Exactly how the fire broke out is now the subject of an intensive investigation by the French authorities, who are so far treating the disaster as an accident. Much remains to be learned. But already it is emerging that Notre-Dame, irreplaceable as it is to France’s heritage, lacked the fundamental fire-prevention safeguards that are required in more modern structures and have been grafted onto other ancient cathedrals elsewhere in Europe.

“There had been a systematic refusal to install anything electrical” within “the forest” because of the risk, said Pierre Housieaux, president of the Paris Historical Association. “Everyone knew that the attic was the most fragile part”...