How Did Medieval Churches Stay Standing?

The past couple weeks on this seminar have involved trips to more churches and cathedrals than I’ve seen in any other short period of my life. From the weathered friezes of Breedon on the Hill to the high-vaulted ceilings of Ely and Peterborough, each building has its own story to tell about the people that built and used it over the years. As an engineer however, one thing that immediately struck me when seeing these churches was the scale and robustness of their construction. Using stone, mortar, and a little wood Norman architects were able to build grand cathedrals reaching over a hundred feet towards the heavens. While the churches of the Anglo-Saxons are comparatively more humble, the fact that their stonework has lasted for over a millennia is a noteworthy accomplishment in itself. With this in mind I would like to offer some explanation into how these engineering feats were achieved and showcase the genius of medieval architects.

Stone has been used as a building material for the entirety of human history, and for good reason. It’s strong, long-lasting, and found nearly everywhere. However, the one caveat of stone is its brittle nature. Unlike wood or metal, which have the ability to deform when put under excessive loads, stone will just fracture. This means that improperly-constructed stonework has the potential for catastrophic failure, such as the total collapse of Ely Cathedral’s central tower in the 1300s. This brittleness gives stone another interesting property: it is about three times as strong in compression (pushing it together) as it is in tension (pulling it apart). This means that were a person hypothetically endowed with the superhuman strength necessary to break a stone brick with their bare hands, it would be far more difficult for them to crush the brick between their palms than it would be to pull it in half. (If you’d like to test this for yourself, using meringues as a substitute for stone works quite well.)

While medieval architects did not know about the microstructures responsible for giving stone this property, they were keenly aware of the property’s existence. As such, medieval church building was centered on the principle of making sure as much of the building’s weight was supported compressively as possible. In some aspects of construction this was quite simple to achieve; for example, solid walls were built by stacking layers of stone block on top of each other. The weight of the top level of blocks was supported by the level below it, continuing downward until everything is compressing against the building’s foundation.

Things get a bit trickier for parts of the church that need to be supported over open air, such as doorframes and windows. While it seems simple to just leave a hole in the wall for such architectural features, this design choice creates a serious structural problem. Because the block at the top of the opening no longer has anything beneath it, its weight plus the weight of anything above it cannot be compressively supported. Instead these loads must be taken by the mortar holding the block to the blocks on either side of it, straining the mortar downward in a far weaker tensile support.

The medieval solution to this problem was the arch – a simple but extremely effective piece of engineering design. An arch is comprised of a semicircular ring of stone blocks stretching over an opening. Its functionality lies in the keystone, the trapezoidal block at the arch’s apex. The keystone’s shape means that when it’s pushed down by the weight of blocks above, it compresses against the neighboring blocks on the arch. These blocks act similarly on the blocks next to them, transferring the weight down along the arch until it can be supported by the sturdier blocks on the side of the opening. With this design the entire weight of the stone above the opening can be supported via compression.

With the help of the arch, medieval churches were able to increase tremendously in size towards the late first and early second millennia CE. Where once arches had just been used to support windows and doorframes, they were now stacked on top of one another to create whole separate floors that looked out into the church’s center. Some churches were even built to have a ceiling made entirely out of stone, held up with a series of arches spanning out from a few central points. Beyond medieval church building, the arch continues to be a staple in construction even in modern times. If you need any proof, just look around main quad at Stanford.