Moisture Management in Historic Brick Masonry

How Water Enters a Solid Brick Wall

Nineteenth-century commercial buildings in Canada were constructed almost universally with solid masonry walls—two to four wythes of brick laid in running or Flemish bond, with no cavity. This construction type manages moisture differently from modern cavity-wall assemblies. Rather than deflecting water at an air gap, a solid wall absorbs precipitation into the outer wythe, holds it during the weather event, and then dries outward as conditions allow.

This "reservoir" behaviour functions adequately when the mortar and brick remain vapour-permeable and when drying is not obstructed. Problems arise when:

• Hard Portland cement repointing or sealant coatings reduce vapour permeability, trapping moisture inside the wall
• Mortar joints have deteriorated to the point that water bypasses the nominal outer face and saturates the interior wythes
• Above-grade damp-proof courses have failed, allowing rising damp from the foundation
• Parapet copings, window heads, or sill flashings are missing or deteriorated, directing bulk water into the wall

Freeze-Thaw Mechanics

In Canadian climates—particularly in the Great Lakes region, Quebec, and the Prairies—masonry walls experience numerous freeze-thaw cycles each winter. Water expands approximately 9% when it freezes. In a saturated brick or mortar joint, this expansion generates internal tensile stress. When that stress exceeds the tensile strength of the material, cracking or spalling results.

The degree of damage depends on the degree of saturation at the time of freezing. A wall that dries adequately between precipitation events is less vulnerable than one that retains moisture near the surface. This is why interventions that reduce the drying rate—such as applying an impermeable sealant—can accelerate freeze-thaw damage even when they appear to reduce water entry.

Common Moisture Entry Points

A systematic survey of a historic commercial façade for moisture vulnerability typically focuses on the following locations, in rough order of frequency:

Location	Typical Cause	Observable Signs
Mortar joints (face)	Deterioration, shrinkage cracks at mortar-brick interface	Hollow-sounding joints, missing mortar, staining below joints
Parapet and coping	Missing or cracked cap flashing; open coping joints	Spalling at upper wall courses; efflorescence on interior
Window and door heads	Absent or failed through-wall flashing	Water staining below lintels; damp interior plaster
Window sills	Sill slope less than 15°; absent drip edge	Staining and mortar loss directly below sill
Base of wall	Failed damp-proof course; soil or paving too high	Rising efflorescence; spalling at lower courses
Horizontal string courses	No drip profile; open joints	Water tracks below projecting courses

Diagnostic Investigation

Moisture problems in historic masonry are often attributed to the most obvious visible symptom—typically efflorescence or surface staining—without identifying the actual entry point. A systematic investigation approach includes:

• Probe rod survey: Tapping across the wall surface with a sounding rod or rubber mallet to identify hollow areas and delaminated brick.
• Visual survey after rainfall: Observing the wall within 12 to 24 hours of a significant rain event to identify active infiltration paths.
• Moisture meter readings: Capacitance or resistance meters give relative moisture readings; they are useful for comparing areas of the wall but should not be treated as precise absolute measurements in historic masonry.
• Infrared thermography: Under appropriate conditions (temperature differential between interior and exterior), thermal imaging can reveal moisture patterns inside the wall assembly without invasive probing.

Appropriate Remediation Approaches

The hierarchy of interventions recommended in heritage conservation practice begins with the least invasive:

1. Address the source first. Repair gutters, downspouts, flashings, and drainage elements that are directing water toward the wall. Repointing a wall that is being saturated from the parapet above will not solve the problem.
2. Repoint deteriorated joints. Where mortar joints are the primary infiltration path, appropriate repointing with a compatible lime-based mortar restores the wall's designed moisture management behaviour.
3. Allow the wall to dry. Before applying any coating or repair, a saturated wall should be allowed to dry—potentially over a full drying season —so that any treatment is applied to a wall with normal moisture content.
4. Use breathable consolidants if needed. Where brick has deteriorated beyond repointing alone, breathable consolidants based on lime or ethyl silicate may be appropriate in specific locations; these should be selected in consultation with a conservation specialist.

What to Avoid

The following interventions are consistently identified as problematic in heritage guidance and post-occupancy surveys of historic brick buildings:

• Application of impermeable coatings (elastomeric paint, silicone sealers, epoxy grouts) to solid masonry walls
• Sandblasting or high-pressure wet blasting, which removes the fired outer surface of the brick and increases water absorption
• Grouting cracks with Portland cement or epoxy without addressing the underlying cause of movement
• Installing interior vapour barriers in buildings with solid masonry walls, which can redirect moisture-driven wetting to wall-floor intersections

References

• Parks Canada. Standards and Guidelines for the Conservation of Historic Places in Canada, 2nd ed. Ottawa, 2010.
• Canada Mortgage and Housing Corporation (CMHC). Renovation series: Moisture and Air.
• National Park Service (US). Preservation Brief 39: Holding the Line — Controlling Unwanted Moisture in Historic Buildings.
• BRE (UK). Repairing Brick and Block Masonry. (Referenced for comparative freeze-thaw methodology.)