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
Catch up with the latest episode of Coast & Country where Andrew meets the sleddog racing team and Ruth goes weather watching in Snowdonia.
What a week it's been! We started with exceptionally warm temperatures, braved Doris and now it looks like we're in for a wet weekend!
Cardiff manager Neil Warnock has agreed to stay on at the City Stadium, whilst Swansea boss Paul Clement prepares for a return to Chelsea.