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A lime-based stucco cladding, scored to look like stone blocks, was put on in 1816, in part to make the exterior of the building appear as a unified whole and hide the different periods of the building. Such stucco was known to have a life span of 50 years and so was replaced by the building’s owner in the early 1870s. However, ivy growing on the north side dislodged chunks of the stucco and helped lead to water infiltration within the walls.

Instead of lime-based stucco, preferred because it moves with the underlying masonry, an owner in the early 20th century applied Portland cement stucco atop an expandable ribbed metal lath nailed onto the masonry. He thought that approach would stop the water damage, but instead, the combination made it worse. Portland cement is rigid and doesn’t breathe in reaction to temperature changes the way lime-based cement does. This causes the bricks underneath to crack.

In addition, the metal lath made pockets between the stucco and the bricks that trapped water from condensation inside the walls. By 1918, cracks showed up in the Portland cement stucco, enabling rain to enter and get trapped against the bricks.

The problem is a familiar one to any homeowner trying to understand cracks and blistering.

“People have old houses with moisture problems and don’t know why,” Mrs. Shafagoj says.