Compact roof systems that are done right, using materials that are not mould-prone between a correctly installed vapour barrier and the roof waterproofing membrane, have typically been considered to be not very moisture sensitive. Moisture in compact roofs has therefore perhaps not received sufficient attention. With large precipitation amounts in southern Norway in the fall of 2000, the theme of built-in moisture in compact roofs became relevant. Both in connection with roof work during periods with heavy precipitation and for example, from leakage, relatively large amounts of moisture can get trapped in the roof. This field study was performed in order to investigate how flat, compact roof behave over time when moisture has been trapped in the construction. The investigation includes 12 roofs. 10 of the 12 roofs were chosen among roofs we knew had considerable problems with built-in moisture during the building period (fall 2000). Two of the roofs did not have any previously known moisture problems, and were therefore to act as reference roofs. Phase 1, which was completed in the summer of 2002, involved the examination of all 12 compact roof constructions. Phase 2 was completed in the summer of 2004 and included an examination of nine of these roofs. The degree of moisture in the roofs was evaluated, and information was obtained about what types and how large the moisture problems have been. Even though moisture had quite clearly penetrated into the roofs, there was less evidence of moisture in several places 1½ year after the roofing period than described by the roof entrepreneurs subsequent to the roofing period. In Phase 2, after a further two years, this impression was reinforced. The results were presented at the 7th Nordic Symposium on Building Physics. The roofs are now examined for the third time (Phase 3, 2007) in order to evaluate the self-drying potential. A more thorough discussion of the self-drying mechanisms of compact roofs as well as a discussion of the risk of mould growth will be given in the paper on the basis of the field survey.