What water-cement ratio does the Code set for an Ontario patio?

The 2024 Ontario Building Code sets a maximum water-to-cementing-materials ratio of 0.45 for garage floors, carport floors and all exterior flatwork, under section 9.3.1.7(1). That is the Code number for exterior concrete.

Whether that ratio binds a patio built without a permit is a different question. The City's list of projects not requiring a building permit includes landscaping, and the words patio, walkway and concrete do not appear on that list. The City's patio page sets out conditions under which a patio structure is exempt from a building permit. We found no City statement on whether the Building Code legally applies to a patio built under that exemption. Calling 0.45 the Code standard for exterior flatwork is accurate; saying it is enforceable law for every backyard patio is a step we cannot take on the information available.

What is clear is that the ratio matters in practice regardless of permit status. The Code's appendix note to section 9.3.1.7 says adding water to concrete on site may make it easier to place but can reduce strength and increase porosity and the propensity to shrinkage cracking. Section 9.3.1.6(1) sets a minimum compressive strength of 32 MPa after 28 days for all exterior flatwork. Those two numbers, the 0.45 ratio and the 32 MPa strength, belong in the contractor's written specification for any exterior slab, and you should ask to see them there before signing.

A few things to ask for in writing before the work starts:

• the maximum water-to-cementing-materials ratio the contractor will hold to
• the specified 28-day compressive strength
• the base preparation and depth
• the plan for rain or cold weather during the pour and curing period

Start with the contractor: ask them to put the mix specification in the quote. If the job involves steps or a landing that serves the entrance to the house, ask the City's Building Code Services whether a permit is needed, because the City's Building Code Services advisory on landscaping structures says those can require a permit, and the Code's requirements would then apply without question.