A John Carroll, a mason and builder from Durham, N.C., responds: The most likely cause for the failure of the mortar is the freeze/thaw cycle. Water expands as it hardens into ice. As people who live in cold climates know, this expansion is a powerful force. It can crush boats and lift buildings, so it’s no surprise that it can separate and break mortar and bricks.

The deterioration of exterior masonry is usually a cumulative process. One year, the ice causes a crack; the next year, the crack admits more water, which in turn causes more deterioration. It’s a downward cycle, with ever increasing levels of damage.

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