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
It will be a breezy day with variable amounts of cloud and some bright or sunny spells.
RAF veteran Andy Davies says the Royal British Legion helped him get back on his feet after problems with alcohol.
Lucy has the latest weather forecast for Wales