Fairly detailed descriptions and interpretations of the Ostian store buildings (horrea) were produced by Rickman (1971). Some can be identified through the presence of large storage-jars, or raised floors (suspensurae) meant to protect grain from damp and overheating. Lanciani reports that in the central-north part of the city "floors of the storehouses have been found still covered with a layer of grain which, on being brought into contact with the air, tumbled into dust" (Notes from Rome, 15 ). In one case the name Horrea is recorded in an inscription in the facade (Horrea Epagathiana et Epaphroditiana). Other buildings can be identified as store buildings on the basis of their resemblance to these buildings. A special category are the buildings with dolia defossa, huge, buried jars, in which wine or olive-oil was stored.
The store buildings are all from the Imperial period, most of them from the first half of the second century, while some date back to the first century. There is a relation with the work of Trajan in Portus, possibly also with that of Claudius. In the store buildings one alteration from the second half of the third century has been traced, and very few activities are documented in late antiquity.
Characteristic for the appearance are buttressing, thick walls, few entrances, slit-windows high up in the outer walls, special locking devices, long rows of rooms (cellae), and ramps leading to the first floor. Rickman distinguishes two kinds of lay-out: cellae around a courtyard with portico or colonnade; cellae along a corridor. The Horrea Epagathiana et Epaphroditiana have an axial lay-out, consisting of two vestibules, a courtyard, and an accentuated room. The axis was emphasized by the mosaic of the courtyard: behind the vestibule is a panel as wide as the vestibule with the depiction of a tiger, meant to be seen from the vestibule; behind is a large panel with meanders and a swastika; in front of the accentuated room is a panel as wide as the room with a depiction of a panther, meant to be seen from the room.
The size varies from quite small to very large. Most of the large horrea are, for obvious reasons, situated to the north of the Decumanus Maximus and Via della Foce, that is along the Tiber, and the main entrance of most of these is turned towards the river. Most of the smaller ones are to the south of the Decumanus and Via della Foce. Concentrations are found to the northwest of the Forum, and to the south of the eastern Decumanus. Store buildings have also been identified in the Ostian Trastevere
Both the number and size of the store buildings show, that they did not serve Ostia alone. They held reserves for Rome, even after the construction of Portus. Smaller horrea south of the Decumanus and Via della Foce, and perhaps others, served the local market. The Grandi Horrea were probably owned by the Emperor, and part of the grain stored there was taken to the bakery in the Caseggiato dei Molini, a bakery most likely working for the Emperor. Horrea I,VIII,3 were a non-Imperial investment, as is shown by an inscription in its facade: Horrea Epagathiana et Epaphroditiana, Epagathus and Epaphroditus being two freedmen. How many store buildings were the property of the Emperor, and in which period, cannot be said.
The workers in the Imperial store buildings may well have been Imperial slaves and freedmen. As Rickman has remarked, "it is a remarkable fact that among the mass of Ostian inscriptions there is no reference to the workers and managers of horrea". Some data from outside Ostia is available however, largely from Rome from Imperial horrea. Here slaves and freedmen only are documented, many of them Imperial. It is not known whether the workers in horrea usually lived in the building, or elsewhere.