Stones Floating Overhead

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While a large group of school children fill the cathedral with chaotic but joyful singing I look straight up at the ceiling. I still feel exhilaration being inside an immense classical Gothic cathedral.  There are thousands of tons of stone overhead.

Looking Straight Up at Cathedral Ceiling, Villefranche-de-Rouergue, France

The stones appear to be unsupported. The lime and sand in the mortar couldn’t keep 100+ pound stones hanging 10 stories above.

How do they remain floating overhead? What keeps them in place? How did they construct those soaring curved ceilings? And what about the weight of the stones above the stained glass windows? How did they keep them in place while the mortar dried?

I can understand gravity and mortar holding stones in place in a plain vertical wall. But these stones gradually and symmetrically curve away from vertical and are just sitting there straight over my head!

It seems that through centuries of trial and (cataclysmic) error medieval stone masons, engineers, and architects implemented three key construction features. They were: the pointed arch, the ribbed vault, and the flying buttress. This is a simplification, of course, (gleaned from a PBS Nova program transcript).

The first problem was that with the typical Roman arch the weight of the stones pushed outward. The solution was the pointed arch which directed some of the weight of the stones in an arch downward.

But there was still sideways pressure. The cathedrals were more than 10 stories tall and were mainly supported by columns in the main hall (nave). The columns and outer walls were still being pushed apart from sideways pressure. The solution was the flying buttress which, if placed correctly on the outside of the cathedral, counteracted the sideways pressure.

But that still left these floating stones over my head unexplained!

The solution as you can see in this photo is the ribbed vault. The ribbed vault was a combination of two intersecting pointed arches. The ceiling stones are placed in a convex arch above the ribs placing the weight onto them. And the ribs distributed the weight of the ceiling stone onto the columns rather than onto the walls.

Tréguier, France

This is a very brief and simplistic explanation. But they are interesting solutions. These advances took centuries to discover and implement. Individual Gothic cathedrals could take several generations to build and the knowledge passed slowly from older stone masons to apprentices.

There are many things I still don’t understand. Did they have to build scaffolding and supports under the ceiling during construction? If so, how long did they have to wait for the mortar to dry? How big of an area of ceiling could they complete before they had to wait for drying? When winter arrived did the rain wash away some of the mortar that hadn’t finished drying?

I look up and around at the ceiling. It is intricate. The stones curve in complex patterns.

The children are still singing. It is a performance of young scouts and they are energetic. Many generations ago their ancestors figured out these engineering problems. They built lasting marvels.

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