How do I waterproof a foundation wall from the inside if my neighbour's fence is right against my house?

When exterior access is blocked by a neighbour's fence or structure right against your foundation wall, interior waterproofing becomes your primary option — and while it manages water rather than stopping it at the source, modern interior systems are effective when properly installed. This is a common situation across the GTA, particularly in older Toronto neighbourhoods like Leslieville, the Danforth, Riverdale, and Beaches where lot lines are tight and side yards are narrow.

The most effective interior approach combines crack injection, an interior drainage system, and a sump pump. Start by identifying where water is entering. In poured concrete foundations — the most common type in post-war GTA homes — water typically enters through non-structural cracks, the cold joint where the wall meets the footing, and around pipe penetrations. Each crack can be injected with expanding polyurethane foam, which fills the crack through the full thickness of the wall and remains flexible to accommodate minor foundation movement. Polyurethane injection runs $300-$800 per crack in the GTA market, and a typical basement with three to five active cracks will cost $1,000-$3,500 to inject.

For more widespread water entry or hydrostatic pressure from GTA's notorious clay soils, a full interior weeping tile system is the standard solution. This involves cutting a trench around the interior perimeter of the basement floor, installing 4-inch perforated PVC pipe in a gravel bed, connecting it to a sump pit with a quality sump pump, and pouring new concrete over the trench. A dimpled membrane is installed against the interior face of the foundation wall to direct any wall seepage down into the drainage channel rather than onto the basement floor. This system costs $5,000-$15,000 depending on the basement's linear footage and whether the floor needs to be broken around the full perimeter or only the affected walls.

It is important to understand what interior waterproofing does and does not do. It does not stop water from reaching your foundation wall — it redirects water that enters so it reaches a sump pit rather than your basement floor. The exterior face of the wall remains exposed to moisture, which means ongoing dampness can still contribute to efflorescence, musty odours, and long-term concrete deterioration. This is why the Ontario Building Code (OBC Section 9.13.2) specifies exterior dampproofing as the standard — interior systems are a practical workaround when exterior access is impossible, not an equivalent replacement.

Before committing to a fully interior approach, explore whether exterior access is truly impossible. Under Ontario property law, you may have the right to access your neighbour's property for necessary maintenance under certain circumstances, though this should be discussed cooperatively and potentially with legal guidance. Some contractors can excavate in tight spaces using mini excavators or even hand-dig, though labour costs increase significantly — expect a 30-50% premium over standard exterior waterproofing. If the fence is the only obstacle and your neighbour is agreeable, temporary fence removal may be the most cost-effective path to a proper exterior waterproofing job at $3,000-$12,000.

Whichever approach you choose, also address surface drainage. Ensure downspouts on the affected side extend at least 6 feet from the foundation, and verify that the grading slopes away from the house at a minimum of 6 inches over the first 6 feet. These low-cost measures can reduce hydrostatic pressure significantly. Toronto Parging can connect you with waterproofing professionals experienced with tight-access GTA properties — get matched for a free assessment through the Toronto Construction Network.