If your Toronto home was built before approximately 1940, there's a reasonable chance it was constructed with clay brick that has a significantly lower compressive strength and higher water absorption rate than modern brick. This distinction matters enormously when it comes to repointing — and understanding it can save you from a very expensive mistake.
The Problem with Portland Mortar on Heritage Brick
Modern masonry mortars — Type S and Type N Portland-lime blends — have compressive strengths in the range of 4 to 17 MPa. Heritage clay brick from the late 1800s and early 1900s often tests at compressive strengths below 10 MPa, sometimes as low as 3–5 MPa for the softest varieties.
When a mortar harder than the brick is used for repointing, the thermal and moisture cycling that would normally be absorbed by flexing mortar joints instead concentrates stress at the brick-mortar interface. The brick, being weaker, fails first — the face spalls, cracks, and progressively disintegrates. This is sometimes called "mortar erosion reversal," and it typically appears within three to seven years of an incorrect repointing job.
The right mortar for heritage brick is softer and more flexible: typically a natural hydraulic lime (NHL 2 or NHL 3.5), a lime putty mortar, or at most a Type N Portland-lime blend with a high lime proportion. These mortars yield at the joint rather than at the brick face, and they're more vapour-permeable — which allows the wall to manage moisture through breathing rather than trapping it.
How to Identify Heritage Brick
There's no foolproof field test, but several indicators suggest you're working with heritage brick:
- The home was built before approximately 1940 - The brick is hand-made looking, with slight variations in size, colour, and surface texture - The existing mortar joints are soft enough to probe with moderate key pressure - The mortar has a warm beige or cream colour rather than the grey of Portland-based mixes
If you're uncertain, a masonry contractor experienced in heritage work can identify the likely brick composition and specify an appropriate mortar match based on the building's age, observed hardness, and existing joint profile.
What to Ask Your Masonry Contractor
Before any repointing work is performed on a pre-1940 Toronto home, ask:
1. How are you determining the mortar specification for this building? 2. Are you using lime-based mortar on the heritage sections? 3. Will the new mortar be softer than the brick?
A contractor who can answer these questions with specificity is one who understands heritage masonry. One who defaults to Type S on everything — because it's easier and cheaper to source — is a risk to your brick.
Related services: Brick Repointing & Tuckpointing · Brick Repair & Restoration · Back to Elite Toronto Masonry
Written by the Elite Toronto Masonry Team