Arwel Jones from the National Library of Wales told ITV News that he is hoping that the damage will be limited - "the most valuable stuff is housed in purpose-built cells at the very bottom of the library, and this fire is at the very top."
He admitted to concern that the damage from water pumped onto the building by firefighters would be as bad as the fire damage.
More top news
Wales got their 2016 Six Nations campaign under way with an entertaining 13-all draw with Ireland at the Aviva Stadium.
Mosques across Wales and the rest of the UK have opened their doors today to encourage people to find out more about the Islamic faith.
Rain quickly clearing this evening with blustery, locally heavy, showers following, merging at times into longer spells of rain.