The Royal College of Nursing in Northern Ireland says some staff at Altnagelvin hospital are concerned about the safety of the tower block where a fire broke out on Friday night.
Three wards on the top floors of the unit are to remain closed after the Western Health Trust said it was difficult to move patients from these levels following the weekend blaze.
RCN officer Dessie Lowry told the BBC today that an "extensive report" into the tower block at Altnagelvin is needed, and they are working with trust management to try and ensure recovery plans that are put in place are fit for purpose.
He welcomed the decision of the trust to permanently close the top three wards, with the trauma, orthopaedic and general medical wards to be relocated.
Patients from those wards are being cared for in other parts of the hospital.
Eighteen procedures scheduled for today have been postponed.
A total of seven wards at the hospital were evacuated on Friday night after a fire started on the roof, the second fire at the hospital in three weeks.
Yesterday, Health Minister Edwin Poots told the assembly the hospital is safe, and he does not believe the fire was caused by anything involving the fabric of the building.
Mr Poots was responding to concerns raised by SDLP MLA Pat Ramsey, who has called for an immediate inspection.