The Roman fort at Housesteads on Hadrian's Wall is located on a high ridge formed by an outcrop of the Whin Sill. As a result of this location, the supply of water to the fort using conventional aqueducts does not appear to have been possible, and yet no alternative methods of supply have been identified. This paper, after analysing detailed meteorological records from the site, concludes that the Romans would have been able to supply a garrison of 800 men with water using rainfall harvesting from the roof structures within the fort. Using an estimated runoff rate from the roofs of 65 per cent means that the soldiers could have been supplied with 10 litres/capita/day or 8 tonnes of water in total each day provided that a storage capacity of 300 cubic metres existed within the fort. This storage capacity is believed to have been provided by stone-lined tanks with volumes of around 2 cubic metres. Perhaps four or six of these tanks would be associated with each of the major buildings within the fort. Excess surface drainage and overflow from the storage tanks would have been led to the south-east corner of the site as a result of local topography to be used for flushing the latrines.