This paper deals with diagnostics of flat roofs with flexible sheets for waterproofing (membrane). The diagnostics is focused on defects and water tightness of membrane. The roof is one of the building's most important building envelope. In case of defect, the other layers of the roof, including the structure frame, are degraded. The introduction gives an overview of individual methods and their division. The following part describes advantages and drawbacks of the individual methods. The next part is focused on the following type of test - High Voltage Membrane Testing. The test is described in detail, including the Conductive detection membrane, which was developed specifically for this method. This test, in combination with other methods, seems optimal, but it is most reliable to prevent possible defects by following technological procedures.