Vertical cracks in mortar joints are almost always thermal movement or minor settlement, not structural emergencies. Stair-step cracks are a settlement signature and are usually stable on older homes. Horizontal cracks in a wall that is bowing outward are structural and need a structural engineer, not just a mason. The crack pattern, not the crack alone, is the diagnosis.
A crack in a brick wall is not a verdict. It is a piece of information. The shape, location, width, and direction of a crack tell you what caused it, whether the cause is still active, and whether you are looking at a maintenance issue or something that needs a structural engineer on the premises.
Most residential masonry cracks are not structural emergencies. They are records of thermal movement, minor settlement, freeze-thaw cycling, and mortar deterioration. They need attention, but on a maintenance timeline, not an emergency one. A small number of cracks are different. Knowing how to read the difference is what this post is for.
The local context changes the reading. A stair-step crack on a Des Plaines home near O’Hare has a different probable cause than the same pattern on an Evanston greystone. Joint-edge cracking on an older Evanston home may not be settlement at all. It may be the result of a prior Portland cement repair on soft brick, a failure mode that looks like structural damage but traces to the wrong mortar chemistry. Those distinctions change what you do next.
A Crack Is a Diagnosis, Not a Verdict
Every crack in a masonry wall has a cause. The cause produces a characteristic pattern. If you can read the pattern, you can narrow down the cause, which tells you what repair is appropriate, how urgent it is, and whether you need a masonry contractor or a structural engineer.
The patterns fall into a few categories based on crack geometry: vertical cracks, horizontal cracks, diagonal or stair-step cracks, and cracks at openings. Each has a different set of probable causes. Within each category, crack width, crack location, and the presence or absence of displacement tell you more about severity. TMS 402, the reference standard for structural masonry, provides the engineering framework that underlies these distinctions.
One principle runs through all of this: monitor before you fill. A crack that is stable does not need to be treated urgently. A crack that is active needs its cause understood before anything is filled. Fill an active crack without understanding the cause and you will see it reopen within one freeze-thaw season. Fill it repeatedly without addressing the cause and you are masking a problem rather than solving it.
A second principle: active masonry problems are typically less expensive to address at the stage you discover them than after one to two more winters. Illinois weather delivers dozens of freeze-thaw cycles each winter, and each cycle works on any existing opening in the wall.
Vertical Cracks
Vertical cracks run straight up through mortar joints or through brick units themselves. They are the most common crack type on residential masonry and frequently the least urgent.
Thermal expansion and contraction cracks: Brick expands with heat and contracts with cold. A long uninterrupted masonry wall without expansion joints will develop vertical cracks at intervals where the cumulative movement exceeds the mortar’s flexibility. These cracks typically run straight up through mortar joints, are narrow (hairline to 1/16 inch wide), and tend to be located near the ends of walls or at changes in wall direction. They are maintenance cracks, not structural warnings. BIA Technical Note 7B addresses the workmanship standards relevant to sealing these joints correctly.
Settlement cracks at building corners or additions: Differential settlement between an original structure and an addition, or between two sections of a long wall, can produce vertical or near-vertical cracks at the junction. These run from the top of the wall downward and are typically wider at the top. If the settlement has stabilized, these are a repair item. If active, the settlement needs investigation before the crack is filled.
Cracks through brick units: Vertical cracks that run straight through the face of a brick, splitting it, indicate tension forces in the wall rather than movement through the joints. Horizontal tension from lateral restraint, or vertical tension from overloading above, can produce this pattern. Through-brick cracks warrant professional assessment when they appear in multiple locations or span significant wall length.
In general: a narrow, vertical crack through mortar joints at predictable locations on a wall that has been standing for decades is a routine maintenance item. Document it, monitor it for active movement, and address it with matching mortar. A vertical crack that is wide, through-brick, or actively growing is a professional assessment item.
Horizontal Cracks: The Most Serious Pattern
Horizontal cracks running along mortar joint lines deserve the most serious attention of any crack pattern in a residential masonry wall. They can indicate conditions that are genuinely structural.
The primary cause of horizontal cracking in a residential masonry wall is lateral pressure. In a basement or foundation wall, this typically means hydrostatic pressure from saturated soil pushing against the wall from outside. A horizontal crack in a basement wall combined with any visible inward bow is a structural warning sign. Get a structural engineer involved before winter. This is not a masonry contractor conversation.
In above-grade walls, horizontal cracking can also result from freeze-thaw damage to the mortar joint opening progressively along its length. This is more irregular in width and depth than structural horizontal cracking. It is a significant masonry maintenance issue but a different category from structural lateral pressure.
How to distinguish: Structural horizontal cracking from lateral pressure tends to be more concentrated at mid-wall height, where bending moment in a retaining or foundation wall is greatest. The wall face may show a slight bow when you sight along it. Freeze-thaw horizontal joint failure tends to be more distributed across the wall height and more irregular in pattern.
When horizontal cracks appear on above-grade wall sections, check whether the wall is bowing outward. If you look along the wall face with your eye close to the surface and the wall is not perfectly flat, the cause and urgency shift. A straight wall with horizontal joint cracking is a masonry issue. A bowing wall with horizontal joint cracking is a structural issue. See bowing and bulging brick walls for the full treatment of that failure mode.
For basement and foundation walls specifically, horizontal cracking is a flag for professional structural assessment. Do not defer this. Brick wall pulling away from the house is an advanced form of the same failure on veneer construction.
Stair-Step Cracks: The Settlement Signature
Stair-step cracks follow mortar joints diagonally, moving one course in one direction and then one unit across, producing the characteristic stepped diagonal pattern. They are almost universally a settlement signature. In most of the Chicago suburbs, that settlement is soil-driven: clay soils contracting in dry summers, frost heave in winter, or gradual compaction under a foundation that was never fully loaded before construction.
Des Plaines adds a cause that does not appear in most masonry education: vibration cracking from O’Hare flight paths. Homes in Des Plaines within a few miles of O’Hare International Airport experience constant low-frequency vibration from aircraft. Over 40-plus years of that persistent vibration, mortar bonds loosen incrementally. The accumulated loosening produces stair-step cracking on upper stories and around window lintels that looks identical to settlement cracking but traces to a different cause. We see more mortar joint failure in Des Plaines homes near the flight paths than in comparable homes a few miles away from the airport. The diagnostic question for a Des Plaines stair-step crack is: which upper floors are affected, and does the pattern concentrate near window openings? Vibration cracking tends to appear at structural transition points where the wall has less continuous support. Settlement cracking tends to track from the foundation upward.
Des Plaines also sits on heavy clay soil, so vibration and settlement can combine. A Des Plaines home near O’Hare may show stair-step cracking from both causes simultaneously. That is a professional assessment situation, not a homeowner monitoring situation.
When one section of a foundation settles relative to an adjacent section, the masonry above experiences differential movement. The path of least resistance is through the mortar joints, producing the stair-step pattern. The diagnostic question is not “is there a stair-step crack” but “is it active or stable and how significant was the movement.” Decades-old stair-step cracking in a 1920s home that stopped when original settlement completed is common across the North Shore. Tuckpointing the open joints with compatible mortar is appropriate maintenance. No emergency.
Active stair-step cracking indicates ongoing foundation movement. This requires structural assessment before any masonry repair. Filling active stair-step cracks without addressing the foundation movement is a cosmetic fix on a structural problem. Wide stair-step cracks, meaning anything over 1/4 inch, deserve professional attention even if they appear stable. For a thorough treatment, see stair-step cracks in brick: what they mean.
Cracks at Window and Door Openings
Cracks at the corners of window and door openings are among the most common masonry defects on older residential buildings. They are also some of the most commonly misdiagnosed as structural when they are frequently a lintel problem. In Evanston, a third cause adds to the diagnostic picture: joint-edge cracking from prior Portland cement repairs on soft brick.
A lintel is the structural element that spans a window or door opening and carries the masonry load above it. Steel angle lintels are the most common type on 20th-century residential construction. IRC Section R703.7 governs masonry veneer attachment and support, and lintel adequacy is part of that framework.
Steel lintels corrode over time. Corroding steel expands in volume, and that expansion creates pressure at the top surface of the lintel where it contacts the first course of masonry above the opening. That pressure cracks the mortar joint directly above the lintel horizontally. As corrosion progresses, the cracks extend diagonally up through the masonry at 45 degrees from the corners of the opening, producing a characteristic inverted-V or wing crack pattern.
This is frequently read as settlement cracking because the diagonal direction looks like movement. But the origin point is the lintel, not the foundation. Check for rust staining on the brick face at or below the window opening, which is corrosion evidence. Check whether the cracking originates directly above the lintel header rather than from the foundation.
The Evanston version of window-area cracking often has a different origin. Evanston has the oldest residential brick stock on the North Shore, with many homes exceeding 100 years. Many were previously repointed with Type S or Portland cement mortar, far harder than the original soft brick. Hard mortar does not flex with the brick; thermal movement causes the brick face to fail instead of the joint. The result is joint-edge cracking and spalling adjacent to openings. Up close, the tell is that the mortar is intact while the brick faces around it are popping off. The repair is removing the incompatible mortar and repointing with lime-based mortar per NPS Preservation Brief 2. Filling the face cracks without removing the incompatible mortar leaves the cause in place.
Evanston greystones carry an additional wrinkle: the Indiana limestone facing on the front facade weathers differently than the common brick on the sides and rear. Mortar joints between limestone blocks need softer formulations than brick-to-brick joints, and crack patterns at window openings on the limestone facade require a different diagnostic lens than those on the brick sections of the same building.
Cracks that originate at the sill of an opening and extend diagonally downward are a different pattern, typically pointing to differential settlement at the foundation under the opening. These require structural assessment.
For a detailed treatment of lintel types, failure modes, and repair options, see lintel repair: steel, stone, and window openings.
Hairline Crazing vs Structural Cracking
Surface crazing is a network of very fine cracks that appear in the surface of the brick face itself, not in the mortar joints. It looks like a map or web of shallow cracks confined to the outer layer of the brick. It is a weathering and manufacturing phenomenon, not a structural indicator.
Crazing occurs when the brick surface layer has different expansion characteristics than the interior of the brick, or when manufacturing conditions produced a surface with higher porosity. The surface layer cracks in a random map pattern under the stress of thermal cycling and moisture movement. On salvaged brick or old brick that has been exposed for many decades, this is extremely common. ASTM C67 brick testing standards capture freeze-thaw durability in brick units and distinguish this surface behavior from structural degradation.
Surface crazing without any underlying structural cracking is a cosmetic issue. It does affect water absorption at the brick face, which is worth noting as part of an overall assessment. But it is not the same thing as a crack through the brick unit or a crack through a mortar joint.
Structural cracking in the brick unit itself, as distinct from crazing, typically has width, depth, and follows a consistent direction related to the stress that caused it. A crack through the full depth of a brick face along a vertical line is different from surface crazing in the same area.
For a full treatment of what brick face deterioration looks like and how it progresses, see what causes brick spalling and how to prevent it.
When to Monitor vs When to Call
The practical decision facing a homeowner who finds a crack is: watch it or act on it now.
Monitor, document, and reassess in three to six months for:
Hairline cracks, under 1/16 inch wide, in mortar joints only, with no displacement between the two sides of the crack.
Stair-step cracks that are narrow, appear in a pattern consistent with old settlement, and where the home has a long history with no related interior issues.
Vertical cracks through mortar joints at predictable thermal-movement locations that have not widened since you first noticed them.
Surface crazing on brick faces.
Get a professional assessment for:
Any horizontal crack in a basement or foundation wall, or any horizontal crack combined with visible wall bow in an above-grade wall.
Cracks through brick units rather than only through mortar joints.
Active cracks, meaning cracks that you can observe widening or that reopen after you fill them.
Cracks wider than 1/4 inch regardless of pattern.
Cracks at window openings with associated rust staining.
Any crack accompanied by door or window binding, floor settling, or other signs of building movement.
Any crack that appeared suddenly after a specific event such as heavy rain, nearby excavation, or foundation waterproofing work.
The threshold for calling is not “am I sure this is serious.” The threshold is “I am not sure this is routine.” In our work across Evanston, Wilmette, Winnetka, and Des Plaines, the homeowners who call when they first notice something uncertain are consistently better served than the ones who wait until the concern escalates. Des Plaines homeowners near O’Hare in particular should not apply a standard monitoring timeline to stair-step cracking: if the vibration source is ongoing and the mortar bond is already loosened, a crack that looks stable today may widen faster than the three-to-six-month monitoring window would suggest.
Emergency situations, meaning displaced brick, bowing walls, or cracks that appeared after an acute event, call for the guidance in emergency masonry repair.
Documenting and Measuring Crack Width Over Time
If you decide to monitor a crack rather than act on it immediately, document it properly so the monitoring means something. Photograph the crack with a reference scale and record the date. Mark the endpoints with pencil or chalk on the masonry surface, then check whether the crack extends beyond those marks on subsequent visits. Measure width at the widest point with a feeler gauge or digital caliper; record measurements with dates.
Check the crack after each significant weather event and after the first hard freeze. Freeze-thaw movement is most visible in spring when the wall returns to its expanded state. A crack that holds its dimensions across two full seasonal cycles is effectively stable. A crack that has grown measurably in three to six months is active and needs professional assessment before another winter. The spring masonry inspection checklist for Illinois is the right tool for that seasonal re-evaluation.
Getting the Right Assessment
Delta - Masonry and Tuckpointing has provided on-site crack assessments for Chicagoland homeowners since 1987. We look at crack patterns in the context of the building age, construction type, and surrounding conditions to distinguish maintenance items from items that warrant structural consultation. On Evanston greystones and older soft-brick homes, we test mortar compatibility before recommending any repair to avoid the Portland cement mismatch problem. On Des Plaines homes near O’Hare, we factor in vibration as a contributing cause alongside the standard clay-soil settlement assessment.
Brick repair and masonry repair work scoped after a proper assessment addresses the actual problem rather than the surface symptom. We see re-repair jobs regularly where a previous contractor filled visible cracks without diagnosing the cause. The cracks come back.
We serve Highland Park, Glenview, Northbrook, Evanston, and Des Plaines, as well as communities across the North Shore and northwest suburbs. Call (847) 713-1648 or contact us online to schedule an assessment. If you have photographs of the crack, send them when you call. Pattern recognition starts with the first look.
A crack is a symptom with a cause. Read the crack before you reach for the mortar gun.