How did a ribbed stone ceiling help Gothic walls turn into windows?

By turning the ceiling into a skeleton instead of a solid lid. A rib vault covers a wide space, like a church nave, with a crossed framework of stone arches, then fills the gaps between those ribs with thin stone panels instead of one heavy continuous shell. That single change cuts the weight pressing outward on the walls dramatically, because the ribs funnel the load down to specific columns and piers instead of spreading it everywhere. Once walls no longer had to brace a massive stone roof along their whole length, they could get taller, thinner, and perforated with far bigger windows, which is exactly the look we now call Gothic.

The idea wasn't new when medieval Europe picked it up: Romans had built rib vaults back in the 2nd century, embedding brick ribs directly into concrete, and a mosque in Córdoba had a ribbed vault a full century before Europe's earliest Romanesque examples. A key early experiment came at Durham Cathedral, started in 1093 under the Norman bishop William de St-Calais. The plan was to use ordinary rounded-arch vaults throughout, but partway through construction, the builders began experimenting instead with pointed arches over the nave, which directed the weight outward and downward, with thin panels of small stones between the ribs.

The early results were genuinely innovative: Durham's aisle vaults were finished by 1096, its choir by 1107, with pointed vaults over the nave itself begun around 1130, light enough that the walls could carry bigger windows than before. But the experiment wasn't perfect. The vault panels over the chancel were made of plastered rubble that turned out heavier than expected, and they started to crack, needing replacement over a century later, in 1235.

Meanwhile, experiments with pointed rib vaults had moved to France, where builders made thinner, lighter panels from small cut pieces of stone instead of rubble. Gothic builders also used another trick the pointed arch made possible: adjusting an arch's height independently of its width, so vaults of different spans could still meet at the same height. That combination, light ribs doing the structural work and pointed arches giving builders control over proportions, is the engineering that let Gothic cathedrals swap solid stone walls for soaring glass.