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 REPOINT Repointing 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 COMPOSITION When 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. Sand in historic mortars includes a wider range of grain sizes and colors. Therefore, matching an original mortar often requires mixing sand from various sources. Generally, it's best to try to match color by finding the correct combination of binder and aggregates. However, some 19th-century mortars were colored with red, black, or brown pigments. JOINT PREPARATION Before repointing, remove any loose, crumbling mortar and rake the joints back to a uniform depth 2 to 2 1/2 times the joint width. Using power tools carries a serious risk of damaging the units and is strongly discouraged in restoring historic buildings.