Hoffmann Architects

US Capitol Overview

Historic Dome Restoration

AIA Virginia Honor Award for Excellence in Preservation, AIA|DC Design Award for Excellence in Historic Resources & Preservation, AIA New England Regional Council Design Award, Engineering News-Record MidAtlantic Award of Merit

A symbol of American democracy, the United States Capitol Dome was constructed more than 150 years ago and has become one of the most recognizable architectural landmarks in the world.  Designed by then Architect of the Capitol Thomas U. Walter in the 1850s to accommodate the newly expanded Capitol building, the Dome was constructed during the Civil War, with construction proceeding continuously despite there being scarcely a dozen people available to work. Each ornamental element was crafted by hand, with as many as 200 richly detailed individual pieces in just a single column capital.

In the first comprehensive restoration in more than half a century, the Dome underwent a multi-year, ambitious project to repair thousands of cracks, improve water-tightness, restore or replace structural and decorative elements, remove layers of peeling paint, and protect the Dome from corrosion and deterioration. Beginning in the 1990s with an award-winning water penetration study, the final restoration began in 2014 and was completed before the 2017 Presidential Inauguration.

Hoffmann Architects served as Architect-of-Record for this historic project, working closely with the Architect of the Capitol to develop a master plan for the restoration of all systems, spaces, and finishes from the floor of the Rotunda to the Statue of Freedom. Pilot tests and field studies helped to refine the restoration strategy and to provide the most accurate cost estimates possible. The Dome restoration project stayed within the congressionally authorized budget.

Constructed primarily of cast iron, the Dome was highly susceptible to corrosion from exposure to the elements. Water infiltration through pin holes in the Statue of Freedom and through cracks and open joints in the exterior shell led to leaks in the interstitial space between the inner and outer domes, promoting the rusting of ironwork and failure of protective coatings. Irreplaceable historic artwork in the Rotunda, including the Apotheosis of Washington and the Frieze of American History, were at risk of damage due to leaks. At the exterior, decorative elements had succumbed to such severe corrosion that many had fallen from the Dome to the roof of the Capitol building, posing a serious safety concern.

Several condition surveys were undertaken to assess the condition of the Dome and to document cracks and other deficiencies. With each passing year, studies found that items identified as in need of repair in earlier reports had further deteriorated, and the requisite repairs would continue to become more complex and costly if restoration were postponed. Some cracks were several feet long, in cast iron plates about three-eighths of an inch thick, a significant threat to the integrity of the structure. Using both innovative technology and historical tradecrafts, the project team repaired more than 1,300 cracks and deficiencies in the cast iron, repaired or recast intricate ornaments, gutters, and balustrades, removed hazardous materials, upgraded mechanical and electrical systems, and repainted the entire Dome, inside and out, according to a historical color scheme.

As a highly trafficked, active government building and visitor site, as well as home to invaluable art and statuary, the Dome required extensive measures to protect the public and the building interior during restoration, while maintaining full accessibility and operation throughout the project. As such, design of the scaffolding and the protective canopy in the Rotunda were a critical part of the project.

Cracked cast iron was repaired using a proprietary metal stitching process, by which a series of steel pins were installed by drilling and tapping to draw the sides of the crack together, resulting in a continuous row of interlocking pins that creates a strong, pressure-tight repair. Then, locks were driven in across the joint line of the pins, pulling the repair together. The stitch repairs are watertight, absorb compression stresses, and spread tensile strains away from the original failure point, for a long-lasting solution.

At the tholos level, just below the Statue of Freedom, the balustrade was in such poor condition as to merit complete removal to a foundry, where historical pieces that were beyond repair could be salvaged and recycled. Original elements were melted down, re-formed, and reintroduced into the balustrade. At the cupola, some windows dating from the original 1850s construction were too damaged to repair. The original slump pour glass was replicated using historically accurate methods, whereby 2,000-degree molten glass was poured between rollers and cut to shape.

Some of the worst damage to the Dome was at the internal gutter system, which was clogged with rust sloughed off from cast iron elements. Many new parts were forged in the foundry, where the entire gutter system was assembled and tested before being shipped back to the Dome, hoisted up through 21 levels of scaffolding, and reassembled in place.

Hundreds of decorative acorn pendants, grape clusters, antefixes, scrolls, and other ornaments throughout the Dome exterior, weighing hundreds of pounds each, were disassembled, blasted to remove peeling paint and rust, repaired, repainted, reassembled, and reattached. Those ornaments that were damaged beyond repair were recreated using 3D scanning technology to create accurate sand molds, then pouring molten cast iron to forge a new part. Once the cast iron pieces had been restored, a special “Dome White” paint was formulated and applied to the exterior of the Dome to replicate the sandstone painting from the original construction.

The compromised condition of the exterior Dome had led to leaks, which in turn caused severe deterioration, rust stains, bubbling and peeling paint, and corrosion at the interior of the Rotunda. Cast iron at the inner Dome was repaired and painted, and artists restored portions of the Frieze that were damaged. As part of the work, lead paint was removed down to the bare cast iron. The interior had never been stripped down before, so the original coats of paint enabled the design team to develop timelines for different color schemes, dating back to the original construction. The final colors are based on the 1906 palette, adjusted to add contrast and definition.

The most comprehensive and detailed restoration since the Dome’s completion in 1866, the project has been described by members of Congress as “symbolic” (Paul Ryan), “part of the history of this magnificent building” (Mitch McConnell), and “preserving the solidness of our country” (Shelley Moore Capito). As stewards of the building most emblematic of American democracy, the project team painstakingly dismantled, cleaned, repaired, replicated, restored, and reassembled the intricate components of this feat of architectural design and craftsmanship, treating the Dome of the United States Capitol with the respect and attention to detail that befits a national treasure.