Restoring Mortar Joints in Historic Buildings Northampton MA

Source: Masonry Construction
Publication date: October 1, 1993

By Kenneth A. Hooker

In a properly designed and constructed masonry wall, mortar joints can last 50 years or more without maintenance. Eventually, though, natural weathering by wind and rain will cause the mortar to erode. Masonry with seriously deteriorated mortar can be repaired by repointing; that is, removing the damaged mortar back to a uniform depth and refilling the joints with new material.WHEN TO REPOINTRepointing should be considered whenever existing mortar joints are eroded 1/3 inch or more from the face of the masonry, visibly cracked, or separated from the masonry units. It is seldom necessary to repoint all the joints in a building. To avoid needless effort and control costs, it's best to repoint only those areas where the mortar actually has deteriorated.MORTAR COMPOSITIONWhen repointing historic masonry, trying to match the original mortar is important. All mortars consist of water, aggregate, and a binder. The mason should match each of these as close as he can to the original mortar. Lime was the sole binder in most mortar produced before the introduction of portland cement in the 1870s. Repointing mortar for old masonry may use lime alone or lime combined with a small amount of white or gray portland cement to speed setting and improve durability. Aggregate makes up the largest portion of mortar and is the most important element in matching color and texture.

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