Phase I multi-stage flash (MSF) unit roofs have recently experienced major structural damage, especially at the heat reject section. Excessive corrosion of the roof structural supporting beams and plates was not detected at earlier stages because of the thermal insulation material covering the entire roof area of these units. Preliminary investigations revealed that the corrosion of the roof supporting beams and plates was mainly caused by (1) trapping of water (rain and or seawater) between the structure support beams and roof plates causing corrosion of the structural roof beams and hence leading to weak structure and roof partial sagging; (2) trapping of rain water under the insulation material for a prolonged period of time; (3) an overlooked area; and (4) insulation material absorbed the water provided good corrosion cell. Temporary corrective repair work was carried out to bring the unit back into service by welding in new supporting beams. Proper repair work is planned for the station rehabilitation.