Preserving an engineering feat: Restoring the Capitol Dome


A gold-accented achievement in St. Paul’s skyline, the Minnesota State Capitol’s dome is the second-largest self-supportive marble dome in the world.

But looks can be deceiving. There are actually three domes.

Designed after Rome’s St. Peter’s Basilica, architect Cass Gilbert’s engineering feat has lasted more than 100 years, but it needs upkeep. It’s currently undergoing $4 million in weatherization repairs.

The initial cost to build the Capitol, which opened in 1905, was $4.5 million.

Department of Administration Communications Director Jim Schwartz said the repairs are to correct aging issues related to water infiltration. Schwartz explains the three-dome structure includes an exterior dome that people see from the outside, an interior dome seen while looking up in the rotunda and an unseen middle dome that functions as the water transport system.

The outer dome was designed to allow water to penetrate it, and on the interior of it there’s a gutter system that collects and removes the water. Over time, the system has deteriorated along with the joints on the outside marble allowing additional water to enter and cause damage to the inside dome, particularly the plaster, Schwartz said. Other repairs include replacing the windows, louvers and installing a drain for the deck at the top of the dome.

Another major issue is humidity on the inner dome that is built of a brick structure covered by a rubber membrane. When water comes in from the outer dome it falls on the membrane and is directed to a gutter system that pipes water out of the building. To correct the humidity issue, a hole was enlarged on the inner dome, Schwartz said.

Engineers and inspectors found further deterioration on outside decorative elements, including the eagles that stand guard around the dome, as well as some of the roof projections. Veined like grains in wood, the marble is long-lasting, but the veins are deteriorating at a faster rate because of Minnesota’s freeze and frost cycles causing cracks, Schwartz said. A report expected to be completed this summer should detail the additional work needed to be completed.

The majority of the initial exterior work has been completed as well as a good share of the interior work.

By early summer, Capitol visitors should be able to again view the dome from inside once the protective sheeting is removed. The 1-ton chandelier that hangs above the Rotunda is to be hoisted later this year at a ceremony when the fixture will be lit, said Minnesota Historical Society State Capitol Site Manager Brian Pease.