In the early hours of 20 April 1974, at 4 am, a devastating fire broke out in the Head-office of the University of Porto, in Praça de Gomes Teixeira. Apparently, the incident began in the south wing of the Grand Hall and spread to from the roof covered by the ceiling and its lining. The flames advanced towards the rest of the roof and lining, sweeping from west to east. On this side, the archives of the University and the Library of the Faculty of Economics were immediately destroyed. On the west side, the roof caved in on the Calculus Rooms and other rooms of the Department of Mathematics of the Faculty of Sciences, which were also destroyed. The first structure to cave in was the skylight, which topped the Grand Staircase.
The north wing of the building, facing Praça de Gomes Teixeira, was the most affected by the fire. In fact, the official report of events, prepared on the day after the fire by a team of experts of the General Directorate for School Buildings and the Northern Directorate of School Buildings, highlights the north wing of the building as being the hardest hit. The damages on the 3rd floor were significant and the 2nd floor also suffered major damages. The Grand Hall was totally destroyed, as well as the Council Room, classrooms and offices. The damages on the staircase were due to the collapse of the roof. The two oil paintings on canvas on the side walls of the main floor, painted by José Maria Veloso Salgado (1864-1945), were also damaged by the fire. The furniture found in all of these places were destroyed or badly damaged, either because of the flames or by the water used to put them put.
In 1974, the building in Praça de Gomes Teixeira was occupied by the Faculty of Sciences and adjoining facilities, by the Rectory and Administrative Services of the University and by the Faculty of Economics, which had its first facilities here. The School Board of the Faculty of Sciences was offered alternative facilities so as not to disrupt classes (for e.g., a private house in Rua da Restauração and the premises of the Academic School, near Rua do Pinheiro). The possibility of occupying a building bought by the State, in Beco do Paço, near the Carregal Garden, was also considered, as well as of the nearly completed building of the Faculty of Economics, in the Asprela Campus. However, it was decided not to abandon the premises in Praça de Gomes Teixeira and to carry out the minimum rehabilitation works so that the building could be made habitable and classes could continue. Three days after the fire, classes were resumed.
The rehabilitation works went on for years and were organised into three different phases. Priority interventions included the reconstruction of floors and ceilings of the 2nd and 3rd floors, which had to be replaced by concrete. A 2nd stage involved the division of spaces in brick walls and not wood, as before, and the lining of ceilings, walls and pavements, except those in the Grand Hall. A 3rd and final stage involved the reconstruction works in the Grand Hall and the staircase. These works were the responsibility of architect Joaquim Marques de Araújo as the author of the project. The restoration of the Grand Hall benefited from the fact that the "Workshop of Decorative Sculpture of Enes Baganha" still kept the original plaster ceiling and wall moulds. Early in 1979, the structural works were completed. The 2nd stage – finishing works and water and sewage installations, air conditioning, electrical installation and lifts – were nearly completed. The 3rd stage – furniture, equipment and decoration – had been handed to the School Buildings for consideration.