Masonry walls are only as solid as their weak links, the mortar joints. Stacks of concrete block, brick and stone that depend on them always outlast them. Hairline cracks are the first sign of trouble.
They let moisture and water into the wall, triggering erosion that gradually widens the cracks, lets in more water and makes the mortar spongy. (During winter in cold climates, freezing water in cracks exerts extra pressure on the wall.) When chunks of mortar start to break away, the wall is weakened, and on the verge of needing major repairs or even rebuilding. To catch the deterioration before that happens, rebuild the joints, a job called repointing.
1 The rule of thumb is to excavate old joints at least as deep as they are wide, and preferably twice as deep. Anything loose or spongy should be removed or it will undermine the repair.
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2 Rake the joint with a scraper or screwdriver to remove loose mortar. Dig in vigorously until you hit solid material. You may need a cold or plugging chisel and hammer (and safety glasses) to chip out mortar that is cracked but still lodged in the joint. If you use a pressure washer, dial down the power or you may remove good mortar with the bad.
3 After the scraping, brush out the joints, or use compressed air to remove dust and debris that disrupts the bond of fresh mortar.
4 Set fresh mortar on a wide trowel or a hawk (a board with a handle underneath), holding the mix next to the wall so you can push it directly into the joint. Pack the mortar firmly into place.
5 Finish by smoothing the surface before the mortar hardens, a process called tooling. Most joints are recessed from the surface with a concave shape. There are special mason’s tools for the job, such as a brick jointer, or you can duplicate the shape with an old spoon or the handle of a paintbrush.