ITV News understands that there were people working on the roof of the National Library of Wales today, when the fire started.
Nobody is thought to have been injured.
More top news
Mild and muggy overnight, but just be aware, we have a weather warning issued for Wales for tomorrow!
Kirsty Williams successfully defended her Brecon seat, but her party lost four others in a disastrous night.
The body of 75-year-old Iris Owens from Ystrad Mynach was found on Tuesday.