Joints: Repointing Mortar Joints
Figure 1. Example of masonry in need of repointing.
When properly designed and constructed, masonry is very durable and requires little maintenance. Still, various repairs are to be expected over the life span of a building. One repair that is generally performed as part of normal building maintenance involves repointing mortar joints with new mortar, also known as tuck pointing in some parts of the country. Under normal exposure, modern mortars may last eighty to one hundred years or more. However, the maintenance cycle for historic or lime-based mortars is generally shorter than that. Other factors, such as freeze-thaw, water migration, biological attack, salt crystallization, foundation differential movement or structural damage can accelerate the natural degradation process and further reduce that time frame. Potential problems with mortar might include spalling, crumbling, cracking, efflorescence, biological growth, etc. Whatever the reason for repointing is in your case, there are some aspects to consider for a successful project.
Repointing is the process of replacing damaged or deteriorated mortar near the surface of a masonry wall with new mortar. Replacement mortars should be as durable as possible, but using mortar that is too strong can cause damage to the existing masonry. Unless structural concerns dictate the mortar selection, the “matching mortar” principle should be followed. Mortar should match the color, texture, sand content, and chemical composition of the original mortar in order to blend well into the existing structure. Laboratory tests can be done to characterize the composition of the original mortar. Pointing mortar of the same or weaker strength as the original mortar should be used. Compatibility with original materials is important to reduce the risk of several undesirable effects, such as localized stress concentrations, alteration of moisture path in the wall, etc., that could result in future damage to the masonry units. It is better to repoint mortar joints more frequently than to replace damaged masonry units. Additional care should also be exercised when specifying proper mortar mix for repointing an old or historic building, since using an incompatible mortar can lead to damage and costly future repairs. Additional information about repointing mortar joints in historic buildings can be found in the National Park Service Preservation Briefs No. 2 (http://www.nps.gov/tps/how-to-preserve/briefs/2-repoint-mortar-joints.htm).
The first step in the repointing process is joint preparation. Deteriorated mortar is cut out by means of a toothing chisel or nonimpact power tool or hand tools to a uniform depth of at least twice the joint thickness or until sound mortar is reached. An angle grinder with a diamond blade is usually used to remove hard, cement-based mortar, but you may be limited to hand tools when working with sensitive or historic masonry. Remember to check the project specifications for which tooI can be used on your project. When removing mortar, take care not to enlarge the joint or damage the surrounding brick. After mortar removal is complete, the joint is cleaned by brushing, blowing with air, or rinsing with water to remove all loose material, dust, and debris.
Figure 2. The joint is prepared for repointing by removing the existing mortar to a uniform depth of at least twice the joint thickness.
The mortar mix should be proportioned following the construction specifications. One key aspect of the preparation process is pre-hydration of repointing mortar to reduce excessive shrinkage. The pointing mortar is prepared by first thoroughly mixing all dry ingredients then adding only enough water to produce a damp mix that retains its form when pressed into a ball. The mortar is kept in this damp condition for one to one and one-half hours for pre-hydration; then sufficient water is added to bring it to a workable consistency for pointing.
To ensure good bond with the brick, the joint into which the mortar is to be placed is dampened before mortar placement. However, freestanding water in the joint should be avoided. Mortar is packed tightly into the joints in thin layers and then tooled to a smooth surface to match the original profile. Shallow and poorly consolidated pointing mortar tends to have a shorter service life.
Figure 3. Select a qualified and experienced contractor to avoid poorly executed repointing. The pointing work at left was done by a homeowner who smeared mortar into the joint. Experienced restoration masons are experts at matching mortar (right image, where new pointing mortar matches original mortar).
Quality repointing requires a qualified and experienced repointing craftsman. Remember, not every mason is a good tuckpointer and you may end up being stuck with a visually unpleasant wall. Check that the contractor has a minimum of 5 years of tuckpointing experience. Ask for a list of other repointing projects that you can inspect. Depending on the size and budget of the project, a sample panel is a good idea to help show the final appearance of repointed mortar joints.