Why Mortar Joints Crack and What That Means for Your Masonry

A familiar sight: crumbling joints between bricks

Walk past an older brick wall or chimney and you will often see it: narrow bands of gray material between the bricks that are crumbling, missing in spots, or marked by fine cracks. The bricks themselves may look perfectly sound, yet the joints are deteriorating. This pattern is so common that many homeowners assume repointing is simply routine maintenance, like repainting trim. But the condition of mortar joints is not just cosmetic. The joints are part of a load-bearing, weather-resisting assembly, and how they fail tells you something about what the wall is actually doing.

What mortar joints actually do

Mortar in a brick or block wall serves several functions at once. It bonds the units together so that the wall acts as a single mass rather than a stack of separate blocks. It distributes compressive loads fairly evenly across the bed area, reducing stress concentrations at hard points where a brick edge might otherwise bear directly on the unit below. It also seals the wall against wind-driven rain, slowing water entry through the face of the masonry. Finally, it accommodates small, virtually invisible movements as the wall warms, cools, and absorbs moisture.

Because mortar is usually softer and more porous than the brick or stone it joins, it is deliberately designed to be the sacrificial element in the assembly. When the wall moves or is exposed to moisture, the mortar sacrifices itself before the harder units crack. That design logic is why you see crumbling joints in a wall whose bricks still look undamaged. The joint is doing its job by deteriorating first.

Why mortar joints crack and crumble

Mortar fails for several distinct reasons, and the pattern of failure gives clues about which cause is at work.

Freeze-thaw damage is the most common culprit

In cold climates, the dominant cause of mortar deterioration is freeze-thaw cycling. Mortar is porous. It absorbs water from rain, melting snow, or ground splash. When that water freezes, it expands by roughly nine percent. If the mortar is saturated and the freeze is rapid, the expansion exerts internal pressure that exceeds the tensile strength of the material. Fine cracks form, and with repeated cycles, those cracks widen and the surface begins to crumble or flake away.

This is why joint failure often appears first on the most exposed faces of a wall: the side that receives driving rain, the top course, or the parapet. It is also why joints on a south-facing wall may fail differently from those on a north-facing wall. The south side gets more wetting from rain followed by rapid freezing at night, while the north side may stay frozen longer but dry more slowly.

Incorrect mortar hardness accelerates failure

A less obvious but very common cause of premature joint failure is using mortar that is harder than the masonry units. Older soft brick and natural stone were often laid in lime-based mortar that was soft and permeable. That soft mortar allowed moisture to evaporate from the joints and allowed the wall to move slightly without cracking the units. When someone later repointed with a modern Portland-cement mortar that is harder and less permeable, the wall's behavior changes. Moisture that once escaped through the joints is now trapped in the bricks or stone. When that trapped water freezes, it cannot push the soft mortar aside, so it spalls the face of the harder unit instead.

This is the classic repointing mistake: repairing the joints in a way that destroys the bricks. The correct approach is to match the new mortar to the hardness and permeability of the original, which usually means using a softer lime-based mix for older masonry. Modern hard mortar belongs on modern hard brick, not on antique soft units.

Poor workmanship and full-bed problems

Mortar joints also fail because of how they were originally installed. A joint that is not fully filled, one with voids or honeycombing, provides a path for water and creates stress concentrations. A tooled joint that compresses the surface may look tidy but can leave a dense skin over a weaker interior. Similarly, joints that are struck while the mortar is too wet or too dry will not bond properly to the brick. Poor bond strength means the joint separates from the unit along a hairline crack that later admits water.

Movement and settlement cracks

Not every crack in a mortar joint is caused by weathering. Walls move. They expand and contract with temperature changes, they shrink as they cure, and they may settle slightly as the soil beneath the foundation compresses. Masonry is strong in compression but weak in tension, so when a wall is pulled or bent beyond its capacity, it cracks along the weakest plane, which is usually the mortar joint.

A vertical crack running through several courses, a stair-step crack following the joints, or a crack that widens at one end and narrows at the other may indicate differential movement or settlement. These cracks deserve attention because they may be telling you something about the foundation, the wall's support, or a concentrated load above. Filling such a crack with fresh mortar will not stop the movement that created it. If the crack reappears after repointing, that is evidence of ongoing movement rather than a failure of the repair itself.

Distinguishing weathering damage from structural cracks

Before you decide on a repair approach, it is useful to separate deterioration caused by normal weather exposure from cracking that may indicate a more serious condition. Surface crumbling, flaking, or loss of material from the face of the joint, especially on exposed walls, is generally weathering. So is a uniform, gradual loss of depth across a large area of the wall.

Cracks that step diagonally through the joints, that run through the bricks themselves, that are wider than about 1/8 inch, that show one side displaced relative to the other, or that are accompanied by a bulge in the wall are more concerning. These patterns suggest movement, load, or foundation issues. A wall that is bowing outward, a chimney that has separated from the house, or a foundation wall with a horizontal crack and inward displacement needs professional evaluation, not just repointing.

If you are unsure whether a crack is active, you can monitor it over a season by marking the ends and noting the date. If the crack widens, lengthens, or the two sides shift relative to each other, that is evidence of continuing movement. A crack that stays unchanged through a full year of weather is more likely to be the result of an earlier event that has stabilized.

Why joints need to breathe

One of the most important concepts in masonry repair is vapor permeability. A brick wall is not waterproof in the way a raincoat is. It absorbs moisture, and it must be able to dry out. Water vapor moves through the wall from the wet exterior toward the drier interior or out through the surface when the sun and wind dry it.

If you cover the exterior with a low-permeability coating, such as a cement-based paint or a thick elastomeric sealant, you may reduce the amount of rain absorbed, but you also trap moisture that is already inside the wall. That trapped moisture can then cause spalling, efflorescence, or interior dampness. The same logic applies to repointing with hard, impermeable mortar. The wall needs its joints to be the most permeable part, not the least.

This is why the common assumption that harder, stronger mortar is better is exactly backward for many older walls. The goal is not to make the joint as strong as the brick; it is to make the joint slightly weaker, softer, and more permeable so that it protects the more expensive and harder-to-replace units.

Repointing: restoring the joint, not just filling it

When weathering has eroded the joints to a depth greater than about a quarter of an inch, the standard repair is repointing: cutting out the deteriorated mortar to a sufficient depth, cleaning the joint, and packing in new mortar that is matched to the original.

The reason for cutting out the old mortar rather than simply troweling fresh material over the surface is that a surface patch has no real bond to the brick below. It sits in a shallow recess and will soon pop out as the wall moves or as water freezes behind it. A proper repair keys the new mortar into the joint by filling the full depth of the cut-out area, usually at least three-quarters of an inch to an inch deep. This mechanical interlock, combined with a good bond to the clean brick surface, is what allows the new joint to function as part of the wall.

Repointing also requires matching the new mortar's physical properties to the old. That means considering the color and texture for appearance, but more importantly, the compressive strength and permeability. For most older brick and stone, a mortar with a higher proportion of lime and a lower proportion of Portland cement is appropriate. For modern hard brick, a standard mortar with moderate lime content is usually fine. Determining the right mix is not guesswork; there are guidelines based on the hardness and porosity of the units, and local masonry supply yards or historical societies often publish recommendations for specific regions.

Because repointing is skilled work, it is easy to do more harm than good. Using a power saw to cut the joints can damage the brick edges. Pounding with a chisel can shatter adjacent units. Packing the mortar too tightly can create stresses. If you are comfortable with masonry and have a relatively small area, such as a patch of wall or a garden wall, repointing can be a satisfying DIY project. But for a chimney, a entire house facade, or any wall that is load-bearing or showing signs of movement, hiring an experienced mason is the safer choice. A professional will also know how to select the right mortar and how to finish the joint to match the existing style.

When mortar failure signals a deeper problem

Mortar erosion is a normal part of a wall's life, but it is also a signal that the wall is being exposed to moisture and weathering. If you see joints deteriorating much faster on one side of the house than on another, consider what is different about that exposure. Is it getting more rain? Is it shaded, so it stays wet longer? Is there a leaking gutter or downspout splashing water onto the wall? Is the grade sloping toward the foundation? These are the kinds of issues that, if corrected, will slow the deterioration of the joints and improve the durability of the entire masonry assembly.

In some cases, the deterioration is a symptom of something larger. A chronically wet wall, indicated by efflorescence (white salt deposits), moss or algae growth, or interior dampness, suggests that water is penetrating the masonry at a rate faster than it can dry. That may be due to failed flashing, a leaking roof, or a damaged parapet, rather than just old mortar. Repointing the face of the wall will not solve a water problem that originates at the top of the wall or behind it.

Weighing repair against replacement

When mortar has deteriorated to the point where it is loose or missing, the joint no longer provides lateral support to the bricks. In a wall under load, that loss of support can allow individual units to shift. In an unloaded garden wall, it may simply look neglected. The question of whether to repoint, partially rebuild, or replace a section of masonry depends on the extent of the damage, the cause, and the function of the wall.

Curious fact: it is often more economical and more durable to repoint a wall than to replace it, because the bricks themselves may still be serviceable for decades if the joints are restored correctly. But if the wall is bulging, if many bricks are cracked or spalled, or if the mortar has completely lost its bond and the wall is unstable, then repointing alone is not enough. In those cases, the advice of a structural engineer or a knowledgeable mason should come before any decorative repair.

Bottom line

Mortar joints are not an afterthought in a masonry wall; they are an engineered component that must be soft enough to protect the units, permeable enough to allow drying, and durable enough to resist weather for decades. Their failure is a normal part of a wall's life cycle, but the pattern of that failure tells you a great deal about the environment, the materials, and the history of the wall. Before you pick up a trowel, ask yourself why the joints are failing. Is it freeze-thaw exposure? Was the wrong mortar used in a previous repair? Is the wall moving? Answering that question first is what separates a durable repair from a recurring one.