Should the expansion joint between my foundation and garage slab be sealed with caulk or backer rod?

You need both — backer rod and caulk work together as a system, not as alternatives. The expansion joint between your foundation wall and attached garage slab is a designed movement joint that allows the two concrete elements to expand, contract, and settle independently. In the GTA, this joint undergoes significant seasonal movement due to frost heave pushing the garage slab upward in winter and thermal expansion cycling the concrete through a 55-degree annual temperature range. Sealing it properly requires backer rod to control the sealant depth, followed by a flexible caulking that can handle the movement without tearing or losing adhesion.

This joint is important to seal because it is a direct path for water, cold air, carbon monoxide (if you warm up vehicles in the garage), and insects into the living space. In GTA homes with attached garages, the foundation-to-slab joint is often the number one source of garage water infiltration during spring thaw and heavy rainstorms. Snow and ice melt that accumulates on the garage floor drains toward this low point, and without sealant, it runs directly against or under the foundation wall.

Here is the correct method. Clean the joint thoroughly — remove any old caulking, debris, road salt deposits, and loose concrete. If the joint has been filled with tar, asphalt patching, or expanding foam in the past, all of it needs to come out for a proper repair. Use a screwdriver, wire brush, or oscillating tool to get down to clean, solid concrete on both sides. Measure the joint width — typical foundation-to-slab expansion joints are 10-25mm wide. Select closed-cell foam backer rod that is approximately 25% wider than the joint so it compresses and stays in place when pushed in. Press the backer rod into the joint to a depth that leaves approximately 6-12mm of space above it for the sealant. The backer rod prevents three-sided adhesion — the sealant should bond only to the two side walls (foundation face and slab edge), not to the bottom. This two-sided bond allows the sealant to stretch like a rubber band as the joint opens and closes, rather than tearing because it is glued to three surfaces.

For the sealant, use a self-levelling polyurethane designed for concrete joints. Self-levelling formulas flow into the joint and create a smooth, flush surface that will not trap dirt or water. Standard (non-sag) polyurethane works too but requires more tooling to create a smooth finish in a horizontal joint. Apply the sealant over the backer rod, filling to within 3mm of the slab surface. Do not overfill — the sealant should be slightly recessed so vehicle tires and foot traffic do not abrade it. GTA pricing for materials: backer rod is $5-$15 per package, self-levelling polyurethane is $12-$20 per tube, and you will typically need 2-4 tubes for a single-car garage and 3-6 for a double. Total DIY cost: $25-$80.

Timing matters in the GTA. Do this work when temperatures are above 10°C and the garage slab is dry. Ideal timing is late spring or early fall when the joint is at a moderate width — neither fully expanded from summer heat nor fully contracted from winter cold. If you seal the joint in the dead of summer when it is at its narrowest, the sealant will be compressed when the joint closes further in extreme heat and may squeeze out. If you seal it in the coldest part of winter when the joint is at its widest, the sealant may be stretched beyond its capacity when summer expansion closes the joint.

Do not use silicone, construction adhesive, or expanding foam for this joint. Silicone does not bond to porous concrete reliably. Construction adhesive is rigid and will crack. Expanding foam fills the space but has no flexibility or water resistance and crumbles under foot traffic. If your expansion joint is severely deteriorated with crumbling concrete edges, or if the garage slab has heaved significantly, professional assessment is worthwhile — the joint may need to be routed (widened with a concrete saw) to create clean, uniform edges before sealing. Toronto Parging can match you with qualified local contractors for this and other foundation work through the Toronto Construction Network.