A mortar joint is the space between bricks, stones, or concrete blocks that’s filled with mortar, a paste made from cement, sand, and water. These joints are what hold masonry walls together, distributing the weight of the structure, sealing out water, and allowing for slight movement as building materials expand and contract with temperature changes. Every visible line of mortar you see on a brick building is a mortar joint, and its thickness, shape, and composition all affect how well the wall performs over time.
What Mortar Joints Actually Do
Mortar joints serve two distinct structural roles depending on their orientation. The horizontal joints, called bed joints, run along the top and bottom of each course of bricks and carry the vertical load of the structure, including the weight of the wall itself, floors, and roof. The vertical joints between bricks in the same row, called head joints, resist lateral forces like wind and seismic pressure. Together, these joints transfer loads evenly across the wall so no single brick bears too much stress.
Beyond structure, mortar joints act as the wall’s first defense against water. Rain hitting a brick wall is channeled through the joints, and properly shaped mortar sheds that water outward rather than letting it seep into the wall cavity. Joints also accommodate the natural thermal movement of masonry. Bricks expand in warm weather and contract in cool weather, and the mortar (which is softer than the brick) absorbs that movement without cracking the bricks themselves.
Common Joint Profiles
The shape of a finished mortar joint isn’t just decorative. It affects how well the joint sheds water and how long it lasts. There are nine standard profiles used in masonry construction.
- Concave: The most common and weather-resistant profile. A curved tool compresses the mortar inward, creating a slightly recessed, rounded surface that channels water away efficiently.
- Vee: Similar performance to concave but with a V-shaped indentation instead of a curve. It sheds water well and works in most climates.
- Weathered: Angled so the top of the joint is recessed and the bottom is flush with the brick face. This slope directs water outward and downward.
- Flush: Cut flat with the brick surface. It’s simple to produce but doesn’t compress the mortar as tightly, making it slightly less durable than concave or vee joints.
- Raked: The mortar is scraped back from the brick face to create a shadow line, giving the wall a more dramatic look. However, the horizontal ledge created by raking can trap water, making this profile a poor choice in wet or freezing climates.
- Beaded: A raised, rounded bead of mortar extends out from the joint. It’s decorative but fragile, since the protruding mortar is vulnerable to chipping and water damage.
- Grapevine: A thin line is pressed into the center of a flush joint, mimicking the look of hand-tooled colonial brickwork.
- Weeping: Mortar is left to squeeze out naturally between bricks without being tooled, creating a rough, rustic appearance.
For most residential and commercial construction, concave and vee joints offer the best balance of durability and appearance. Raked and beaded profiles are typically reserved for sheltered walls or interior applications where water exposure is minimal.
Types of Mortar Used in Joints
Not all mortar is the same. Four standard types are classified by compressive strength, and choosing the right one matters more than most homeowners realize. The general rule is to use the lowest-strength mortar that meets the structural requirements of the project, because softer mortar is more flexible, easier to work with, and less likely to damage the surrounding bricks over time.
Type N is the standard choice for most residential brickwork, including exterior veneer walls, chimneys, and parapets. It bonds well, handles normal weather exposure, and is easy for masons to work with. Type S offers higher compressive and flexural strength, making it the go-to for load-bearing walls and buildings in areas with high wind or seismic activity. Type M has the highest compressive strength and is used primarily below grade: foundation walls, retaining walls, and exterior paving. Type O is the weakest and is limited mostly to interior partitions and restoration work on older buildings.
That last point is worth highlighting. When repairing historic masonry, using mortar that’s too hard can actually destroy the original bricks. Older bricks are softer than modern ones, and pairing them with high-strength portland cement mortar forces the bricks to absorb all the stress from thermal movement and moisture changes. The result is spalling and cracking in the bricks rather than the joints, which is far more expensive to fix.
Why Mortar Joints Deteriorate
Mortar is designed to be the sacrificial element in a masonry wall. It’s softer than the bricks or stones it surrounds, so it absorbs stress and wears out first. That’s actually a feature, not a flaw, because replacing mortar is far simpler than replacing bricks. But several factors accelerate that deterioration.
Freeze-thaw cycles are the most common culprit in cold climates. Water enters tiny pores in the mortar, freezes, expands, and cracks the joint from the inside. Over dozens of cycles each winter, this gradually erodes the mortar surface. Thermal movement is another major factor. South- and west-facing walls absorb more heat and experience greater expansion and contraction, which builds up stress that can crack joints in predictable patterns around windows and doors.
Foundation settlement causes diagonal cracking that typically runs through mortar joints in a stair-step pattern. Failed lintels (the supports spanning window and door openings) create cracking and displacement directly above those openings. In older buildings with wood lintels, the wood sags or decays, and in buildings with iron or steel lintels, rust expansion pushes the surrounding masonry apart. Even salt deposits can cause damage: moisture carries dissolved salts into the mortar, the water evaporates, and the growing salt crystals exert pressure that breaks the mortar apart from within.
Repointing and Tuckpointing
When mortar joints crack, crumble, or erode to a noticeable depth, the standard repair is repointing. This involves grinding or scraping out the damaged mortar to a depth of roughly twice the joint width, then packing in fresh mortar and tooling it to match the existing profile. Done properly, repointing restores both the structural integrity and weather resistance of the wall and can extend its life by decades.
Tuckpointing is a related but distinct technique focused on appearance. It involves applying a thin layer of mortar in a contrasting color over existing joints, then shaping it to create the look of crisp, uniform lines. Tuckpointing can dramatically improve the visual appeal of older brickwork, but it’s essentially a surface treatment. If the underlying mortar is failing, tuckpointing alone won’t solve the structural problem.
Matching the mortar is the critical step in any joint repair. The replacement mortar should match the original in both color and strength. For older brickwork built with soft lime mortar, Type O is often appropriate. For newer construction, Type N typically works. Using mortar that’s too hard for the surrounding masonry is one of the most common and damaging mistakes in brick repair, since it transfers stress to the bricks and accelerates their deterioration.
Signs Your Mortar Joints Need Attention
The clearest sign of joint failure is visible erosion. If you can press your fingernail or a key into the mortar and it crumbles easily, the joint has lost its integrity. Gaps between the mortar and the brick face, even hairline ones, allow water in and signal that the bond has broken. Stair-step cracking along the joints usually points to foundation movement or thermal stress, while horizontal cracking near the roofline can indicate the roof structure is pushing the walls outward.
Water stains on interior walls, especially in basements, often trace back to deteriorated mortar joints on the exterior. Efflorescence, the white powdery deposits that appear on brick surfaces, is another indicator. It means water is moving through the wall and carrying salts to the surface, which typically points to compromised joints that are no longer keeping moisture out.