The Rib Vault
The flying buttress was one important advance in church construction that became part of the gothic style. The flying buttress allowed more wall space to be opened up for windows. It also made taller churches possible because of the way it supported the stone vaults of the church ceiling.
Those stone vaults also went through an important change in the gothic age: they got lighter.
Before gothic, churches were built either with flat wooden ceilings or with heavy, stone barrel vaults. A barrel vault is a long, curved ceiling of heavy stone and concrete. Barrel vaults press so heavily against their walls and buttresses that those walls and buttresses had to be very thick and heavy to support them.
Windows in such walls are few and small. It was too much work to cut windows through thick walls and the holes for large windows would have weakened the walls too much for them to do their jobs.
The vaulting problem improved with the invention of the groin vault. A groin vault is two barrel vaults crossing each other over a square area of ceiling (a bay).
The groin vault did not press down as heavily on the walls. Instead of pressing all along the walls, the groin vault pressed most against the corners where its crossing barrels came together. Builders placed heavy piers at the thrust points of groin vaults. The piers held part of the vault's weight while the walls and buttresses held the rest. This meant the walls could be thinner and have larger windows.
Gothic builders improved on the groin vault. True, the groin vault allowed for thinner walls and larger windows than a regular barrel vault, but the groin vault was still very heavy. This is because the groin and barrel vaults were made up of lots of heavy stone held together by a thick layer of concrete.
In the gothic age, builders reduced the weight of the ceiling by building what was called a rib vault.
In a rib vault, the builders create a skeleton of heavy stone on which the vault will rest. The stone in this skeleton forms the ribs.
Rebuilding the rib vaults in the nave of Soissons Cathedral after they were damaged in World War 2. You can see the rib skeleton and the wooden frame that holds it up. The wooden frame would be removed after the vault is covered in cement (below).
First, the builders put the ribs together. Then, building over a removable wooden form, they place lighter weight stone between the ribs.
This is covered with a thin layer of concrete. The ribs help the concrete hold the vault together. The result is a much lighter vault that does not press as hard against the walls and buttresses. The walls can be thinner, taller, and have more space for windows.
With rib vaults, most of the ceiling's weight is carried to supporting piers and to the flying buttresses. And since the vault is lighter, the piers can be more slender, blocking less light and freeing up even more wall space for windows. That is why gothic churches with rib vaulted ceilings often seem to have no real walls at all, just a thin skeleton of stone piers holding a curtain of stained glass windows.
Rib vaulting in the nave of Bourges Cathedral, France.