Curators are putting finishing touches on exhibits at San Francisco’s Conservatory of Flowers, following a $25-million restoration that involved almost complete disassembly and reconstruction.

Erected in Golden Gate Park in 1879, the city-owned Victorian-era greenhouse is the oldest public conservatory in the Western Hemisphere, but it has been closed since a fierce winter windstorm ripped through the 12,000-sq-ft redwood and glass structure in 1995.

To speed palm room reconstruction, the contractor built the upper portion of the dome on the ground and lifted it into place. (This photo and above courtesy of the Conservatory of the Flowers)

The storm shattered 40% of the glazing, damaged some of the structural components and destroyed a portion of the collection of rare tropical plants. But inspection of the building revealed that the conservatory had been deteriorating for years, exacerbated by a series of modifications and high humidity. Much of the wood was like "Swiss cheese," says Deborah Cooper, project manager for locally based design and conservation firm Architectural Resources Group.

The decay came from many sources. Use of iron nails to join wood components in the original construction had caused ferric degradation of the wood around the nails from an acidic reaction. During a 1959 renovation, operable windows used for ventilation were filled in, contributing to excessively humid conditions inside the building. In the 1970s, some redwood was replaced with laminated members. The glue "just turned to mush" in the greenhouse environment, says Nancy Tennebaum, principal of San Francisco-based structural engineer Tennebaum-Manheim Engineers.

The conservatory's oldest and largest specimens required temporary enclosure during the Palm Room restoration. (This photo and bottom by Joann Gonchar for ENR)

The structure has a domed pavilion at its center that is roughly 55 x 55 ft in plan and 68 ft tall. The pavilion, or palm room, is flanked by two symmetrical L-shaped wings, approximately 19 ft tall and 35 ft wide. The wings are made of a series of wood arches, roughly 7 ft, 6 in. on center, connected by purlins at three points on each side.

One of the chief challenges facing the design and construction team was selecting the material for replacement of the deteriorated redwood components. This was complicated by a city ordinance prohibiting the use of old-growth redwood. The team considered aluminum. But this likely would have meant losing the building’s landmark status. Pressure-treated young-growth redwood was another option. However, the pressure treatment and drying process would have required between three and six months.

Wing arches and purlins wre milled from redwood "buckskin" logs.

Eventually the team decided to use redwood "buckskin" logs—old-growth trees that fall naturally or are standing stumps abandoned by loggers. The buckskins were considered the "most historically correct and the most durable option," says Edgar Lopez, project manager for the Dept. of Public Works.

The city assembled its own team to find and collect the buckskins and it supplied the wood to the contractor. Each piece was hand graded and tested for density and structural integrity after drying from 30 to 60 days and before milling to the sizes and shapes required. "Since the strength of the material was not known, we had to develop our own [testing] program," says Tennebaum.

One of the most logistically complex parts of the project was reconstruction of the palm room. The pavilion’s exhibits include a 35-ft-tall imperial philodendron and a 20-ft-tall pygmy date palm, both about 100 years old. The plants are among the oldest and largest in the collection and could not be relocated for construction. To work around them, the contractor, ISEC Construction Services, Davis, Calif., devised a temporary enclosure supported by scaffolding, complete with irrigation and climate-control systems.

To speed construction, the contractor built the top portion of the dome on the ground and then lifted it into place by crane. "Constructing the dome in the air would have taken us twice as long," says Ken Zad, general superintendent for ISEC.

Since the dome forms a triangle, "it has great stability," says Tennebaum. Still, the 29,000-lb lift was nerve-wracking for the contractor, who lifted it from four points inside the structure. "But once we got if off the ground we could see that it wasn’t deflecting or moving," says Zad.