Dome of St Peter's Basilica

The dome of St Peter's Basilica in Vatican City is one of the largest masonry domes ever built and the dominant feature of the Rome skyline. It was designed principally by Michelangelo and completed in 1590.

Design history

The new basilica, replacing a 4th-century church founded by Constantine, was begun on 18 April 1506. Donato Bramante drew the first design for the dome that year. After several changes of architect, Michelangelo took over in 1547 and redesigned the dome. He did not live to see it built: Giacomo della Porta and Domenico Fontana completed it in 1590, in the last year of the reign of Pope Sixtus V. Fontana finished the lantern under Pope Gregory XIV, with inscriptions honouring Sixtus V around its opening. The basilica was consecrated by Pope Urban VIII on 18 November 1626.

Structure

• Two shells: like Brunelleschi's dome in Florence, St Peter's dome has an inner and an outer shell, both of brick.
• Ribs: the outer shell carries 16 stone ribs.
• Height: about 136.57 m (448.1 ft) from the basilica floor to the top of the external cross.
• Span: an internal diameter of about 41.47 m (136.1 ft), slightly less than the Pantheon's roughly 43.3 m (142 ft).

Cracking and iron chains

Masonry domes push outward at their base. Cracks appeared in St Peter's dome, and in the mid-18th century four iron chains were installed between the two shells to hold it together. As many as ten chains have been installed at various times.

Care

The Fabbrica di San Pietro, founded in 1506, is still responsible for maintaining the basilica. The whole of Vatican City, about 44 ha (109 acres), is a UNESCO World Heritage site, with Michelangelo's cupola named among its key works.

Significance

With the Pantheon and the dome of Florence Cathedral, St Peter's is one of the three great domes of Rome and Italy, all of similar span.