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.
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It will become drier in the early hours setting us up for a fine start to Wednesday
Showers in the north will spread southeastwards overnight. Temperatures will be higher so not as chilly as list night.
Chilly start to Tuesday, turning into a largely fine and dry day.