By Col. Peter Mueller, P.E., PMP, F.SAME, USA (Ret.), Joseph Abriatis, CCM, PMP, and Justin Kieffer  

The U.S. Capitol Dome is one of the most recognizable structures in the world and stands as a symbol of democracy. It sits atop a building that was once smaller than it is today, but expanded as our nation spread west across the continent.

The original dome, built of wood and covered with copper, was not large enough for the extended building and created a fire hazard. On Dec. 16, 1854, as the Capitol’s extensions were under construction (from 1851 to 1865), the Architect of the Capitol (AOC), Thomas Walter, displayed a drawing outside his office that showed what the completed building would look like. In place of the existing Bulfinch Dome, there was a much larger, cast iron structure that closely resembles what we see today. Members of Congress who observed the design quickly embraced it and on March 3, 1855, legislation was signed by President Franklin Pierce approving the funds for construction of the new dome.

Montgomery Meigs, a captain in the U.S. Army Corps of Engineers was appointed Principal Superintendent of Construction, and along with Walter, the two were responsible for the iconic structure. During construction, significant engineering challenges were solved through ingenuity and drive of the combined team. For example, to raise the cast iron into place, Meigs erected scaffolding in the rotunda and added two long “sticks” to act as mast and boom. A steam engine, initially fueled with wood from the previous dome, was used to lift up to 20,000-lb of cast iron. The dome stood for almost 100 years before its first restoration in 1959-1960.


By the 1990s, weather and age were slowly destroying the dome. Water threatened to infiltrate the building and create further complications, including causing potential damage to artwork and the historic fabric housed under the dome’s cast iron skin. Without a major restoration, the condition of the building would worsen.

To achieve its mission “to serve Congress and the Supreme Court, preserve America’s Capitol, and inspire memorable experiences for all who visit the buildings and grounds,” AOC moved forward with the Dome Restoration Project, the most comprehensive and challenging work the structure has seen since its original construction.

Capitol Dome Restoration


The project was awarded on Sept. 30, 2013, to Turner-Smoot, a Joint Venture, and came with a strict timeframe to be completed before the 2017 Presidential Inauguration—with no option for extending work until after the event.

The restoration comprised of three phases: exterior, interior (interstitial space between the two domes), and the rotunda. The exterior phase required the removal of more than a dozen layers of paint, application of new paint and sealants, and restoration of the cast iron shell and ornamentation, while maintaining water resistance to protect it from effects from the elements. The interior phase addressed similar issues as to the exterior, but within the space between outer and inner domes. Scaffolding was required around the entire dome, as well as a “donut-shaped” safety-netting system inside the rotunda. The final phase included new lighting, removal of lead paint in the rotunda, and repainting it to historically appropriate colors.

The exterior phase began in 2014, with the erection of 25 levels of scaffolding— weighing 1.2-million-lb—around the entire dome. The scaffolding was supported at three points on the dome structure itself and not on the roof of the Capitol. Scaffolding was constantly monitored during the course of the project to ensure that its capacity was never exceeded by environmental or construction forces.

Additionally, the installation of the scaffolding started at the peristyle level (base of the dome) and was erected upwards, eventually around the Statue of Freedom. Scaffold level one was hung over the side of the peristyle from scaffold level two, which rested on the peristyle floor. Scaffold levels two to seven were supported from the peristyle floor. Then scaffolding from level eight to level 25 was entirely self-supported from the boiler plate floor. Restoration work and paint abatement began from the bottom up, following the sequence of scaffold erection. Inside the rotunda, the “donut-shaped” safety-netting system protected members of Congress and visitors to the Capitol.

By the 1990s, weather and age were slowly destroying the dome. Water threatened to infiltrate the building and create further complications, including causing potential damage to artwork and the historic fabric housed under the dome’s cast iron skin. Without a major restoration, the condition of the building would worsen.

With scaffolding in place, containment areas were assembled for the removal of approximately a dozen layers of paint, some of which were lead-based, as well as remediation of hazardous materials in the caulking. The cast iron shell and ornamentation was repaired using a metal stitching process to best preserve the historical fabric of the building. The 36 peristyle and 36 secondstory windows were removed and individual panes replaced. Five Cupola windows were replaced and finally, the exterior was repainted and then weatherproofed.


The interior phase began in 2014 with a complete 3D scan and point cloud. This allowed for the precise measurement of all existing conditions to design and fabricate new personnel catwalks that were weaved through existing structural supports of the dome, providing maintenance workers with access to the backside of the dome’s exterior.

In 2015, the final phase of work began and consisted of installing new energy-efficient lighting within the rotunda, new ductwork, abatement of lead paint, and repainting the dome’s cast iron colonnade and coffered dome to a historically appropriate color determined through paint analysis.

The rotunda was required to be closed in order to erect and later disassemble the scaffolding for the restoration and repainting. The two closures started a few weeks before the summer Congressional Recess and were completed on time, minimizing the impact to staff and visitors. The safety netting that was installed for the exterior phase had to be extended to conceal lower levels of the rotunda scaffolding where work was to take place. The scaffolding was designed so that work could be completed, but could also safely allow visitors views of the artwork, statues, and The Apotheosis of Washington, the famous fresco painted by Constantino Brumidi in the eye of the rotunda.


While the Capitol remained open during renovation, the project team arranged work to minimize the impacts on the occupants and visitors. Modifications were made to adjust to unique events and facility requirements. Safeguards were established for the removal of hazardous materials.

Another challenge was ensuring that when cast iron was stripped bare of its 12 layers of paint (approximately a quarterof-an-inch thick) in containments, it was prime painted within eight hours to avoid flash rusting. Workers had to stop blasting, clean up the containment area, and paint the cast iron before the building opened for business. If they missed the eight-hour window, the cast iron required re-blasting.

To preserve the artwork in the rotunda, AOC and the contractor ensured that each time a section of the dome’s balustrades, ornaments or sealants were removed, the building remained watertight.


When the dome was constructed more than 150 years ago, Thomas Ustick Walter did not design how the individual ornaments would be constructed, nor out of how many pieces. This was left to the foundry. Ornaments on the exterior vary in weight from 1-lb to 680-lb and are generally not monolithic but assembled from many small components. The ornaments at the top of the peristyle columns are made up from between two and 18 individual castings and vary depending on the type of ornament.

Resurfacing Dome

The records of how these ornaments were constructed and attached did not survive. It was not until scaffolding encompassed the columns that the team could begin disassembling these ornaments to investigate how they were made and fastened to the building so that they could start developing repair strategies. The restoration process involved disassembly of each ornament; tagging and transporting the pieces to a work platform on the roof; blasting paint off from each piece; and priming, painting, reassembling and reattaching the ornaments to the dome structure. The project team used the 1860s original drawings and performed mock-ups to determine how best to accomplish the work.


The repair methods selected for the cast iron dome consistently followed a preservation approach based on studies from the 1990s in coordination with the National Institute of Standards and Technology. Cold processes such as metal stitching were chosen over welding or brazing to eliminate the permanent change and damage that heat would cause to the iron. Repairs such as backer plates and tabs were performed based on methods used by the original builders to correct defects that occurred during the dome’s construction.

Since the metallurgy of historic cast iron from the 1850s is different than modern day cast iron, the modern metal stitching process for today’s carbon steel or cast iron would not work. Special, double-hooked stitching pins were designed and fabricated for the specific use in historical cast iron, which has a lower tensile strength compared to modern day material.

Repair of the cast iron deterioration involved over 1,000 cracks measuring more than 8,000-in. The metal stitching process entailed drilling closely spaced holes along the length of cast iron crack.

The next steps involved inserting threaded stainless steel tabs in the drilled holes, shearing off the heads, and grinding the tables flush with adjoining surface. After the pins were installed, a series of 18 holes were drilled perpendicular to the crack spaced at intervals. These created a cavity for a “lock” to be hammered into place. Once the lock was installed, the locks and pins were ground smooth. Then the repair was sealed with epoxy before the surface was primed in preparation for painting. It took an average of one-man-hour to stitch a 1.25-in of crack. Where sections of plates were badly damaged, new sections were cast to match. The damaged area was cut out and replaced. These “Dutchman” repairs left the maximum amount of the original fabric in place while proving to be a strong repair.


The Dome Restoration Project may be one of the most visible restorations in recent history. The work was successful because of the dedicated effort, ingenuity, technical expertise, and diligence of hundreds from government and the private sector. This team worked with the attention to detail required to deliver quality, within budget, and ahead of the inauguration in January 2017. The renovation should protect this icon for the next 50 to 70 years.

The AOC will continue caring for the Capitol Dome and will make certain its historic fabric is preserved for future generations of Americans and others around the world who see this structure as a shining beacon for democracy.

Col. Peter Mueller, P.E., PMP, F.SAME, USA (Ret.), is Director of Planning and Project Management, Joseph Abriatis, CCM, PMP, is Project Controls Manager, Planning & Project Management Division, and Justin Kieffer is Senior Communications Specialist, Planning & Project Management Division, Architect of the Capitol. They can be reached at;; and

[Article originally appeared in the September-October issue of The Military Engineer.]