Mid and West Wales Fire and Rescue Service says 11 of its crews are continuing to fight the fire at the National Library of Wales.
It is advising people "to stay away for safety."
This video - from viewer Edward Mulgrew - shows firefighters pumping water onto the library roof earlier.
More top news
A dry night is expected throughout the country. During the early hours clear spells are likely to develop allowing the temperatures to fall.
More than a thousand people gathered for the funeral of Wales international rugby league player Danny Jones in Halifax.
A chef from St Gwladwys Bargoed Primary School has won School Chef of the Year in a National competition