Does Ontario's Code require dampproofing under a basement slab?

Yes, with three exceptions. Ontario's Building Code says floors-on-ground shall be dampproofed (9.13.2.1). The exceptions are floors in garages, floors in unenclosed portions of buildings, and floors installed over granular fill that meets 9.16.2.1, which is the layer of at least 100 mm of coarse clean granular material under a floor-on-ground.

Where dampproofing is installed below the floor, the Code lists what it can be (9.13.2.6): polyethylene at least 0.15 mm thick with joints lapped at least 100 mm, Type S roll roofing with joints lapped at least 100 mm, or rigid polystyrene with sealed or shiplapped joints that is strong enough to support the floor and meets the Code's vapour permeance rule.

Waterproofing is a different step. Where hydrostatic pressure occurs, waterproofing is required (9.13.3.1). A basement floor that is waterproofed has a membrane between 2 layers of concrete, each at least 75 mm thick, continuous with the wall membrane (9.13.3.5). The Code gives no number or test for when hydrostatic pressure occurs, so ask the contractor which protection their quote covers for the floor and the walls, and why.

The walls have their own rule: exterior dampproofing runs from finished ground level to the top of the footing where the ground outside is higher than the floor inside (9.13.2.1, 9.13.2.4), unless the walls are waterproofed.

For an existing slab, ask the contractor to say in writing what is under it now, if they can find out, and what the quote does about it. If the job is a new slab or a slab being replaced, ask which of the Code's under-floor options the quote uses, and whether the granular layer is part of the scope.

Before you sign, have the quote name the floor protection, the wall protection, and how the two meet.