Historical Roofs: The United States Capitol Building Dome


The United States Capitol building is one of the most symbolically important and architecturally impressive buildings in the world. It has domiciliated the US Congress (the Senate and US House) for more than two centuries. The building was started in 1793. Ever since, the building has been built, burned-over, reconstructed, extended, and repaired. In the present day, the Capitol stands as a monument to its builders and the people of American and their government.

The building is located in U.S. Capital, Washington D.C. It has been ranked among the most emblematically significant and architecturally remarkable buildings in the United States. The Capitol has had numerous construction phases. Being an example of nineteenth century neoclassical architecture, the building represents the paragons that helped the founders as they created the new republic.

William Thornton's original Capitol building design.

William Thornton’s original Capitol building design.

Before the construction kicked off, the secretary of State Thomas Jefferson and US president George Washington suggested a competition that would award a city lot and $500 to whosoever came up with the winning plan within a specified period. All the seventeen plans submitted were rejected as they were not satisfactory. Later, Dr. William Thornton submitted his plan that was accepted by the Commissioners in charge.

The United States Capitol building has been the most place able historic edifices in Washington, DC. It is situated at the opposite side of the National Mall. It is an outstanding landmark and a perfect example of neoclassical architecture of the 19th-century. The building has 540 rooms within 5 levels. The Capitol is an imposingly massive building. Its ground level is apportioned to the offices of the Congress. The second level houses the Senate in the north wing and House of Representatives chambers in the south wing. Placed in the center of the US Capitol is the dome, the roofing that we will be discussing. Under the dome, there is the Rotunda, a round space that is used as a gallery of sculpture and paintings of US historical events and figures. The third level serves as the space in which visitors are able to watch the Congress proceedings. In the basement and fourth floor, there are more offices and machinery rooms.


The Capitol Dome Roofing

US Capitol Timeline

The US Capitol dome roof refers to the double-dome that is above the US Capitol building. It reaches upward to 88 m (288 feet) in height and 29 m (96 feet) in diameter. Its design was created by Architect Thomas U. Walter. The roofers started the construction in 1855 and ended in 1866. The dome cost $1,047,291.The dome is not made of stone but cast iron that is carefully coated with paint to look like it is built with the same stone as the most parts of the capitol building. The dome is actually two domes: there is one inside the other. The entire weight of the dome is 14.1 million pounds.

Constructed in 1793, the Capitol roof (dome) has its height from its engineering prestidigitation. The bigger outer dome is basically a thin shell, supported by curved iron ribs that make a ring. On the lower side is a smaller dome that supports itself. The smaller one is visible only from the inner side.

In addition, the Capitol dome roof is among the first domes constructed using preassembled cast-iron ribs. The dome itself symbolizes the switch to lightweight metal ribs from heavy masonry.

The Capitol dome roof is among the oldest great domes that crowned an entirely political building. It, thus, has had a rather eventful history. From the time George Washington laid the foundation stone in 1793, the US Capitol has been built, destroyed by fire, reconstructed, extended, and repaired.

The construction of the first dome of the US Capitol, whose design was created by Boston architect Charles Bulfinch, was concluded in 1824. However, it was a total failure. Other than its poor appearance, the dome constantly required repair. Besides, it was also a fire hazard. In 1855, architect Thomas U. Walter was asked to replace the first dome with a larger, more improved, fireproof dome.

The new dome being constructed on the U.S. Capitol during the inauguration of President Abraham Lincoln in 1861

The new dome being constructed on the U.S. Capitol during the inauguration of President Abraham Lincoln in 1861

A heavier and taller dome whose walls would not push outwards or crack required a lot of iron chains similar to the St. Peter’s Basilica and monumental walls such as the Pantheon. However, Walter did not want that. He made a double dome design — a smaller dome within a taller dome. This formed the illusion of height. The tall outer dome is really a thin shell. It is supported by a ring made of thirty six iron ribs that are architecturally curved. A smaller dome, which is underneath the bigger dome, is open at the top similar to the Pantheon. While it appears like it is made of stone, every part of the Capitol dome is constructed using cast iron.

Eventually, the Capitol dome construction was concluded in December 1863. Nowadays, it stands as the virtuoso symbol of the US democracy.

The dome stacks up higher against a number of the largest domes in the globe.

The First Dome

The modern day cast iron roof (dome) of the US Capitol is actually the second dome to be constructed above the building. The construction of the initial dome started with the Capitol design contest which was sponsored in 1792 by Thomas Jefferson (Secretary of State) under the authority of President George Washington.


Benjamin Henry Latrobe, the second Capitol’s Architect, who replaced Thornton, changed Thornton’s design plan on the outer side by introducing an octangular drum to visually separate the pediment of the top of the building from the bottom of the dome. The third Capitol’s Architect, Charles Bulfinch, changed the plans’ exterior profile further by increasing the height of the dome, which he afterward wrote was at the pressure of the US Congress and President.

In 1822, funds were requested for the building of the center of the building. President Monroe approved the allocation of $120,000. This consisted of the construction of a double-dome structure  (to rise 29 m (96 feet) above the rotunda floor, wooden interior dome, brick and a stone as well as a wooden outer copper-covered dome that would rise to 43 m (140 feet). An oculus 7.3 m (24 feet) wide was set at the outer dome’s crown. It provided illumination to the below rotunda floor. The project was completed in 1823.

For at least two decades, the Capitol’s green copper dome welcomed visitants to the Washington DC until the 1850s. Owing to the United States growth and the expansion and addition of more states, the US Congress size had consequently grown and this pushed the capacity limits of the US Capitol. As such, extensions were constructed onto the south and north wings of the Capitol. In that process, the new, expanded building made the initial dome look aesthetically poor (it faced a lot of criticism just like before). The Congress enacted legislation to construct a larger dome in 1855.


The Second (Modern) Dome

The construction of the second dome roof kicked off after the Inauguration of President Abraham Lincoln in1861.


The Statue of Freedom being cleaned during the dome restoration.

A bronze statue known as the Statue of Freedom has crowned the U.S. Capitol dome since 1863.

Plans were put in place in May 1854 to construct another more improved cast-iron dome for the US Capitol. This was because of the need for the roofer to come up with aesthetics of a new dome, and the usefulness of a fire-retardant one.Walter was influenced by the groovy domes of Europe. He paid special attention to the London’s St Paul’s Cathedral, Paris’s Pantheon and Rome’s St. Peter’s Basilica. Walter’s initial design was described by William Allen (a Capitol Historian) as,

This is the design by Walter

This is the design by Walter

“…a tall, ellipsoidal dome put up on a two-story drum with a ring of 40 columns creating a peristyle that surrounds the drum’s lower half. The upper segment of the drum was meliorated with adorned pilasters upholding an attic that is bracketed. Moreover, a statue crowns the structure, standing on a columned, slender tholus…”

Walter outlined a two-meter (7-foot) drawing of the aforesaid design and exhibited it in his office, where it stirred the attention of Congress members in 1854. One year afterward, in March 1855, US President Franklin Pierce approved the setting aside of $100,000 to construct the dome. Building started after some practical modifications to the initial design (e.g. the reduction of the columns to thirty-six from the initial forty) in September the same year. A distinctive scaffold was made inside the rotunda, made to handle weight considering the weak center area of the floor. There were other major changes introduced to the roofing design.



Apotheosis of Washington

The eleven years that followed saw the dome’s design created with an exterior dome and interior dome rising over the US capitol. In December, 1863, the Statue of Freedom was set on the top the dome. This was not completed until Walter was compelled to redo the dome’s design to accommodate the statue, which had been made heavier and taller than requested. It is ironical that the man who created the design of the dome did not witness its completion, as Thomas Walter resigned in 1865. He was replaced by Edward Clark, who oversaw the completion of the dome. Slightly more than a month afterward, in January 1866, he removed the scaffolding utilized as he was carrying out his work on the Apotheosis of Washington. This symbolized the conclusion of building the US Capitol dome.


The Apotheosis of Washington painting inside the dome.

Some 4,041.2 metric tons (8,909,200) pounds of iron were eventually used by the roofers in the building that ran almost took 11 years. The inside of the dome interior rises to 55 m (180 feet) over the rotunda floor. The outer dome is 88 m (288 feet) tall including the Statue of Freedom’s height. It cost a total of $1,047,291 to build the dome.

Visitation of the dome is extremely restricted. Only Congress members and their chosen guests are allowed. Looking up from the floor of the rotunda, the railing situated 55 m (180 feet) above is hardly visible. Visitants go up a series of metal staircases between the outer and inner domes. They then weave their way to a balcony just below the Apotheosis of Washington. Looking from this position, the painting is distorted and curved. From the balcony, metal stairways lead visitants to the painting and up to the outer side of the balcony underneath the tholos straight below the Statue of Freedom. Some more staircases lead up to the statue and this helps when maintenance is required.

The Statue of Freedom stands in the columned tholus. Two lights act as signifiers to the people of America and Washington that either one or both Congress houses are in nighttime sessions. The white light symbolizes the presence of night session of the House of Representatives. The red light symbolizes the presence of night session of the Senate. When both the red and white lights glow together, they show that the two houses are in joint night session or both Congress houses are in night sessions.

Conservation and restoration of the US Capitol Dome’s skirting and cantilevered peristyle took place in 2012. In the year 2013, the Capitol’s Architect said that a provisory 4-year, $10 million project to conserve and repair the US Capitol dome roof. The project consisted of both exterior work on the dome as well as interior work to the rotunda. The proposal would need installing new lighting, setting up white scaffolding around the dome, repairing the ironwork, stripping the paint and repainting the dome. The project was requires since the dome, which got repair and conservation last in 1960, was corroding and some of the structure’s ironwork had fallen. Congress had not set aside any funds for the project, though.

Quick Facts Regarding the US Capitol Building (Summary)

The US Capitol building has around 540 rooms and 658 windows. The dome alone has 108 windows. The Capitol has roughly 850 doorways.

  • The roofing (dome) was built with 8,909,200 pounds of iron.
  • The crowning feature of the US Capitol dome is the bronze Statue of Freedom. It weighs roughly 15,000 pounds and stands 19 feet 6 inches tall.

Thus, the US Capitol building is a monumental and significant building in America as it has housed the Congress for over two centuries.

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