Double-Helix Stair
A cherry spiral built with divine inspiration, a little steam and a lot of clamps.
Synopsis: This article describes the construction of a double-helix circular stair that rises 16 ft. to an observation tower in a house in the Colorado Rockies. Unlike most circular stair designs, this one uses two helical stringers instead of a central post. It’s patterned on a famous staircase in a New Mexico chapel. You’ll need plenty of clamps to tackle this job.
The house is majestic, sitting on a bend of Castle Creek, 8,000 ft. up in the heart of the Colorado Rockies. And a castle it seems, with its medieval lines and three towers. The central, and highest, tower was designed for solitude, meditation and glimpsing the sunset. When the clients looked at the architect’s model of the house, they liked the tower. It was just what they wanted, but with the tower centered over the living room, they wondered how they’d get up there. In response, Steven Conger and Michael Martins, who had designed the house, conceived a sculptural, double-helix stair.
The design and construction of the stair was a group effort that evolved over many months. On hearing the architects’ concept for the stair, construction foreman Ivar Eidsmo said he’d seen one just like what they had in mind. He was referring to the Miraculous Staircase in the Loretto Chapel in Santa Fe, N. Mex. It was built about 1878 by an itinerant carpenter who appeared out of nowhere, apparently in response to prayers to St. Joseph, carpenter of Galilee. The anonymous carpenter worked four months and disappeared as mysteriously as he had arrived, without finishing the railing and without payment for his work.
Eidsmo went down to Santa Fe and photographed the Miraculous Staircase. Although the stair we built is very different structurally and is shorter, narrower and much less ambitious, the Miraculous Staircase did serve as inspiration.
Rising over 16 ft. around a 2-1/2 ft. radius and revolving 1-1/2 turns along the way, our stair was a major undertaking. It was begun by Chuck Miller, one of the carpenters working on the house. I came along later, after the layout had been done and the forms built. Initially my job was to speed up production, but when Miller had to leave the project, I took responsibility for its completion.
Specifications
During the design stage, an engineering firm, Nicol and Giltner, was hired to do structural specifications for the stair. To match the interior woodwork, the staircase is made of cherry. So the engineers based their calculations on the strength of that wood.
They determined that the stringers should be 2 in. thick, composed of eight layers: four 1/4-in. plies in the middle, two 5/16-in. crossband plies with their grain running perpendicular to the others, and two 3/16-in. face veneers.
Because of the wedge-shaped treads, the inside stringer needed to be 7 in. wide, while the outer one had to be 14 in. wide. At the top and bottom of each stringer, a 1/4-in. steel plate had to be sandwiched across the full width and securely lag-bolted to the landing and floor system. To hide these plates and cover the various structural laminations, we had to glue 1-in. thick caps, made with eight layers of solid cherry, to the upper and lower edges of the stringers.
In addition, the engineering firm also specified the 2-in. thickness of the treads and their installation details. The treads are held in place by five 1/2-in. dia. lag bolts 6 in. long-two through the inside stringer and three through the outside one. The heads of the bolts are concealed beneath the final ply.
Since the stringers are so long, each ply is composed of sections, 6 ft. to 8 ft. long, glued up to form one full-length piece. For maximum strength, the adjoining ends of each ply were scarf-jointed on a huge industrial table saw at a local millwork shop.
To read more on the construction of the double-helix staircase, and to see conceptual drawings, click the View PDF button below.
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