One reason so many of us admire older buildings is that they are durable — icons of stability in a fleeting world. Decade upon decade, these silent witnesses to our history withstand hot and humid summers followed by dry and bitterly cold winters. We see that familiar architecture still standing each year, and yet may not realize how those cycles of freezing and thawing may eventually bring water inside the structure.
We can see evidence of this cycle in the wooden lintel we recently removed from a main doorway on East Orange Street. This large timber lintel had gradually shifted out from the brick façade surface over the years. Several courses of brick above had begun settling, and mortar joints had progressively widened. The lintel began protruding in front of the face brick, and that newly-exposed top ledge of wood began collecting water, snow and ice, year after year. The lintel’s top edge became soft and green in appearance, as moisture and mold accelerated their wood-destructive growth. In a few years, this could have become dangerously unstable, as bricks might have fallen forward to the ground, and even more costly to correct than today.
Fortunately the building’s owner noticed something wasn’t right with the façade, and asked for our assistance to determine the correct approach in repairs. We were able to identify the problem and assemble a skilled team to address it. Our solution involved selective brick removal by the experienced hands of a senior brick mason. He numbered the bricks sequentially so their replacement will be the same. Then the lintel was carefully removed, and transferred to a local carpenter’s millwork shop, so a new one could be made to match its dimensions exactly. Next, carpenter and mason worked together to hoist the new lintel back into the open slot. The original bricks, carefully cleaned of old mortar, have been be re-laid over the new lintel and re-pointed using a high-lime mortar.
Using high-lime content produces a finished mortar that is nearly the same hardness as the old mortar, maintaining a flexible joint that can both take in and let out a little water vapor. This is important because old bricks swell and shrink ever so slightly with changes in temperature and moisture levels. Modern mortars, made with Portland cement instead of lime, are too hard to be used with old brick and mortar (5-10 times as hard as the 19th century mortar) and do not permit this kind of microscopic adjustment. In time, cement-based mortar can cause serious damage to old brick walls. We see this in many places across the city.
Older buildings do change and respond to all that changing weather, and we need to be attentive to keep them healthy and strong. Every so often, some real hands-on work is needed. If we overlook this, we do so at some risk to ourselves, our budgets, and our historic city. An informed approach and the right experience make all the difference between a further-deteriorating wall and one that will stand securely for another century.
For more info:
National Park Service Preservation Brief: Repairing Mortar Joints in Historic buildings