Why arches last

A stone arch is a strange thing when you think about it: a curve of loose blocks with nothing gluing them together, standing for three hundred years under carts, lorries and floods.

The trick is that every block is pushed against its neighbours. Weight from above does not pull the arch apart; it squeezes it tighter. Take away the load and an old arch is, if anything, slightly less stable.

What kills arches is rarely the stone. It is water getting into the fill behind the spandrels, freezing, and pushing the face out. Look for bulges in the side walls. That is where the trouble starts.