The Duomo di Milano's basic floor design consists of large square tiles made of Candoglia marble with a geometric floral design (a large round central rose, shells, and floral motifs arranged axially along the vertices of the square) that repeats alternatingly. From the time it was created in the 15th century, it has always been made of only three materials: pink Candoglia marble with inlays made from small slabs of black marble from Varenna (Como) and red marble from Arzo (Canton Ticino).
The first mention of the Duomo di Milano's floor is from 1392 when the order was given to complete the flooring "a medoni", or "with bricks", from the back of the choir all the way to the door that probably led to Santa Maria Maggiore, the area around the apse in which the floor was described in white marble alternating with black.
In 1404 the project was entrusted to Marco da Carona who completed the flooring of the northern sacristy in soft pink Candoglia marble with geometric inlays in black marble with red brocatelle.
Following the orders from San Carlo for the Cathedral's decoration, on 24 July 1567 the design for the entire marble flooring was entrusted to Pellegrino Pellegrini, known as Tibaldi, however construction didn't begin until 27 May 1575. The archbishop of Famagosta Ragazzoni bitterly criticized the condition of the floor on 12 April 1576 and in his decree suggested that a decoration in keeping with the design created by Pellegrini in the central nave and in the two smaller external naves be completed, leaving the medians as they were "for the passage of wagons for resupplying the construction site".
Thanks to a resolution by the Fabbrica, dating from 29 December 1585, Bernardo Robbiano was once again hired (after having previously turned down the offer) to complete the Duomo's entire "solatura" [flooring] in marble according to Pellegrini's designs, making the raw marble available to the architect. The contract was taken from Robbiano after two years and given to Lelio Buzzi in the second half of the 17th century.
The floor completed according to Pellegrino Tibaldi's design was begun in 1584 and was the object of an important renovation from 1914-1920. The purpose of the renovation, ordered by Luca Beltrami, was to reinforce the most resistant coloured inlays in the visible surface.
In fact, due to abrasion caused by pedestrian traffic, the white Candoglia marble is worn down more quickly and more evidently than the coloured marbles, creating recesses that can even lead to the detachment of the inlays.
For this reason it has been necessary to perform regular maintenance, restoration, and renovation as needed since the middle of the 20th century. So far the restoration has included work to repair the marble slabs in the areas that show the most wear and to strengthen and re-grout the adjacent areas. The floor has been examined, whitened, reinforced, re-assembled, levelled to match the original, sanded, and re-grouted.
In the three years from 2012 through 2014 restorations of the inlaid floors at the rear choir, in the Duomo's entrance foyer, and in the central nave were completed. Certain parts of the original floor are on display and preserved in the Grande Museo del Duomo and maintenance and substitution of the marble goes on, with the Veneranda Fabbrica continuing, since 1387, to oversee the safeguard and restoration of the Cathedral, the international symbol of Milan.