Hudson said he told the physical plant shop, which put the problem on a list of things to be fixed. But the problem isn't urgent because it rarely happens, and so far, hasn't caused any damage.
"The problem only rears its ugly head in huge storms," he said.
Heavy winds can drive the rainwater into the building through a broken seal between the glass door and the building, Hudson said.
A room full of current periodicals in Hill Hall also flooded -- but for the music library, it was business as usual, said librarian Diane Steinhaus.
"It's just what we expect whenever we get rain," she said. Steinhaus said the library staff keeps a wet/dry vacuum and several fans on hand for every storm.
Steinhaus said the water extended about halfway across the floor, which is standard. "It never gets above the bottom shelves," she said.
At least one UNC student was taken by surprise when the rain started.
"I usually look at weather.com but (today) I didn't," said junior Anna McLean, walking into Greenlaw Hall with her hair dripping wet.
"I got out of class about 3:15 p.m. and ran to the (Student Recreation Center) really fast," she added. "I was slipping and sliding. I found a towel in there."
Meanwhile, Chapel Hill officials also dealt with the sudden downpour that caused at least four accidents by late Thursday afternoon.
"If it's raining when (people) go to their cars, they're in the mind-set of `Hey, I need to be careful,'" said Maj. Tony Oakley of the Chapel Hill Police Department. "We seem to have more wrecks when it starts raining."
Only one injury and no fatalities had been reported by early Thursday afternoon.
To get the day's news and headlines in your inbox each morning, sign up for our email newsletters.
Oakley said the department reacts differently to inclement weather depending on its severity. "Sometimes we have to count on the common sense of the motoring public. There are a lot of intersections in Chapel Hill," Oakley said.
"People need to be patient, take a deep breath and count to 10."
The City Editor can be reached at citydesk@unc.edu.