The historical roof was heavily damaged during World War II. Before that the roof had undergone multiple maintenance interventions. Serious problems in the roof were reported in the 1970s, and some of them were solved in the 1980s. The roof that we see today has gone through different historic modifications and represents complicated conservation issues. In 2015, in preparation for Expo Milano 2015, the facades, statues, and mosaics underwent detailed cleaning and repair, using a giant moving crane scaffolding system.