The mills consisted of two floating rafts: on the main one, nearest to the bank, were located the millstones inside the characteristic small house, often with a cross on the top. Between the two rafts was supported the bladed wheel. The working principle is shown in the animation carried out at the Technical College Meucci in Rome. The rafts were moored to the banks by long chains; a masonry ramp and a wooden footbridge allowed access to the bank (pict. E1 e A3).
In every mill four workers were normally employed: two "loaders" that brought the grain and the flour with beasts of burden, a "servant" that worked at the millstone and was assigned to repairs and a generic "boy" for various services.
In a mill about 4500 kg of grain per day were ground and the production was daily recorded by the food administration; we have this data from 1823 to 1845 (pict. D1).
The Roman millers located the religious see of their corporation, "Romana Molendinariorum", in the S. Bartolomeo church on the Tiber Island, building a chapel decorated with images of their activity (pict. E9); a plaque still commemorates one of the many restorations of the chapel (pict. D2).
The mills obstructed the normal water flow of the river and often the strength of the current broke the moorings and dragged the mill downstream with disastrous results. As early as the middle of the 1700s it was proposed that they be moved downstream from the town and in 1870 the Public Works Ministerial Commission included the floating mills among the causes of the Tiber floods in Rome. After the big flood of December 1870 and the successive construction of the embankment the Tiber mills definitively disappeared.