Understanding Thermal Cracking in Masonry

Every solid material changes dimension when its temperature changes. Masonry units are no exception. Clay brick typically exhibits a coefficient of thermal expansion (CTE) between 4 and 6 × 10⁻⁶ in/in/°F (7–11 × 10⁻⁶ mm/mm/°C), while concrete masonry units (CMUs) range from 5 to 7 × 10⁻⁶ in/in/°F (9–13 × 10⁻⁶ mm/mm/°C). Mortar often has a slightly higher CTE, and the differential expansion between units and mortar can itself generate internal stresses, particularly in long, uninterrupted wall runs. Clay bricks also undergo irreversible moisture expansion after firing—a permanent growth of 0.1% to 0.2% that occurs over months to years—while concrete blocks shrink as they dry and carbonate over time. These permanent volume changes interact with thermal movement, creating a complex environment that demands careful accommodation from the earliest design stages.

Consider the physical stress accumulation: when a wall is heated by direct solar radiation, its outer wythe can become significantly hotter than the interior, causing the exposed face to expand more than the backup. Temperature differentials of 50°F to 70°F (28°C to 39°C) between the exterior surface and the interior backup are common on south- and west-facing facades during summer afternoons. If movement is restrained—by rigid connections to steel or concrete frames, by intersecting walls, by improperly sized shelf angles, or by masonry laid tight against columns—stress accumulates until the material cracks. The tensile strength of masonry is low, typically only 50 to 100 psi (0.3 to 0.7 MPa), far below the compressive strength. Once tensile stress exceeds this threshold, cracking is inevitable.

These cracks often appear as fine vertical fissures at regular intervals along long straight walls, at the corners of openings, or in head joints above windows. Without proper movement accommodation, cracks widen over time, allowing moisture penetration that reduces durability, accelerates freeze-thaw deterioration, and compromises insulation value. Water that enters through even hairline cracks can cause efflorescence, corrosion of embedded ties, and staining that mars the building's appearance. The financial implications are significant: repairing thermal cracking in a typical commercial facade can cost $50 to $150 per linear foot, and premature replacement of an entire wall system runs far higher.

Risk factors vary by orientation, climate, and material choice. South- and west-facing walls in sunny climates absorb the most radiant energy and can experience surface temperatures 50°F (28°C) or more above ambient. Dark-colored masonry absorbs more solar radiation than light-colored surfaces, increasing thermal excursion. Poorly insulated walls that allow large interior-exterior temperature gradients tend to bow and crack. Regional climate also matters: in arid climates with wide diurnal swings, thermal movement can be extreme, while in humid climates, moisture expansion may dominate. Recognizing these factors helps designers anticipate where movement joints will be most essential and which materials will perform best over the building's service life.

Key Prevention Strategies

Preventing thermal cracks requires an integrated strategy that respects the inherent movement of materials. The following five categories—movement joints, material selection, structural design, environmental control, and construction quality—form a complete framework for durable masonry. Each category reinforces the others; neglecting any single element can compromise an otherwise sound design.

Designing and Installing Movement Joints

The most direct method for avoiding thermal cracks is to provide properly designed movement joints that allow the wall to expand and contract without restraint. For clay brick masonry, vertical expansion joints are the primary tool. These soft joints, filled with a compressible backer rod and sealed with a durable elastomeric sealant, are located at regular intervals and at any points where geometry would otherwise lock up movement. The sealant must be capable of accommodating the full calculated movement range—typically 25% to 50% of the joint width—and should have a low modulus to avoid transferring stress back to the masonry.

According to the Brick Industry Association’s Technical Note 18A, vertical expansion joints should be placed at all corners, offsets, and wall intersections; at maximum center-to-center spacings of 25 ft (7.6 m); and at returns where the length of wall beyond a corner exceeds 15 ft (4.6 m). The same source recommends that expansion joints be continuous through the full thickness of the brick wythe, free of mortar droppings, and properly sized to accommodate calculated movement. A common joint width for low-rise brick veneer is ½ in. (13 mm), though taller buildings demand wider joints—sometimes up to 1 in. (25 mm) for structures over six stories—and the design must account for both thermal movement and irreversible moisture expansion. For buildings in regions with extreme temperature swings, perform a site-specific movement calculation rather than relying on default spacing values.

"Vertical expansion joints should be located at all corners, offsets, and wall intersections; at maximum spacing of 25 feet (7.6 m) on centers; and at returns in the wall where the length of wall extending beyond the corner exceeds 15 feet (4.6 m)." — Brick Industry Association, Technical Note 18A

Horizontal movement joints are equally important in multi-story construction. Shelf angles that support brick at each floor line must be detailed with a soft joint beneath the angle to permit vertical expansion of the brickwork below. The joint should be left open and filled with sealant; the brick must not be wedged tightly under the angle. A gap of ½ in. to ¾ in. (13 mm to 19 mm) is standard, with a compressible filler such as closed-cell polyethylene foam installed before placing the brick above. Flexible ties or anchors that allow vertical slip preserve integrity, typically using two-piece adjustable anchors with a slot or sleeve that permits ½ in. (13 mm) of vertical movement without binding.

In concrete masonry walls, the analogous feature is the control joint—a weakened plane designed to attract cracking to a predetermined, caulked location. The National Concrete Masonry Association’s TEK 10-2B recommends maximum control joint spacing of 20 ft (6.1 m) on center, or 1.5 times the wall height (whichever is less). Joints must be placed at openings, changes in wall height or thickness, and intersections. Horizontal joint reinforcement in the mortar bed can be interrupted at the joint to allow movement, and the joint is then sealed with backer rod and high-performance sealant. For long walls where drying shrinkage is expected to exceed 0.03%, consider intermediate control joints every 12 ft to 15 ft (3.7 m to 4.6 m) during the first year after construction, then seal or retrofit after most shrinkage has occurred.

Optimizing Material Selection

Choosing materials with well-matched thermal and moisture movement properties minimizes internal stresses before they ever develop. Clay bricks with low CTE and moderate irreversible moisture expansion cause fewer problems than highly expansive units. Specify bricks tested for these properties per ASTM C67 and C216, and request manufacturer data on the specific CTE for the brick under consideration. Store bricks on site dry and protected from rain to prevent pre-placement moisture absorption, which can exacerbate expansion after installation.

Mortar mix design plays a role. Type N mortar—with its good balance of workability, bond strength, and flexibility—is preferred for most exposed above-grade brickwork. Mortars with high lime content impart greater extensibility, allowing the assembly to move slightly without cracking. High-cement mortars (Type S or M) are more brittle and amplify crack formation, often leading to cohesive failures within the mortar itself. Use well-graded sand with clean, angular particles to improve paste characteristics and reduce drying shrinkage. The mortar's aggregate-to-cement ratio should be maintained between 2.25 and 3.0 parts sand to 1 part cementitious material by volume for optimal performance.

For concrete masonry, select aggregates with low CTE, such as limestone or certain lightweight aggregates like expanded shale or clay. This reduces overall wall movement, especially in regions with extreme temperature swings. The grout and concrete used in bond beams should match the unit's thermal properties to avoid differential movement. Additionally, specify ASTM C90 block for consistency in dimensions and compressive strength, and request that the manufacturer provide shrinkage test data per ASTM C426. For projects where movement-sensitive cladding is involved, consider using autoclaved aerated concrete (AAC) units, which have a CTE of roughly 4.5 × 10⁻⁶ in/in/°F and significantly lower drying shrinkage than conventional CMUs.

Color is an underappreciated variable. Dark red, brown, or black brick absorbs far more solar radiation than buff or white units, leading to higher surface temperatures and greater expansion. Measurements show that a dark-colored brick facade can reach 160°F (71°C) on a summer afternoon, while a light-colored facade on the same building remains below 120°F (49°C)—a difference of 40°F (22°C) that translates to roughly 0.02 in. (0.5 mm) less expansion per 10 ft (3 m) of wall. Where dark facades are desired, use external shading, ventilated cavities, or high-emissivity coatings to offset the increased thermal load. Solar reflectance index (SRI) values can guide material selection; coatings with SRI of 29 or higher reduce surface temperature significantly, and SRI values above 50 are available for light-colored finishes. For a comprehensive list of approved materials, consult the ASTM C216 standard for facing brick.

Structural Design and Detailing Practices

Beyond movement joints, the overall structural configuration can either restrain or accommodate thermal movement. Thin, tall, unbraced walls are more susceptible to bowing under differential temperature, which opens horizontal cracks at mid-height or floor lines. Ensure adequate thickness-to-height ratios per building code and Portland Cement Association guidance on concrete volume change. For example, the Architectural Institute of America's TMS 402/602 code specifies minimum nominal wall thicknesses of 6 in. for bearing walls and 4 in. for nonbearing walls, but thicker sections may be needed for tall, slender elevations.

Reinforcement restrains movement and increases tensile stress, so it must be detailed carefully. Properly placed vertical and horizontal reinforcement controls crack widths and keeps cracks hairline—typically limiting crack widths to less than 0.01 in. (0.25 mm)—which prevents water ingress. Joint reinforcement—ladder or truss type placed in every other bed joint—distributes stresses and should be continuous through all phases of construction. Two-piece adjustable veneer ties allow differential movement between brick wythe and backup wall, preventing frame movement from transferring into the veneer. Ties must be spaced at a maximum of 24 in. (610 mm) horizontally and 16 in. (406 mm) vertically, with an overlap of at least 1½ in. (38 mm) for adjustment.

At critical intersections (masonry wall meeting concrete column or steel beam), install a bond breaker: building paper, self-adhering flashing, or a manufactured isolation joint such as a preformed rubber or PVC strip. This slip plane prevents dissimilar materials from locking together and transferring restraint. Lintels and shelf angles need sufficient bearing length—at least 4 in. (102 mm) for standard openings—so their ends can rotate without damaging adjacent brick. Continuity of expansion joints around corners is non-negotiable; a joint that stops short forces cracking at the turn, often producing a vertical step crack at the corner. In L-shaped buildings, provide a joint at both legs of the turn, spaced at least one joint width apart.

Openings for windows and doors are common crack initiation sites because the wall panel above and below the opening moves differently from the panels at the sides. Place a vertical movement joint on one side of each major opening, or frame the opening with a soft joint at the head to accommodate differential movement between panels. The junction between a parapet wall and roof deck also demands a horizontal expansion joint, often combined with through-wall flashing, to release differential movement that causes parapet cracking. For parapets taller than 3 ft (0.9 m), provide an additional vertical joint at intervals no greater than 20 ft (6.1 m) to prevent bowing and separation from the roof structure.

Managing Environmental Exposure

Reducing temperature extremes that a wall experiences is a proactive, cost-effective strategy. Overhangs, canopies, and projecting cornices shade the facade and lower peak surface temperature on sunny sides. A properly sized overhang extending 2 ft (0.6 m) can reduce solar heat gain on a south-facing wall by 30% to 50% during summer months. Landscaping with deciduous trees shades south-facing walls in summer while allowing solar gain in winter—but avoid root damage near foundations by planting trees at least 10 ft (3 m) from the wall.

Light-colored coatings or paints with high solar reflectance can reduce surface temperature by 20°F (11°C) or more compared to dark uncoated masonry. Elastomeric, breathable coatings suitable for masonry provide waterproofing as well, preventing moisture from entering the wall while allowing residual moisture to escape. These coatings should have a permeability rating of at least 5 perms to avoid trapping water behind the film, which can cause peeling or blistering. In cavity wall construction, leave the air cavity ventilated with weep holes at the bottom and vents at the top; the chimney effect dissipates heat and reduces thermal gradient across the outer wythe. Weep holes should be spaced at a maximum of 24 in. (610 mm) on center and be at least 3/16 in. (4.8 mm) in diameter to prevent clogging.

Placing primary thermal insulation on the exterior side of structural backup keeps the masonry wythe closer to outdoor air temperature throughout the day. Exterior insulation and finish systems (EIFS) or rigid insulation boards behind brick veneer in cavity walls both serve this function, maintaining the masonry at a more stable temperature and reducing net thermal movement. With a stable temperature, the masonry's daily expansion cycle is minimized, and long-term cracking risk diminishes. For retrofit projects, adding exterior insulation or a ventilated rainscreen can dramatically improve thermal stability of existing masonry, often reducing surface temperature variation by 30°F (17°C) or more. The rainscreen approach also reduces moisture accumulation, which can exacerbate freeze-thaw damage at thermal crack sites.

Cold climates bring freeze-thaw challenges. When moisture-saturated brick freezes, ice expansion spalls the surface and widens thermal cracks. Ensure good drainage details: weep holes at the base of every cavity, cavity flashing that extends at least 2 in. (51 mm) beyond the wall face, sloped sills with a minimum 2% slope, and a continuous water-resistive barrier behind the masonry. A heated interior that prevents the wall from reaching freezing temperature on the inside face also reduces thermal gradient, though care must be taken to avoid condensation within the wall assembly. In severe climates, specify clay brick with an absorption rate less than 8% and use Type N or Type O mortar with air entrainment to improve freeze-thaw resistance.

Construction Quality and Workmanship

The most carefully designed joint fails if not built correctly. Train masons to understand why joints are placed where they are and to keep them free of mortar blockages. Insert a preformed, non-absorbent backer rod to correct depth—typically one-third to one-half the joint width—before applying sealant; if the joint is partially filled with mortar droppings, movement capacity is compromised by 50% or more. Tool sealant to a concave profile against the backer rod to allow stretching and compression without peeling, and ensure the sealant's width-to-depth ratio is at least 2:1 for proper performance. Schedule sealant application after at least 28 days of masonry curing to allow for initial moisture expansion, and avoid sealing during rain or when temperatures are below 40°F (4°C).

Fresh masonry is vulnerable to thermal cracking during curing. In hot, dry weather, mortar can lose water too rapidly, causing shrinkage cracks that become thermal crack loci. Cover the wall with wet burlap, use fog misting at intervals of 2 to 4 hours, and avoid direct sunlight during the hottest part of the day—typically between 10 a.m. and 4 p.m. in summer. In cold weather, heat materials to at least 40°F (4°C), use low-temperature admixtures like calcium chloride (limited to 2% by weight of cement), and erect temporary enclosures with heaters to keep mortar above freezing and allow proper curing for a minimum of 72 hours. The International Masonry Institute provides training resources and field guides that reinforce these standards, including detailed cold-weather and hot-weather construction procedures.

Phased construction can mitigate movement-related cracking. For long clay brick walls, leave masonry to cure for several days (or even weeks for very long elevations) before connecting final tie-downs or shelf angles. This allows initial moisture expansion and early thermal cycling to occur before full restraint is applied. In practice, a 7- to 14-day curing period before installing shelf angles above can reduce cracking by 30% to 50% compared to continuous construction. Similarly, introduce temporary movement joints in long concrete masonry walls until most drying shrinkage has occurred—typically 60 to 90 days after construction—then fill them later with backer rod and sealant. During this period, monitor crack development with simple tell-tale gauges to ensure movement is occurring as expected.

Masons should inspect control joints and expansion joints at the end of each workday, removing any mortar that squeezed into the gap using a raking tool or compressed air. Quality assurance checklists that include verification of joint locations, widths, and cleanliness help embed best practices into the construction culture. Include team meetings before wall construction begins to review the movement joint plan, material specifications, and installation sequence. Document any deviations with photographs and corrective actions to ensure accountability and continuous improvement across the project.

Inspection, Maintenance, and Repair

Even with all preventive measures, periodic inspection remains essential. Facility managers and homeowners should examine exposed masonry after severe weather—particularly after a sudden temperature drop following a sunny period—and at least once a year, preferably in late spring when thermal damage from winter conditions becomes visible. Look for hairline vertical cracks that follow mortar joints, cracks radiating from window or door corners, and step cracks at wall intersections. Use a crack width gauge to measure openings; cracks wider than 1/16 in. (1.6 mm) should be sealed promptly to prevent water ingress. Early sealing with high-quality, low-modulus urethane or silicone sealant prevents water intrusion and stops crack propagation, extending the wall's service life by 10 to 20 years.

If cracks reappear shortly after sealing—within one to two years—the joint, or lack of one, is not accommodating movement, and a structural engineer should be consulted. In some cases, retrofitting additional movement joints by diamond-blade saw cutting can relieve stress before more serious deterioration occurs. The saw cut should be made to a depth of at least one-third the wall thickness, then filled with backer rod and sealant. Install crack monitors or tell-tales with a precision of 0.01 in. (0.25 mm) to record ongoing movement, distinguishing between one-time settlement (which stabilizes within months) and continuous thermal cycling (which repeats seasonally). Data collected over 12 to 24 months can guide whether additional joints are needed.

For larger cracks that extend through the wall thickness, use a repair mortar with similar thermal expansion properties to the original masonry. Open the crack into a V-groove at least ¼ in. (6.4 mm) wide and ¼ in. deep, then fill with flexible sealant rather than rigid patching material—rigid repairs almost always crack again at the bond line within one to two years. Follow manufacturer instructions for surface preparation and primer application to ensure adhesion, and allow the sealant to cure fully—typically 24 to 72 hours—before exposing it to water or heavy loads. For cracks in load-bearing masonry, consult the TMS 402/602 code for structural repair requirements, which may include installation of helical ties or carbon fiber reinforcement.

Conclusion

Thermal cracking in masonry walls is not an unavoidable consequence of temperature change; it is a sign that movement was underestimated or restrained in the building's design or construction. By combining strategically placed expansion and control joints, well-matched materials, thoughtful structural detailing, and climate-responsive construction practices, design professionals and builders can keep masonry facades intact for decades. The cost of implementing these strategies upfront is typically 1% to 3% of the total wall construction cost, while the cost of repair or replacement can be 10 to 20 times higher over the building's lifetime. Regular inspection and prompt maintenance preserve the investment, protecting both structural performance and aesthetic quality. Committing to these proven strategies is one of the most reliable paths to durable, low-maintenance masonry construction. For further reading, consult the ASTM C216 standard for facing brick and the Masonry Standards Joint Committee's TMS 402/602 code. These resources provide additional technical depth for designers seeking to eliminate thermal cracking entirely from their projects.