After the fifth century collapse of the Roman Empire in the west, Europe was assailed by a series of invasions from Germanic tribes. Visigoths, Ostrogoths, Vandals, Huns, and Franks swept into what is now Italy, France, and Spain. These invasions were actually migrations of entire peoples and they settled in the conquered lands, adopting local religions and cultures while adding their own influences.. In the seventh century, the Franks became Christians and created a powerful political base that culminated in the empire of Charlemagne in the 9th Century. Under the Franks and especially under Charlemagne, there was a great Christian revival.

This Carolingian revival of religion was a combination of two great forces – the strong centralized political empire that could defend its lands and a flowering of monastic orders. The monasteries, often endowed by Carolingian leadership, were centers of learning and industry. Towns sprang up around the monastic settlements and wealth would accrue. Churches were built for both the monastic communities and for the towns.

After the fall of the Carolingians, tenth century Europe underwent another series of invasions, but these were not great tribal migrations, they were pillaging expeditions from warrior cultures. Saracens from North Africa, Magyar tribesmen from Central Europe, and Norse raiders from Scandinavia poured into this land of relative wealth and sacked it time and again. The enemies were so widespread that they fought each other over the spoils. In the north, the ranging Saracens fought Norsemen. In the east, in the canton of Vaud, the Magyars fought a pitched battle with the Saracens. Europe was at the mercy of the invaders.

One of the favorite targets of these raiders were the churches, cathedrals and monasteries of the Christian world. Early medieval communities of monks wanted to build stone churches to resist the sacking of these invaders. Wooden churches were burnt to the ground and had to be rebuilt. Stone walls would resist the sacking and provide for defense. The early builders built stone walls with wood roofing structures across the nave. But these wood ceilings burned easily and fiercely, and many a church was destroyed by those determined to sack the Christian churches. Builders needed stone to cover their churches.

Medieval builders were confronted with two problems in building their stone churches. The first was size. In order to span an open space like a nave, they ran up against limitations on lintels. Stone lintels could span fifteen feet at the most. This meant that the width of any volume that could be spanned with a lintel was smaller than most living rooms in an American house. Needing to build larger structures, the builders looked for ways to overcome the size limitation. There were models from antiquity available to guide the builders – the Romans used barrel arches and domes to span larger spaces and the Romanesque builders copied those forms to construct the church roof with something more durable than wood.

The idea of a vault is so simple, a series of arches assembled in sequence to make a tunnel-like effect for a roof. But let’s look at this carefully. A single arch is a collection of shaped stones called voussoirs. They are like wedges and they make the curve of the arc. At the very top, they are capped by a central stone, the keystone. Leonardo da Vinci said that “an arch consists of two weaknesses, which, leaning on each other, become a strength.” The keystone is the source of that strength, it holds the tension of the other stones and makes the arch stable. In fact, the barrel vault is not only stable but it has the ability to span spaces of 150 feet, a ten-fold improvement over stone lintels. It stands to reason that if you have a series of arches, you further increase the stability because you are giving lateral support as well as the vertical support.

But the problem begins to arise when you think of the weight of all this stone when it covers a significant amount of space. The stone barrel vault is pretty much a “dead-load” solution to architecture. The huge mass of stone rests on heavy supporting walls which must be big and strong enough to carry the weight.

The weight, however, does not fall vertically, but thrusts sideways as well. This thrust is continuous for the entire length of the vault, and on both sides. That weight seeks to collapse, and the only thing that prevents the collapse are the massive side walls that contain the outward thrust from the vault. The higher the nave, the wider the wall at the base, because the base had to be two to three times as wide as the width at the top. This put a practical limit on the height of the nave, because the walls could only be so thick before they were ludicrously inadequate.

There were two solutions to this problem – changing the structure of the vault itself (see this earlier post to summarize those structural changes) and buttessing the walls to counteract the thrust of the vault.

The decision to protect the nave of the church with a vault of stone is probably the defining architectural choice for the Romanesque style. It certainly led to a church more protected against fire and pillage, but it created major challenges for the builders.

These thick supporting walls led to the second problem in constructing large stone churches – allowing adequate natural light into the large enclosed volumes. In order to have adequate light there needed to be large openings for windows. However, large openings compromised the structural integrity of the supporting walls, causing the vaults to collapse. Small windows were possible, but they didn’t generate much light at all. With the development of the rib vaults, the thrusts were directed to a series of pillars instead of the entire wall. That freed up the space that used to be used for walls to be used for windows instead.

The history of Romanesque architecture, then, is the history of the attempts to solve these two problems of size and light. Some scholars believe that Romanesque is the history of searching for the solutions, and that Gothic is the result of finding the solution. In the Gothic world, the architectural challenge changed – the purpose was to gain maximum height and maximum light. And the disaster of the cathedral of Beauvais demonstrated that the lessons were learned at great cost by even the most sophisticated builders.