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Working with Old Brick and Mortar - The "Damp Proof Course"

As discussed in our prior post, "Revitalizing History: Ozer Engineering's Innovative Timber Solutions for Somerville's Museum," we have been working on preserving a historic building in downtown Somerville, TN. Recently, our focus has been on restoring the old brick-and-mortar facade.

When we initially uncovered this exterior wall, we found the bottom three feet of brick and mortar completely decomposing and crumbling. What went wrong?

This is a common condition in older brick buildings with brick foundations. In this case, there was a brick basement. Moisture from the foundation will wick up the wall through capillaries in the brick-and-mortar. This is a condition called "rising damp." Traditionally, before the era of concrete foundations, this was addressed during construction with the insertion of a "damp proof course." Usually, this was a granite or slate course just above the foundation, which would break the wicking action.

The extreme level of disintegration in this particular building had a second factor that exacerbated the situation. A previous owner applied corrugated sheet metal to the inside of the wall over the interior plaster to keep the interior dry. This created a barrier that prevented the drying of the wall to the interior of the building, causing the wall to become waterlogged and pressure to increase. That situation was extremely destructive to the old and insufficiently fired interior brick and lime mortar.

So, at this stage, we have determined what went wrong. How do we preserve this historic building and have our repairs serve future generations?

Our strategy includes four main components:

  1. Replace the damaged brick and mortar, including the header courses, with new modern brick and natural hydraulic lime mortar.

  2. Unload the wall. This wall is unsuitable as a bearing wall, especially in our seismic zone. We are installing an interior heavy timber frame to support the floor and roof. We also will anchor brick walls to each floor and roof with through-wall anchors. You have likely seen these cast iron stars on the exterior of other historic buildings in the area.

  3. Allow the wall to dry out naturally to the interior. An 8' stud wall will be installed in front of the brick. It will be open at the top and vented at the bottom. Also, a dehumidification system is planned for installation in the basement.

  4. Install a new damp-proof course. Europe has developed new technologies for injecting into masonry specifically for this purpose. The injection fluid fills and seals the capillaries to prevent wicking. The product we have specified for this project is Koster Crisin 76.

Stay tuned for updates on the construction progress. Several aspects of our design are new or uncommon in the mid-south, including using Natural Hydraulic Lime mortar and the Damp Proof course technologies. Finding subcontractors experienced with using these is an important step in the process.

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