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Historic Mortar Matters

For historic repointing, hardness or high strength alone is not a measure of appropriateness.

NPS Preservation Brief 2 states that new repointing mortar must have greater vapor permeability and be softer, measured in compressive strength, than the masonry units. It must also be as vapor-permeable and as soft as or softer than the historic mortar.

The Brief explains that wall stresses from expansion, contraction, moisture migration, or settlement should be relieved by the mortar rather than by the masonry units. If those stresses break the bond between the mortar and masonry units, that condition is easier to correct in the joint through repointing than if the break occurs in the masonry units.

Mortar selection is project-specific. NPS calls for examination of the existing mortar and masonry when developing an appropriate replacement mortar.

Source: National Park Service, Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings, Robert C. Mack, FAIA, and John P. Speweik (October 1998).