An Arch Undermined

That’s a reasonably good-looking brick arch. Why is it shored?

The short answer is found by looking at the left spring-point of the arch, conveniently obscured by the shadow of a vertical pipe. There’s a gap between the base of the arch and the pier below – the curve of the lower edge of the arch continues past the edge of the window jamb for about four inches, so that the last bricks of the arch are hanging in air instead of being properly seated on the pier. This seems like a really dumb mistake: everyone knows that arches have to be supported at their ends, so how could someone build it like this?

The long answer is found by looking at another window nearby:

This one still has the window frame so it has been interfered with somewhat less. The masonry is not in quite as nice condition but it’s good enough. But the interesting thing at the second window is that scaly black thing between the arch and the window frame. That is what’s left of a wood lintel. The outside (veneer) wythe of masonry has a stone lintel, and the arch is in the two interior wythes; these wythes were built over a wood lintel with a curved top*. The curve served as the form to build the structural arch and as a flat nailer to fasten the window frame.

That wood lintel may have burned, but more likely it’s rotted. There are rot fungi that produce a scaly, blackened appearance in wood that looks very much like charring. As the fungi have eaten the wood, the lintel has drifted down, away from the brick it once supported. The lintel was made with an extension at each end past the window jambs, so that it could bear on the piers. What’s left of the bearing is visible on the far right of the second picture. And, when rot has completely destroyed this lintel, it will look like the first picture: there will be an odd gap between the bottom of the arch and the pier that should be supporting it.

* Sometimes called a “fish-belly” lintel.