tensioned stone structure merges ancient material with modern engineering, transforming 50 large-format treads into a sculptural, gravity-defying ribbon of marble. Designed for a private residence in historic Sag Harbor, Long Island, the three-flight staircase anchors the home’s coastal elegance with a centerpiece of timeless craftsmanship and technical precision.
The building was bestowed one of the Natural Stone Institute’s Pinnacle Awards 2025.
Clad entirely in Danby Fantastico, a domestically quarried New England marble, the stair expresses refined minimalism through bold veining and luminous tone. Structural analysis confirmed that veining orientation was critical – each block had to be hand-selected and marked with quarry masters to ensure both strength and visual cohesion.
Every one of the 50 treads (770 lb each) was modelled with precision to accommodate complex internal features:
* four routing holes for post-tensioning (PT) cables,
* a shallow soffit channel for recessed LED lighting,
* and a deep alcove to conceal the aluminum shoe of the glass guard, and its wiring, which snakes through a discrete bore to feed the light trough below.
The cross-section profile was optimized to maintain generous radius curves on the soffit and vertical faces, with smooth radiused transitions that both emphasize the stone’s natural grace and ensure sufficient thickness to distribute stress around internal PT tendons while preserving the overall vertical stiffness of each flight.
Fabricated using 5-axis CNC and 7-axis robotic machining, each tread was hand-finished to architectural-grade precision. On-site installation demanded millimetric accuracy, sequencing each placement, tensioning, and grouting in a carefully choreographed process. The stone fabricator and installer was Precision Stone Inc..
The result – the first post-tensioned stone staircase in the United States built entirely from domestically quarried marble – stands as a testament to sustainable sourcing, technical precision, and creative collaboration. Seamlessly integrating light, geometry, and natural stone, it exemplifies craftsmanship, material expertise, and what is possible when architectural ambition meets exceptional fabrication.
Source: Pinnacle Awards 2025 brochure
Stone Consultant: PICCO Engineering, Concord, ON
Design: Thomas Juul-Hansen, LLC, New York, NY
GC: Donadic Inc., Long Island City, NY
Stone Installer + Fabricator: Precision Stone Inc., Westbury, NY
Stone Supplier + Quarrier: Vermont Quarries Corp, Mendon, VT
Photos: Julie Florio Photography
