Where does parging end behind a Toronto interlock patio?

Parging should end at or just above the finished surface of the interlock patio — typically 50-75mm (2-3 inches) above the patio surface level, never buried beneath it.

This is one of the most common and damaging installation mistakes on Toronto homes with interlock driveways and patios. When interlock is installed flush against a parged foundation wall, or when the patio surface sits at or above the bottom edge of the parging, you create a moisture trap that accelerates parging failure dramatically.

The core problem is termination and drainage. Parging needs a clean, exposed bottom edge that allows any water running down the face of the foundation to drip free rather than wick into the joint between the parge coat and the patio surface. When interlock butts directly against parging with no gap, water sits in that joint through every rain event and snowmelt. In the GTA, with 50+ freeze-thaw cycles per winter, that trapped water freezes, expands, and mechanically levers the bottom edge of the parging away from the foundation wall. Within 2-3 winters, you'll see the classic pattern: parging delaminating and flaking from the bottom up, often in a band 100-200mm high along the entire patio perimeter.

The correct detail is to maintain a minimum 50mm gap between the finished interlock surface and the bottom edge of the parging, with the parging terminating in a clean, tooled edge rather than a ragged break. Ideally, a bead of polyurethane caulk is applied in the joint between the foundation wall and the interlock edge — this flexible sealant accommodates seasonal movement while keeping water from migrating behind the patio and against the foundation. That caulk joint should be inspected and renewed every 3-5 years.

GTA clay soil makes this worse. Interlock patios on clay soil shift seasonally as the ground heaves in winter and settles in spring. This movement constantly works at the joint between the patio and the foundation wall, opening gaps that allow water infiltration. If your interlock patio has risen or settled unevenly over the years, there's a good chance it's also compromised the parging termination detail and created a moisture pathway against the foundation.

Grading matters here too. The interlock surface should slope away from the foundation at a minimum 2% grade (roughly 25mm drop over 1.2 metres). If the patio pitches toward the house — which happens frequently as clay soils settle unevenly — water pools against the foundation wall regardless of how well the parging terminates. Correcting the grade by re-levelling the interlock base is often the most important fix, separate from any parging repair.

If you're having interlock installed new, make sure your interlock contractor and your parging contractor coordinate on the termination detail before either trade starts work. The interlock contractor needs to know the finished parge coat height; the parging contractor needs to know the finished patio elevation. This coordination rarely happens spontaneously and the homeowner usually pays for it later in premature parging failure.

For existing homes where interlock is already tight against the foundation, the practical fix is to carefully remove the first row of interlock pavers closest to the house, lower the base material to create the 50mm clearance gap, re-set the pavers, and then apply a polyurethane caulk joint between the patio edge and the foundation wall. A parging contractor can also cut a clean termination edge and apply a drip edge detail if the existing parging bottom is buried or in poor condition.

Need help finding a parging or masonry contractor who understands this detail? Toronto Parging can match you with a local professional for a free estimate — or browse the Toronto Construction Network directory at torontoconstructionnetwork.com/directory?trade=masonry.