The fire that damaged part of the roof at the National Library of Wales was started by accident by workers using a blow torch.
An investigation into the cause of the fire has found the device used by workers on the roof caused some wood behind external cladding to ignite.
Two workers were carrying out repairs on the roof at the time, but nobody was injured in the blaze.
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Organisers say there will be increased security at the Urdd National Eisteddfod, which began in Bridgend today.
Any remaining heavy or thundery showers will clear eastwards this evening.
Over 70 firefighters are currently tackling the fire, which is now confirmed to be at a scrap yard.