What concrete strength does Ontario's Code set for a garage floor?

Section 9.3.1.6(1) of Ontario's 2024 Building Code sets a minimum compressive strength of 32 MPa after 28 days for garage floors and carport floors.

That same section also covers all exterior flatwork, so the 32 MPa figure applies across those categories. For interior floors other than garages and carports, the minimum is 20 MPa, and for all other applications it is 15 MPa.

A few related Code requirements bear on a garage slab. Section 9.3.1.7(1) sets a maximum water-to-cementing-materials ratio of 0.45 for garage floors and exterior flatwork; the appendix note to that section says adding water to concrete on site may reduce strength and increase porosity and the propensity to shrinkage cracking. Section 9.16.4.3 sets a minimum slab thickness of 75 mm, exclusive of any topping, with a tolerance that allows no point to fall below 65 mm and requires the mean to be 75 mm. Section 9.35.2.2 requires the garage floor to slope to drain liquids to the outdoors. Where an attached garage is connected to a dwelling, section 9.35.3.2 calls for a construction joint between the main building slab and the garage slab, and in clay-type soils subject to significant movement with moisture, the foundation depth of the garage should be approximately the same as the main building's.

On air entrainment: section 9.3.1.6(2) specifies 5 to 8 per cent air for site-batched concrete used in garage floors and exterior flatwork. For ready-mix concrete, the air content requirement comes from CSA A23.1, which the Code references, and we do not quote a percentage for ready-mix.

When you review a quote, ask the contractor to put the concrete strength, the water-to-cementing-materials ratio, the slab thickness, and the slope-to-drain detail in writing. We found no City statement that the hardscaping contractor licence covers work inside a garage; ask the City whether it applies to your project. If the job involves any digging, whoever digs is the excavator under the Ontario Underground Infrastructure Notification System Act, 2012, and a locate request must go through Ontario Dig Authority before the ground is broken.