Insulae, like domus, had running water and sanitation. But this kind of housing was sometimes constructed at minimal expense for speculative purposes, resulting in insulae of poor construction. They were built in timber, mud brick, and later primitive concrete, and were prone to fire and collapse, as described by Juvenal, whose satiric purpose in writing should be taken into account. Among his many business interests, Marcus Licinius Crassus speculated in real estate and owned numerous insulae in the city. When one collapsed from poor construction, Cicero purportedly stated that Crassus was happy that he could charge higher rents for a new building than the collapsed one.
Living quarters were typically smallest in the building's uppermost floors, with the largest and most expensive apartments being located on the bottom floors. The insulae could be up to six or seven stories high, and despite height restrictions in the Imperial era, a few reached eight or nine stories. The notably large Insula Felicles or Felicula was located near the Flaminian Circus in Regio IX; the early Christian writer Tertullian condemns the hubris of multiple-story buildings by comparing the Felicles to the towering homes of the gods. A single insula could accommodate over 40 people in only 3,600 sq ft; however, the entire structure usually had about 6 to 7 apartments, each had about 1000 sq ft.
Because of safety issues and extra flights of stairs, the uppermost floors were the least desirable, and thus the cheapest to rent. Often those floors were without heating, running water or lavatories, which meant their occupants had to use Rome's extensive system of public restrooms (latrinae). Despite prohibitions, residents would sometimes dump trash and human excrement out the windows and into the surrounding streets and alleys.