Repairing crumbling mortar joints, a process called repointing, involves removing the damaged mortar to a proper depth, packing in fresh mortar in layers, and tooling the surface to match the existing joints. It’s a straightforward DIY project for small areas, but getting the details right makes the difference between a repair that lasts decades and one that fails within a few years.
Assess the Damage First
Before mixing anything, take a close look at what you’re working with. Mortar joints that are cracked, crumbling, recessed, or missing chunks are all candidates for repointing. On older buildings, a low-pressure water wash (no grit or chemical wash) can help you see the true condition of the joints underneath surface grime. Pay attention to how widespread the damage is. A few joints around a window or along a foundation wall are a manageable weekend project. If mortar is failing across an entire wall, the cause may be structural movement or chronic moisture, and that’s worth investigating before you start filling joints.
Choose the Right Mortar
This is the single most important decision in the entire process, and it’s where many repairs go wrong. The new mortar needs to be softer or equal in strength to the brick it sits between. If the mortar is harder than the brick, the brick itself will crack and spall as the wall expands and contracts with temperature and moisture changes.
Walls built before the mid-20th century typically used lime-based mortar paired with softer, coal-fired clay bricks. Those bricks absorb and release moisture freely, and the original lime mortar acted as a cushion for that movement. Portland cement mortars are much harder and resist moisture transmission. While Portland cement pairs well with modern hard-fired brick, using it on older handmade brick causes the soft masonry to deteriorate rapidly. Lime mortars readily accommodate older brickwork. When in doubt, use a softer mortar.
For most residential repointing on modern brick, a standard Type N mix works well. A common volume ratio is 1 part Portland cement, 1 part hydrated lime, and 4 to 6 parts sand (1:1:4 or 1:1:6). For historic buildings with soft brick, a pure lime mortar or a lime-heavy blend is the safer choice.
Matching the Color
New mortar that’s the wrong shade will stand out immediately, even if the repair itself is solid. The color of a mortar joint comes from three things: the cement, the sand, and any added pigment. Sand has the biggest effect on texture and shade. If the sand contains clay or silt fines, it also shifts the hue and intensity of the finished joint. All of your sand should come from the same source to keep things consistent.
Mineral oxide pigments, usually natural or synthetic iron oxides, are the standard way to tint mortar. They’re chemically stable in cement and lime and won’t fade. Mix a few small test batches, let them cure fully, and compare them to the existing wall in both wet and dry conditions before committing to a ratio. Mortar always looks darker when wet.
Remove the Old Mortar
Removing deteriorated mortar cleanly is the foundation of a lasting repair. The goal is to create a rectangular channel in the joint, not a V-shape or rounded groove. A flat-bottomed void gives the new mortar a solid surface to bond against on all sides.
Use a plugging chisel and hammer for small jobs, or an angle grinder with a diamond blade for larger areas. Work carefully to avoid chipping the edges of the brick. The industry standard is to remove old mortar to a depth of 3/8 to 1/2 inch, or until you reach solid mortar. The Brick Industry Association recommends the depth equal or exceed twice the joint width. So a 3/8-inch joint should be cleaned out to at least 3/4-inch deep. At the same time, don’t remove mortar deeper than one third the depth of the brick above the joint, which risks destabilizing the masonry.
Once the mortar is removed, brush out all loose debris and dust with a stiff bristle brush or compressed air. Any crumbs left behind will prevent the new mortar from bonding properly.
Dampen the Joints Before Filling
Dry brick pulls water out of fresh mortar too quickly. That premature drying weakens the bond and leaves you with chalky, poorly adhered joints. Before you start packing mortar, mist the joints and surrounding brick with water until they’re damp but not dripping. For extremely absorbent masonry like common brick, limestone, or sandstone, the National Park Service recommends applying a continual mist for a few hours before repointing begins.
This step is easy to skip and tempting to rush. Don’t. It’s one of the biggest factors in whether your repair holds up or crumbles within a season.
Pack the Mortar in Layers
Load your mixed mortar onto a hawk, which is a flat handheld board that acts as a portable work surface. Use a thin tuckpointing trowel to press mortar into the joints. The key technique here is layering. Don’t try to fill a deep joint in one pass. Pack mortar in lifts of about 1/4 inch, compressing each layer firmly against the back and sides of the joint before adding the next. This compaction develops a strong bond with the existing brick and mortar.
Do horizontal (bed) joints first, then fill the vertical (head) joints. Let each layer firm up slightly before adding the next. On a warm day, that might be 15 to 20 minutes between layers.
Once the final layer is nearly flush with the brick face, let the mortar stiffen until it holds a thumbprint without smearing. Then tool the joint to match the profile of the surrounding original mortar. Common profiles include concave (most weather-resistant), V-shaped, and flush. A jointing tool or the back of a spoon works for concave joints. Consistent tooling pressure gives the repair a uniform look.
Cure the Mortar Properly
Fresh mortar needs moisture and moderate temperatures to cure. If it dries too fast, especially lime-heavy mixes, the chemical process that gives mortar its strength (carbonation in lime mortars, hydration in Portland cement mortars) gets cut short. The result is poor adhesion, reduced durability, and sometimes visible color variations across the wall.
Cover the repointed area with damp burlap for three days after finishing. You can use plastic sheeting, but tent it away from the wall rather than pressing it flat against the surface. Periodically mist the joints after they’ve been tooled to keep them from drying too quickly. This is especially important for lime mortars, where ongoing moisture significantly accelerates proper curing.
Temperature matters too. Mortar needs sustained warmth to cure. Avoid repointing if nighttime temperatures will drop below freezing, as ice crystals forming inside uncured mortar will destroy the bond. On the other end, keep wall temperatures below 120°F. In hot weather, increased evaporation can prevent proper curing, so working in shade or during cooler parts of the day helps. The ideal window for repointing is moderate weather: 40 to 90°F with no rain in the immediate forecast.
Clean Up Mortar Smears
Even careful work leaves some mortar smears on the brick face. Let any stray mortar dry enough to crumble, then scrub it off with a stiff nylon brush and water. For stubborn smears on hard, modern brick, a diluted muriatic acid solution can dissolve cement residue, but it must be well diluted and used cautiously. Never apply acid at full strength. Soft, porous, or historic brick can be damaged or dissolved by acid. For those surfaces, try diluted white vinegar or plain water with a bristle brush.
Test any cleaning method on a small, hidden area first. Rinse the wall thoroughly afterward to remove any chemical residue.
Common Mistakes That Cause Failure
- Not removing enough old mortar. Packing new mortar over a shallow scrape creates a thin cap that pops off within a year. Get to the full 3/8 to 1/2 inch minimum depth.
- Using mortar that’s too hard. Portland cement on soft old brick is one of the most common and destructive errors in masonry repair. Match or go softer than the original.
- Skipping the dampening step. Dry brick robs mortar of the water it needs to cure, leaving weak, dusty joints.
- Filling joints in one thick pass. Without layered compaction, the mortar shrinks as it cures and pulls away from the sides of the joint.
- Tooling too early or too late. Too early and the mortar smears. Too late and you can’t compress the surface, leaving it rough and porous.