The roof of Notre-Dame Cathedral wasn’t just a roof. Sure, it kept the rain out. But what burned away in Paris in April was a technical marvel, the height—literally—of 12th- and 13th-century engineering. “If one imagines the stresses on a large sail of timber and lead rising over 100 feet from the ground, one can only marvel at the ingenuity and skill of these early builders,” as the historian Lynn Courtenay writes in an essay for the Society of Antiquaries of London. The wooden trusses—made of trees cut down in 1160 or so—were specially braced with an extra plate linking them to the walls, and clasps to keep them from sagging across the span. The wood was in tension, helping to hold the tall, thin walls; this charpentre, or roof carpentry, was about a quarter of the cathedral’s total structure. And now it’s gone.