Repointing brickwork, also known as pointing or tuckpoint, is a means of removing damaged mortar and replacing it. This helps to restore the brickwork structurally and aesthetically. If you are repairing a damaged area, you need to use mortar that is similar to the original mortar used or a weaker mortar. Using a stronger mortar may damage weak areas around the part that is repaired if the stronger mortar puts stress or extra load on surrounding areas, especially during times of thermal expansion and contraction.
Determining original mortar
If you are repointing a historical site, you may want to take a small piece and have it analysed. Testing will determine the petrographic or chemical composition to indicate which mortar it is. Although this may be costly, it is important on historic buildings to use the proper mortar. A general rule of thumb: If the building was built in the 20th or 21st centuries, the mortar is most likely made of Portland cement, comprising common lime, sand and water. These main ingredients with more or less the same ratios are used in mortar today. If the building predates the 20th century, the portions of lime and sand are different, making it a different composition.
Type of mortar
The most commonly used mortar in pointing and repointing projects is Type N, O and K. Type N is 1 part Portland cement, 1 part lime and 6 parts sand. Type O is 1 part cement, 2 parts lime and 9 parts sand. Type K is 1 part cement, 4 parts lime and 15 parts sand.
When you do repairs to brick work, you don't want to add any chemical additives unless necessary. For instance, if the original mortar analysis indicates additives, usually oyster shells or horsehair, you should add those into the pointing mortar. Use the proportions indicated by the analysis. The oyster shells should be thoroughly washed and dried before crushing them. If the original mortar has a pigment added, you need to add some to your pointing mortar. Use metallic oxides instead of chemicals.
You must remove the defective mortar from the area and clean the joint to prepare the area. Use a screwdriver and hammer or a mortar rake for removing old mortar. As a general rule, you should remove mortar from inside the pointing site at a depth that is at least the same thickness as the mortar joint. Be careful not to break or harm the surrounding brickwork while you are removing the mortar. Clean dust and debris from the area with a paintbrush. You must hydrate the old mortar before you start your pointing brick work. Hydrating the old mortar area helps reduce the occurrence of mortar shrinkage due to the bricks absorbing moisture from the new mortar. When you apply the mortar to the joint, make sure to push it into the joint so it makes contact with the existing brick and mortar. Use a small pointed trowel for this to ensure a good bond.