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Lime Mortar

The mortar on our 1840s Victorian home is falling out and the bricks seem soft. I have heard that repointing with currently available mortar supposedly would crush the bricks as it cures. — New Holland, Ohio

If used for repointing, modern high-strength Portland cement mortar will not allow for the inevitable movement in older brick walls. As the wall moves slightly the edges of the relatively softer bricks can be chipped off because the cement mortar is much harder than the bricks. The cement mortar repointing can also trap moisture in the wall causing it to come out through the bricks damaging them. Use a low-strength lime-rich mortar which is soft and will crack as the wall shifts. This cracking is acceptable because much of the lime remains “unreacted” and the crack will “heal” as it reacts with moisture and air entering the crack. A soft (Type “O”) mortar with 1 part Portland cement, 2.5 parts lime and 8-10 parts sand may be soft enough. A very soft mortar is 1 part Portland cement, 4 parts lime and 11-15 parts sand. The key is to match the strength of the original mortar used in the wall’s construction and for the mortar to be softer than the bricks.

John Leeke is a preservation consultant who helps homeowners, contractors and architects understand and maintain their historic buildings. You can contact him at 26 Higgins St., Portland, Maine, 04103; or by E-mail: johnleeke@HistoricHomeWorks.com; or log onto his website at: www.HistoricHomeWorks.com

© John Leeke