In historic restoration there has been a great deal of effort made to get the word out that modern practices such as sandblasting can do tremendous harm to historic masonry. But there is one practice that may not be as well known – the practice of using modern cement for repairs to masonry from before 1933.
According to the National Park Service’s Preservation Brief, ‘Repointing Mortar Joints in Historic Masonry Buildings,’ “Mortars for repointing should be softer or more permeable than the masonry units and no harder or more impermeable than the historic mortar to prevent damage to the masonry units. It is a common error to assume that hardness or high strength is a measure of appropriateness, particularly for lime-based historic mortars.” So determining the existing mortars components is a very important step. If the wrong mortar is used, the masonry units can split or break, moisture can penetrate into wall, or the units could start to deteriorate.
The key to prevent damage and get the best results is to match the existing mortar. As restoration consultant Simon Leverett put it, “if the original mortar lasted 100 years, why don’t we match that, as using modern materials will cause damage and need repairing again in 10 or 20 years if you’re lucky.”
Preserving or restoring your masonry building is not as simple as slapping some cement in the joints and calling it good. Mark Liebman, a forensic investigator, says it best: “Mortar is a very complex subject. … Lime putty, hydrated lime, natural hydraulic lime, natural cement and portland cement can all be found in heritage structures. All have their own characteristics; strengths and weaknesses.” And if your mason does not know this, find another one!
To take an excerpt from the “Bill of Rights for Masonry Structures” (ebook by Larry D. Jones, manager of the Historic Preservation Group) … “Repair me only where I need it, and with materials just like me.” Larry also points out that “Closely related to repointing with the wrong mortar, is the blatant practice of arbitrarily removing good sound mortar joints that have survived the test of time.” Or as another excerpt from his ‘Bill of Rights’ states it: “Know what is wrong with me, before you plan how to fix me” and “Respect all that is left of me, sacred as it is, my historic fabric…” I like that!
So be sure to talk with your masonry consultant before any work is started, confirm they understand the importance of proper care of historic mortars, and if they have a ‘mortar is mortar’ philosophy, look for someone else.
Do-No-Harm to Historic Buildings – Do not destroy anything and preserve as much as possible. Be a steward of our historic buildings and show a deep level of respect for them. – paraphrased from a quote by Todd Westrick
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