Iowa State Capitol Dome Restoration

Des Moines, Iowa
  • Client
    State of Iowa
  • Year
    2019
Awards
  • 2019 • Preservation Iowa
    Best Public Structure Preservation

The Iowa State Capitol, designed by architects John C. Cochrane and Alfred H. Piquenard, was dedicated in 1884 and completed in 1886. The 250,000 square-foot building is largely comprised of sandstone and brick. The original sandstone was from Carroll County, Iowa, and the brown sandstone from Saint Genevieve County, Mo. The brick masonry originated from multiple local sources while the original mortar was Portland based, which is unusual for buildings constructed during this time period. Modified Renaissance in style, the Capitol’s design incorporates rectangular forms, high ceilings and large windows. The roof is commanded by a central towering dome, covered in gold leaf, and surrounded by four smaller, corner-domes. The main dome has steel ribs with brick structure infill. The brick masonry tapers as it ascends from three, two, and then one wythe at the top of the dome. The brick, original to the building, had deteriorated over the years with noted acceleration in 2014 and 2015.

OPN Architects work on the Capitol domes began with a study of the domes’ unique conditions in 2015. The study included exterior investigations to understand the condition and cause of deterioration of the copper skin and masonry. Temperature and humidity monitors were placed throughout the dome; each section was scanned with an infrared camera. The root cause of deterioration was moisture. The immense 275-foot-tall volume lacked an appropriate thermal barrier between the interior and exterior of the space, allowing moisture to travel through the masonry. In addition, there were areas of the upper dome below the lantern that, over time, began to allow bulk water leakage onto the back side of the masonry, further deteriorating the dome.

Following the study, the State of Iowa was able to acquire funding for the project through state appropriations in 2016. Construction began in the spring of 2017, with the project recently completed in the spring of 2019.

The restoration included the replacement of the more that 12,000 deteriorated interior bricks, insertion of a monitoring platform, thermal separation from the volume below to prohibit moisture transfer, exterior limestone and masonry restoration, sheet metal upgrades interior to the lantern level, protective painting of the steel structure, exterior copper restoration and repairs and painting.

Given the construction method and historic significance of the building, the project approach was to replace old materials in kind. Materials were restored wherever possible or replaced to match the original aesthetic. The restoration and repair work included the replacement of the deteriorated brick masonry on the interior of the dome. Brick face had spalled and collected at the attic space around the base of the drum and on the sky dome. Much of the original brick needed to be completely replaced, but it was not a standard size – often made up of fragments broken in the field. The replacement brick is handmade in North Carolina and most closely matched the original in size and composition. All exposed steel was painted with a high-performance coating protecting the surface and extending the structure’s longevity.

One of the biggest challenges to maintenance at the Iowa State Capitol is one of access. The immense scale and height of the building make it difficult and cost prohibitive to access the dome on a frequent basis. The project included the insertion of a mid-platform allowing for up-close investigation and monitoring of the masonry, which will increase the building’s longevity. The addition of the thermal separation of the overall building volume below from the dome will help prohibit the stack effect and moisture transfer from occurring, prolonging the life of the brick masonry.

On the exterior, the original brown sandstone was patched, tuckpointed and resurfaced in areas of deterioration. When stone replacement was required, new stone was supplied from Oklahoma, offering the closest color match available. The scope included copper restoration, window upgrade to thermal units at the lantern level, and replacing window surrounds with mahogany to match the original wood species. The exterior lighting of the Iowa State Capitol was also addressed; designed to better illuminate the architectural elements.

The Capitol is one of the most recognizable buildings in the state of Iowa. It is a symbol of perseverance, prosperity, and a sense of community for the state and people that it serves. Increased access, sensitive restoration of exterior stone, wood and copper elements, has helped to ensure the future longevity of this very symbolic and historic building.