Jerash (Gerasa) in northwestern Jordan is an important Decapolis city displaying urban development for more than a millennium beginning in the late Hellenistic period (1st century CE). Despite more than a hundred years of archaeological explorations at the site there are stillmajor questions about the city's urban development which are left open due to the nature of the archaeological investigations undertaken at the site. This in particular pertains to questions about the city's extension in the Roman period. During the investigations undertaken by theDanish–German Northwest Quarter Project since 2011 it has become clear that this area of the city partly has undergone extensive phases of reuse and therefore a strict chronology is difficult to obtain through the archaeological strata themselves. In the 2013 campaign excavation was undertaken in the largest cistern within the city walls, which is located in the Northwest Quarter. The cisternwas in parts lined with several layers of mortar belonging to different phases of use. Furthermore water pressure pipes embedded in mortar were found in-situ on the hill. Sincemethods for datingmortar have becomemore refined over the later years it was decided to have 14C AMSdating of 25 samples done in order to test chronology and relative phases in the cistern and the construction of thewater pressure pipe systemin order and clarify dating aswell as possible relation between the cistern and the pipes. These results together with the archaeological evidence show that the urban development of Gerasa and its extension in the Roman period needs to be reconsidered and that there now seems to be hard evidence for among other things water supply in the Northwest Quarter dating to the Roman period which is earlier in this part of the city than usually assumed.