Police at Strand Road in Derry are appealing for information about the theft of lead from the Gransha hospital site.
In two incidents last week, lead to the value of £18,000 was stolen.
Constable Devlin, the investigating officer said the amount of lead stolen would have necessitated the use of a vehicle.
She said she was keen to hear form anyone who saw suspicious activity or unusual vehicles at the old site between Tuesday and Friday.
Information can be given on the police non-emergency number, 101, or by calling the Crimestoppers Charity anonymously on 0800 555 111.