Future Failure Portrait: Rubble Masonry

by Don Friedman on June 12, 2017

Let’s say it’s the nineteenth century and reinforced concrete doesn’t exist in anything remotely like its modern form. And let’s say you’re an ordinary builder trying to construct a foundation wall. And let’s say you’re not a psychic and therefore don’t know that the entire world of construction is going to be turned upside down after 1900 by the emergence of new materials. What do you do?

You know you can’t build an exterior foundation wall in brick, because it’s simply too porous. You might use brick for interior bearing walls within a cellar and you might even use it for party walls between cellars because in both of those cases the brick is not in direct contact with earth. You know that if you put the brick in contact with earth, even if you try to waterproof it with the best material available to you (coal-tar), it is likely to leak soon after construction and deteriorate to the point of collapse within a few years. So you rule out brick.

You could use cut stone, but you’ve got a problem there. You want hard stone with low porosity to try to make the wall as watertight and as long-lived as possible. But you want soft stone to make it easy to cut and build with. The most common building stones around you in New York are sandstone (brownstone), limestone, and granite. Granite is not very porous but hard to work, sandstone is very porous but easy to work, and limestone is somewhere in the middle. None of them is an ideal choice.

There’s a fourth stone readily available in New York, but it’s not usually considered to be much of a building stone: schist. It’s available because it’s a lot of the city’s bedrock and it’s being cut out of excavations all the time. It’s hard to work and not very porous, like granite, but it has significantly different material characteristics depending on whether you measure with or against the grain – a property of its schistosity. But it’s very cheap because it is local excavation spoil.

So you take the schist and make no attempt to square it off into rectangular prisms because such an attempt would be futile: the stone would break irregularly and leave you with a smaller misshapen lump than the misshapen lump you started with. In order to be able to lay it up in a wall you therefore have to use very thick mortar joints to allow for the irregularities. A common brick wall is about 25 percent mortar assuming common brick and 3/8-inch joints; a wall built of your salvaged junk schist is more like 35 percent mortar. Congratulations! You’ve built a “rubble” wall.

The schist won’t admit much water and won’t deteriorate (much) when it gets wet, but the mortar will. And does. You’ve created a huge problem for me and everyone else dealing with this kind of foundation wall in the twenty-first century, but you don’t care because we’re coming along 140 years after you finished construction.

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