The story unlocks once you make your call.
A myth. That's how long it would take to sag. Blame the glassmaker.
Where it came from
Panes in old churches really are often thicker at one edge, and glass really is an unusual material without a crystal structure. Put the two together and the story writes itself.
What’s actually true
Before modern float glass, panes were cut from spun discs of “crown glass” or from blown cylinders, which were never evenly thick. Glaziers tended to set the heavy edge at the bottom, but not always: some old panes are thicker at the top or the side.
Glass is an amorphous solid. Calculations of its viscosity at room temperature put any visible sagging far beyond the age of the universe, and Roman glass and old telescope lenses show no sign of flow.
Further reading
- Zanotto, “Do cathedral glasses flow?”, American Journal of Physics (1998)