What is the strength rule for Ontario garage floors versus patios?

The 2024 Ontario Building Code sets a minimum compressive strength of 32 MPa at 28 days for garage floors, carport floors and all exterior flatwork. Interior floors other than garages and carports need only 20 MPa, and all other applications need 15 MPa. These figures come from section 9.3.1.6(1) of the Code.

The difference between a garage slab and a backyard patio is where the Code clearly applies. A garage floor is part of a building. For a free-standing backyard patio, 32 MPa is the Building Code standard for exterior flatwork; we found no statement on whether it binds a patio built without a permit. Steps that serve a house entrance and a garage slab are clearly within the Code's reach. For those, 32 MPa is the floor, not a suggestion.

The water-to-cementing-materials ratio matters alongside strength. Section 9.3.1.7(1) sets a maximum ratio of 0.45 for garage floors, carport floors and exterior flatwork, and 0.65 for interior floors other than garages and carports. The Code's appendix note to that section says adding water to concrete on site may make it easier to place, but can reduce strength and increase porosity and the tendency to shrink and crack. For a garage slab, ask your contractor to confirm the specified strength and the water-to-cementing-materials ratio in writing before the pour.

Air entrainment is a related point. Section 9.3.1.6(2) sets an air content of 5 to 8 per cent for site-batched concrete used in garage floors, carport floors and exterior flatwork. For ready-mix concrete, the Code references CSA A23.1 for air content requirements rather than quoting a percentage directly, so the figure for a ready-mix pour comes from that standard, not from 9.3.1.6(2).

A few things to confirm with any contractor quoting a garage slab or exterior concrete:

• the specified compressive strength (32 MPa for a garage floor or exterior flatwork)
• the maximum water-to-cementing-materials ratio (0.45 for those applications)
• whether the mix is ready-mix or site-batched, and how air entrainment is specified
• the base preparation, since section 9.16.2.1 requires not less than 100 mm of coarse clean granular material beneath floors-on-ground, though that article says it need not be installed under slabs in garages, carports or accessory buildings
• cold-weather protection if the pour happens when the air is below 5 °C, since section 9.3.1.9 requires concrete to be kept at not less than 10 °C for 72 hours after placing

If replacing a garage floor slab is part of the plan, we found no City statement on whether that work needs a building permit. Ask the City's Building Code Services before the old floor comes out, and if any digging is involved, submit a locate request through Ontario Dig Authority before breaking ground.