Later she came on the loud speaker again and in a more excited voice told us to evacuate. She said something about "the roof" but I didn't hear exactly what, because of the noise the alarm was making.

I asked one of the staff persons putting on an orange neon vest where the fire was and she said, "I don't known, one of the upper floors".
One staff person, a middle aged woman without an orange vest, zipped into the washrooms to make sure there was no one in them. She kept leaning on people lingering over the first floor computers to get a move on.
I wasn't paying close attention but it seemed to me that the entire library was evacuated in under five minutes. At the risk of sounding like I have Stolkholm Syndrome after being a taxpayer held hostage by library staff this summer, I will say I was impressed by the performance of library staff.
At about 8:40 p.m. staff were allowed back inside the library. A few patrons were lined up in the lobby in front of the locked doors at about 8:45 p.m. when I left.