The Lowther Road club says it will now need to purchase a completely new air conditioning system and replace water-damaged wallpaper in several rooms, with an expected cost of around £20,000 to put things right.
Club members initially feared a worst case scenario in which bookings for reception events might even have to be cancelled due to the work needed in the function room, but they are now confident that it can be undertaken in time.
Steve Clarke, club secretary and licensee, said: "This isn't about stealing lead or copper from the roof, because we had to replace it with aluminium flashing a while back.
"It is pure vandalism, either kids just mindlessly wrecking things for fun, or even someone who may have a vendetta against the club.
"We will have to buy a new air cooling and heating system because we are told our interior system will not be compatible with the new exterior units.
"That alone is likely to cost at least £17,000, we've been told.
"It is very frustrating, with the hassle and stress if trying to put thing right in time for impeding functions and also the matter of our insurance premiums now going sky high."
The vandalism is thought to have occurred some time before last week's heavy rainfall, with the intruders likely to have gained access to the roof via the fire escape.
Steve said: "We had no idea the damage to the roof had even occurred at first.
"But the problem became clear when we had heavy rain.
"We thought we had a serious leak and got someone to go up there and take a look.
"He said he couldn't believe it when he got up there and saw the damage.
"They just trashed the three air cooling systems and then ripped away the flashings.
"They knew what they were doing, it wasn't just smashing a window."
The club is appealing for anyone with information to contact the police.
Steve added: "We're also going to install CCTV cameras up there."