Building without calculation
Medieval builders did not have the mathematics of structural analysis that engineers use today. They did not calculate stresses in a vault or the strength of a pier. What they had was geometry, rules of proportion passed from master to apprentice, close observation of earlier buildings and, when something went wrong, hard experience. The result was remarkably successful: many buildings stand after eight or nine hundred years. Some did fail, and those failures taught lessons too.
Who did the work
A major project involved many trades. Quarrymen extracted stone, carters or boatmen moved it, and carpenters built scaffolding, roofs and the temporary frames needed for arches. Smiths made and sharpened tools and produced iron clamps and ties. Plumbers laid lead roofing, and glaziers fitted glass. Laborers mixed lime mortar, carried materials and did much of the heavy lifting.
The core skilled group was the medieval stonemasons. Masons who cut and set stone were divided between rough work and fine carving, and the senior figure, often called the master mason, acted as designer and site manager. He might produce drawings, set out the plan on the ground using cord and stakes, and make full-size templates for the carved pieces. Masons often marked their stones with personal signs, probably to identify work for payment or quality control. The building lodge, a workshop on or beside the site, was where templates were kept and stones were shaped before being raised into place.
Large projects were funded by bishops, abbots, kings and town communities, and money could run short. This is one reason many cathedrals took generations and show mixed styles. A site could stand idle for years and then resume under a new master with different ideas.
Geometry as a tool
Master masons used compasses, set squares, straightedges, plumb lines and levels. Many plans were based on geometric relationships, such as squares and circles, rather than measurements in a modern sense. Because medieval measurements varied from place to place, proportion mattered more than a standard unit. Drawings on parchment survive for some great buildings, and masons also scratched or incised plans on plaster floors or on boards. Full-size templates let different workers cut identical pieces, so that a window tracery or vault rib fitted together on site.
Lifting and scaffolding
Moving heavy stone upward was one of the main tasks. Builders used ramps, ladders, pulleys, windlasses and cranes. A treadwheel crane, powered by workers walking inside a large wheel, could lift loads far greater than the people could carry. A rope ran from the wheel to a pulley at the top of the crane arm. Some of these cranes survive in cathedral roofs, and historians point to them as evidence of how stone was hoisted. For smaller loads, hand windlasses sufficed.
Scaffolding was usually timber, built from poles lashed together with rope or fixed to the wall using holes left in the masonry. Those small square holes, sometimes called putlog holes, are still visible in many old walls. Because it was cheaper and more available than metal, wood was used for nearly everything temporary, which is also why fire was such a danger on building sites.
Mortar and masonry
Walls were often built with two faces of cut stone and a core of rubble set in lime mortar. Lime mortar is slow to set and relatively flexible, which let buildings tolerate some settling. Fine ashlar, carefully squared stone, was used for important surfaces, while rubble served for hidden parts. Iron or lead was sometimes used to tie stones together, although its use varied by region and period.
Arches and centering
An arch is only stable when complete, so builders needed a temporary support while it was under construction. This was the centering: a curved timber frame set in place, on which the stones of the arch were laid. Once the keystone was in and the mortar had hardened, the frame was removed. Vaults were built the same way, with the centering supporting the entire curved surface. Making centering took skilled carpentry and a good deal of timber, and reusing it from bay to bay saved money.
Vaults and the problem of thrust
A stone vault pushes outward as well as down. Romanesque builders handled this with thick walls and heavy piers, which led to the limited windows and solid look of the style. Gothic builders found ways to concentrate the forces. Ribs, the arched stone bands under a vault, formed a skeleton, and thinner stone panels could be filled in between. Pointed arches generally produce less sideways thrust than round ones of the same width, and they let vaults of different spans be brought to the same height.
The outward push was then carried to the ground by buttresses. Flying buttresses are arched supports outside the building that carry the thrust from the high vault over the aisle roofs to heavy piers. They allowed taller, thinner walls with large windows. The pinnacles often placed on top of buttresses added weight that helped keep the structure stable, though they were also decorative.
Learning from failure
Experiments sometimes went too far. The choir vaults of Beauvais Cathedral in France, among the highest ever attempted, collapsed in the later thirteenth century, and the building was repaired and later modified. Towers fell in other places. Historians generally see such events as part of a process in which builders tested limits, with surviving buildings representing the successful cases.
Other engineered works
Engineering skills carried over to non-church work. Castle builders dealt with wall thickness, towers and moats. Bridges used stone arches on piers set in rivers. Mills, including watermills and windmills, applied wheels, gears and timber frames to harness power. Shipbuilders and carpenters used their own tested rules of thumb for hulls and roofs. Each trade accumulated knowledge in a similar way, through apprenticeship, practice and reputation, and the best masters were in demand and could travel to advise on distant projects.