Should I use hydraulic cement or polyurethane injection to fill foundation cracks before parging?

For most poured concrete foundation cracks in the GTA, polyurethane injection is the superior choice over hydraulic cement when the goal is a lasting repair before parging. Hydraulic cement sets rapidly and stops active water leaks in the moment, but it is a rigid material that cannot flex with the natural thermal movement of a GTA foundation — and with over 50 freeze-thaw cycles per winter, that movement is constant. Polyurethane injection foam, by contrast, remains flexible after curing, expanding and contracting with the concrete through seasonal temperature swings ranging from -20°C to 35°C.

The key difference comes down to how each material behaves inside the crack. Hydraulic cement is a surface patch — you mix it, press it into the crack opening, and it hardens within minutes. It works well as an emergency stop for active water seepage, but it only fills the visible portion of the crack, typically the outer 15-25mm. The crack behind it remains open, and water can still travel through the wall and find a new path. Polyurethane injection, on the other hand, is injected through ports drilled along the crack at intervals, and the expanding foam fills the crack from the interior face to the exterior face through the full thickness of the foundation wall. This creates a complete, flexible seal that moves with the concrete. A typical polyurethane crack injection in the GTA costs $300-$800 per crack, depending on length and accessibility, while hydraulic cement patches cost $20-$50 in materials for a DIY repair — but the DIY patch has a much higher failure rate in Toronto's climate.

When hydraulic cement makes sense is for non-structural surface spalling, minor honeycombing in poured concrete, or as a temporary measure to stop water while you arrange for professional injection. It is also appropriate for filling small voids in concrete block foundations where injection is not practical. For block foundations, a mason can parge directly over properly prepared block with bonding agent — injection is not applicable to block walls.

Before parging over any repaired crack, the repair must be fully cured and the surface must be flush with the surrounding foundation. Polyurethane injection ports should be shaved flush after curing. If hydraulic cement was used, ensure it is solid and not crumbling — failed hydraulic patches must be chipped out and the area properly prepped before parging. In both cases, apply a quality SBR latex or acrylic bonding agent to the entire surface before the parge coat goes on. Skipping the bonding agent over crack repairs is a guaranteed path to delamination within one to two GTA winters.

For structural cracks — step cracks, horizontal cracks, or any crack wider than 5mm — neither hydraulic cement nor polyurethane injection alone is sufficient. These indicate ongoing foundation movement that must be assessed by a structural engineer before any cosmetic parging. Covering structural cracks with parging hides the problem and allows it to worsen. WSIB-covered professional contractors should handle all crack injection work, as improper injection can create pressure that widens cracks or forces material into places it should not go. If you are unsure whether a crack is structural or cosmetic, a qualified masonry contractor can assess it during a free estimate — Toronto Parging can match you with local professionals through the Toronto Construction Network.