Longreach Mental Health Unit in Redruth has been closed temporarily and patients moved elsewhere, including out of Cornwall, after a discovery that there was no fireproofing in the roof space.
All 26 patients at have been found alternative care, either at Bodmin Hospital, out of the county or at home, with NHS managers arranging for the company which owns the building to carry install the fireproofing.
Maria Edgcumbe, associate director of inpatient services at Cornwall Foundation Trust, said: "As part of a major refurbishment to improve the patient facilities at Longreach House in Redruth, the trust has uncovered a serious safety issue.
"We were disappointed to identify that inadequate fire proofing was put in place in the roof area when Longreach was built in 2003.
"As the roof space is a sealed area, it falls outside the remit of routine fire inspections, which we undertake annually.
"As tenants of the building, we have taken expert advice and immediate action to ensure the on-going safety of patients and staff."