Unity Temple, Frank Lloyd Wright’s 1909 masterpiece in Oak Park, Ill., is considered one of the key works in the development of modern architecture. It is known for its rich but simple surfaces, its monumental but intimate character, and its serene but dynamic interior spaces. It’s also a key work in the development of concrete architecture: one of the first major buildings to use reinforced concrete as both a structural and finish material.

After more than a century, Unity Temple continues to serve the religious congregation that originally commissioned its construction. The temple also draws thousands of tourists from around the world each year and houses a wide range of concerts, lectures, and social events. The building is widely admired, heavily used, and in constant need of repairs and restoration.

The Unity Temple Restoration Foundation (UTRF), working in concert with the congregation, has developed a master plan for the restoration, which is divided into three phases:

1) building envelope and stabilization
2) interior climate control
3) interior finishes

The complete restoration is estimated to cost $20 to $25 million and to proceed over time as funds are available. One recent Phase 1 project demonstrates both the seriousness of the building’s problems and the ability of knowledgeable, dedicated design and construction experts to solve them.

Emergency preparedness

When a 4x4-foot section of plaster and concrete came loose and fell into the Unity Temple sanctuary, it was clear the structure needed emergency repairs. The damage, which occurred in September 2008 after several days of heavy rain, accelerated efforts to address longstanding drainage problems unsolved by previous restoration.

An inspection showed serious deterioration of concrete and reinforcing steel in the building’s roof slab. Some of the concrete was disintegrating and some rebar was so badly rusted that it flaked away like sawdust. After a full engineering investigation by Skokie, Ill.-based CTLGroup, it was decided to immediately remove all the ceiling plaster and concrete that could present a falling hazard. Then, as a safety precaution, plywood boards were bolted to the west, north, and east ceilings and painted to match the existing colors.

The south roof, however, was so severely deteriorated by 100 years of water infiltration that it threatened the stability of the entire structure. The south roof slab would have to be almost completely rebuilt and its inadequate drainage system replaced to prevent further damage.

Preserving structure, protecting occupants

The major challenge of the project involved accomplishing this significant structural repair, while keeping the building open and protecting its occupants and contents from harm. Steps were taken to prevent work on the south roof from causing damage to the pulpit, organ loft, original lighting fixtures and art glass windows below.

Extensive shoring was installed from the basement to the roof to support the south wall during demolition and reconstruction of the concrete roof slab. Ground-penetrating radar was used to locate and map the original heating tunnels under the building, so shoring posts would not be placed above hollow tunnels. The designed shoring supported all dead loads, superimposed dead loads, construction loads, and live loads.

Interior and exterior scaffolding gave workers access to the roof and south wall. Plywood boxes were built to encase lighting fixtures in the work area, and a partition constructed to surround and protect the organ.

The irreplaceable art glass clerestory windows and their steel frames just below the roof slab were documented in place, then carefully removed and transported to Botti Studio of Architectural Arts in Chicago. While there for safekeeping during the roof repair, they were inspected, cleaned, and restored.

Up on the roof

Contractor Zera Construction, Niles, Ill., already worked on a project at Unity Temple, restoring concrete soffits and fascias 10 years ago, so chairman Alex Zera was aware of its unique challenges. “This project had a small footprint (the south roof plan dimensions are only 41x21 feet), but a complicated process. The biggest technical challenges were building the shoring and keeping the building interior dry during the roof reconstruction,” Zera says.

The roof slab is supported by large columns in the corners of the sanctuary and by a row of ornamental columns located outside of the art glass clerestory. Because of considerable concrete deterioration around these supports, the repairs took place in stages to maintain the roof slab’s integrity and its connection to the supporting columns.

“We built a temporary roof above the existing roof, composed of four panels that we could remove one by one, while keeping the rest of the area covered. When doing the demolition, we used small air hammers and worked only in small areas at a time, to reduce the amount of vibration and minimize the potential for further damage,” Zera says. “Then, after we placed concrete in one area, we had to wait for it to gain a specified strength before moving on to the next section.”

The existing slab was approximately 9 1/2 inches thick, consisting of 8 1/2 inches of structural “cinder concrete” (as described in original design documents) with a 1-inch “portland cement facing bottom layer.” There was also a 1/4-inch-thick “portland cement facing layer” on top of the roof slab. The original top and bottom slab reinforcement consisted of 1-inch square twisted bars and 1/2-inch smooth round bars. Much of the original slab reinforcement was salvageable, and it was supplemented by new No. 4 reinforcing bars spaced to match existing bar spacing. Before placing new concrete, the contractor wire-tied galvanic anodes to the steel rebar to prevent further deterioration of the concrete and corrosion of the reinforcement.

The new lightweight concrete slab is 9 1/2 inches thick to match the original. CTLGroup’s concrete specification called for a compressive strength of 3000 psi at 28 days, equilibrium density not to exceed 115 pounds/cubic foot, a maximum 3/4-inch aggregate size, maximum water-cementitious ratio of 0.42 by weight, and 5% to 8% air content.

After curing, the slab was topped with tapered insulation to modify the roof pitch and a modified bitumen roofing system installed.

Addressing the drainage issue

UTRF’s restoration architect, Gunny Harboe of Harboe Architects, PC, Chicago, says water infiltration has been an issue in many of Wright’s buildings, including Unity Temple. “The roof drain system had deteriorated over time, but it was inadequate to begin with, because the original pipeline was only 2 inches in diameter.”

Mechanical engineer Mark Nussbaum of Architectural Consulting Engineers, Oak Park, Ill., designed a new drainage system that was installed on the south roof and will eventually be implemented on all four. The existing roof drain head and downspout in the sanctuary’s southeast column were removed, and a new drain assembly built and tied into the sewer system. A cleanout was installed in the downspout at the second-balcony level to facilitate future maintenance.

Interior finishes

Once the new concrete was cured, Zera’s crew was able to dismantle the structural shoring and leave only the access scaffolding in place. The underside of the roof slab, which serves as the ceiling of the sanctuary, was then sand-float plastered to match the original design and finish. The plaster is a historically accurate material based directly on Wright’s original specification—lime, portland cement, and goat hair used as a binder.

The plaster cured for two weeks, and then it was faced with kraft paper using wheat glue. The paper facing was painted to match the Temple’s current interior colors. In the future, when critical Phase 1 and 2 restoration projects are complete, UTRF will allocate funds to paint the interior in accordance with Wright’s original plan. The paper and its interim paint job will be easy to remove and replace at that point.

The final steps of the south roof restoration project included the replacement of architectural oak trim on the interior, reinstallation of the restored clerestory windows, and removal of the access scaffolding. This phase of work was completed in May 2010.

Funding restoration

In 2009, Unity Temple was listed by the National Trust for Historic Preservation as one America’s 11 Most Endangered Historic Places. The south roof repair described here was a critical step, but only one of many needed to restore Unity Temple to its proper condition and preserve it for future generations to enjoy. UTRF and the Unity Temple Unitarian Universalist Congregation worked to raise more than half a million dollars to fund this project, including grants from Save America’s Treasures and Partners in Preservation, $125,000 in Illinois state capital funds, and donations from hundreds of individuals around the world.

To learn more about the restoration of Unity Temple, visit www.utrf.org.

Kenneth A. Hooker is a freelance construction writer based in Oak Park, Ill.