What does the Ontario Building Code say about foundation waterproofing for new construction?

The Ontario Building Code (OBC) Part 9, Section 9.13 sets clear, mandatory requirements for foundation waterproofing and dampproofing on all new residential construction in Ontario, and understanding these standards is essential for any GTA homeowner dealing with foundation moisture issues on newer homes or planning waterproofing upgrades on older homes.

OBC Section 9.13.2 requires that the exterior surface of all below-grade foundation walls must be dampproofed or waterproofed. The distinction matters: dampproofing resists moisture in the absence of hydrostatic pressure (water not actively pushing against the wall), while waterproofing resists moisture under hydrostatic pressure. For new construction in the GTA, where clay soils hold water against foundations for extended periods and create significant hydrostatic pressure, most builders now apply full waterproofing rather than minimum dampproofing — though the OBC technically allows either depending on site conditions.

For dampproofing, the OBC accepts a coating of bituminous material (asphalt-based damp proofing compound) applied to the exterior of the foundation from the footing to finished grade. This is the minimum code requirement. The coating must be continuous and free of gaps, holes, or thin spots. For decades, many GTA builders used only this minimum standard — a single coat of black tar-like dampproofing — which is why so many 1970s through 1990s suburban homes in Mississauga, Brampton, Markham, and Vaughan now have basement moisture problems as that thin coating deteriorates.

For waterproofing, the OBC requires a continuous waterproof membrane applied to the exterior of the foundation wall from the footing to above finished grade. Acceptable materials include liquid-applied rubber membranes, sheet membranes (self-adhering modified bitumen), and spray-applied coatings that meet CSA standards for waterproofing performance. Modern best practice in the GTA — and what quality builders now specify — is a liquid-applied rubber waterproofing membrane covered with a dimpled HDPE drainage board (dimpled membrane). The drainage board creates an air gap between the waterproofing and the backfill soil, which directs groundwater down to the foundation drain instead of pressing against the waterproofing membrane.

Foundation drainage is covered under OBC Section 9.14, which works hand-in-hand with the waterproofing requirements. All new construction must have a perimeter foundation drain (weeping tile) installed around the exterior of the footing. The drain must be minimum 100mm (4-inch) diameter perforated pipe, bedded in and covered by clear granular material (washed gravel), and must drain to a sump pit, storm sewer, or daylight outlet. The granular drainage layer must extend from the top of the footing to at least 150mm above the top of the weeping tile. This system collects groundwater before it reaches the waterproofing membrane, reducing hydrostatic pressure against the foundation.

OBC Section 9.13.3 addresses foundation wall moisture protection from the interior, requiring a moisture barrier on the interior face of below-grade walls before insulation and finishing. This prevents interior humidity from condensing on the cold concrete wall and causing mould within the finished basement assembly.

For homeowners with existing homes considering waterproofing upgrades, the OBC does not retroactively require older homes to meet current standards. However, when excavation is performed for any reason — parging, crack repair, weeping tile replacement — it is strongly recommended (and often required by local building departments) to bring the waterproofing up to current OBC standards while the foundation is exposed. Excavating a foundation is expensive ($150–$250 per linear foot in the GTA for excavation alone), and doing the work twice because you skipped waterproofing the first time is a costly mistake.

Exterior foundation waterproofing on a GTA home typically costs $3,000–$12,000 depending on depth, linear footage, soil conditions, and accessibility. This includes excavation, surface preparation, membrane application, dimpled board, weeping tile inspection or replacement, granular backfill, and grading. Always ensure the contractor carries WSIB coverage, as excavation near footings is hazardous work regulated under Ontario construction safety standards.