Building Skin Care: Tips to Maintain Historic Masonry Walls

By on January 22, 2019

From the Journal of the National Institute of Building Sciences:  by Frank Rosario, Trivers Associates

March breezes eddied in the mansion’s courtyard. Two chattering sounds drew my eyes away from my tablet screen to small mortar chunks scattered among windswept leaves at the base of the exterior’s rubble wall.

I am the architect tasked with repairs to this grand old house. Despite years of spot-repairs to mortar and sealant joints, leaks through its limestone walls are getting worse. Gazing upward to a parapet 30 feet above my head, I notice the wall surface is pocked with missing mortar and spalled stone. Despite its formidable construction and craftsmanship, this 19th-century stone edifice is giving way to the effects of 150 years of freezing, thawing and rainstorms.

The walls are solid masonry, laid up as multiple wythes of rubble limestone to a thickness of nearly a foot and a half. It is clear that the deterioration is the result of several decades of repointing with a mix of mortar too hard for the stones it was meant to protect. Repairs using 19th-century stonemasonry methods and materials — not 21st-century methods — should go a long way to extend the useful life of an extraordinary example of Americana.

Mortar loss beyond simple weathering is a clue to underlying stresses. Poorly executed attempts at repointing are a predictor of future degradation. To maintain the solid masonry of historic buildings, expertise outside the norms of contemporary construction is necessary.

Time-Tested vs. Contemporary Methods

Solid masonry walls exemplify time-tested methods of dry-stack and mortared-wall construction dating back millennia. Stable height-to-thickness arrangements of stone use friction its and interlocking patterns to deliver gravity forces to the ground. Mortar facilitates the stacking and ills the joints. In contrast, the stacking of veneer masonry (stone or brick) relies on anchor ties to the back-up framing for stability, though outwardly the veneer may look very similar to a solid wall.

Mortar well-packed into joints keeps wind and rain from entering both wall types. Veneer construction includes an air-and-drainage cavity from which water that gets past the brick or stone is removed via lashing and weeps. Water vapor exits via the mortar joints in both wall types. And while there are voids inside walls of “solid” masonry, water is eliminated by evaporation, not drainage. The solid wall gets wet, holds the moisture within itself for a while, then dries out as relative humidity levels drop and evaporating water escapes via the most accommodating solid material, ideally the mortar. he rate at which this water vapor moves through a solid material is referred to as the material’s permeability. Use of a mortar that is harder than the original mortar can change this moisture mitigation dynamic.

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