The flagpole the flag was on had also been "destroyed" and would need to be replaced, she said.
She said the internal height of the building was 11m and the outside would be higher so the vandal would have had to have been quite fit to clamber up on to the roof.
She had figured out how they managed to get up their and were reviewing CCTV footage from that side of the building.
Police had also been to fingerprint the hall today.
"It puts pressure on us. The hall is a not-for-profit organisation. All the funds we make we put back into the hall...It's a real nuisance."
Ms Wilkinson said she was determined to find out who did it and prosecute them.