Brick Pointing and Repair
A common maintenance task for brick and stone masonry is restoration of the mortar joints. The longevity of mortar joints will vary with the exposure conditions and the mortar materials used, but a lifespan of more than 25 years is typical. The longevity of brick, however, may well exceed 100 years. Consequently, occasional repair of the mortar joints is expected over the life of the brick or stone. It is our observation that the most common reason for repointing brick and stone is to improve water penetration resistance. Repointing deteriorated mortar joints is one of the most effective and permanent ways of decreasing water entry into brickwork. This is because the most common means of water entry into a brick or stone wall is through de-bonded, cracked or deteriorated mortar joints.
Repointing is the process of removing deteriorated or distressed mortar from joints between masonry units and installing new mortar into the joints. Repointing is likely to be necessary for most exterior masonry work at some point in its life, and is one of the basic processes in the restoration of all masonry. Repointing will improve the weather resistance by reducing the amount of water that penetrates the masonry, thus increasing its longevity. Proper repointing can last 25 to 30 years, however, improper repointing will do little to extend the life of the masonry and may lead to irreversible damage to masonry units. Thus, proper repointing procedures are important to protect historic masonry structures and enable current and future generations to appreciate them.
The most common misconception regarding mortar and repointing is that stronger mortar is better mortar. In fact, not only is this not true, but when the mortar is stronger than the adjacent masonry materials, it can actually induce damage to the masonry units and reduce the wall’s long-term durability. If the repointing mortar is too strong, stresses induced into the walls from applied structural loads to thermally induced stress are concentrated along the edges of the stone or brick in contact with repointing mortar. This concentration of stress may cause fracturing of the face of the masonry units. Mortar must have some flexibility or softness to accommodate movement while still maintaining a bond with the adjacent masonry materials. Mortar used for building a wall and for repointing should always be softer and more permeable than the adjacent masonry materials. The strength of the masonry units and of the existing mortar should be taken into consideration when specifying the strength of the repointing mortar. “When considering repointing, especially on masonry of artistic, architectural, cultural, or historical significance, guidance from a specialist experienced in historic masonry and repointing should be sought.”
In every repointing project, proper joint preparation is critical to the long-term durability and performance of the repointing work. Frequently, improper joint preparation only consists of hand-scraping loose mortar from joints and then applying a new, very thin layer over the existing mortar. Mortar applied with this technique typically falls out of the joints within five years. Mortar installed into a properly prepared joint can last between 25 and 30 years.
Proper preparation of mortar joints should include the removal of old mortar to a minimum depth of two to three times the width of the joint to ensure an adequate bond and performance of the mortar. With joints typically being 3/8 in. (10 mm) wide in most brick masonry, this requires removing existing mortar to a depth of at least 3/4 in. (20 mm) from the face of the masonry. Additionally, deteriorated mortar encountered behind the minimum depth should also be removed. The mortar should be removed to a uniform depth from all joints in the area to be repointed, although this can sometimes be very difficult particularly at the intersection of vertical (head) and horizontal (bed) joints.
Mortar can be removed using hand tools such as hammers and chisels, but typically, electric grinders are used. Extreme care should be taken when using power tools to prevent damage to the adjacent masonry material. The thickness of the blades used on electric grinders should never be more than half the thickness of the joint being cleaned. The grinder should be used to remove the mortar in the center of the joint; the mortar adjacent to the masonry, on either side of the cut, should be removed using hand tools. Once the mortar has been properly removed and mortar dust and debris have been properly cleaned out of the joints, an appropriate mortar should be selected. For most walls, a Type N mortar provides good durability as well as enough flexibility to not damage the masonry. If the wall is constructed of sandstone, brownstone or other soft stones, or soft masonry units, a softer mortar may be necessary.