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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A yellow warning for snow and ice on Thursday night and Friday has been issued.
Some wintry showers and staying windy with a significant wind chill and widespread frost developing