Understanding Basement Waterproofing

A waterproof basement is the foundation of every other finish you will ever install down there. A leaking foundation will ruin millwork, cup hardwood floors, and create the conditions for mold. Doing waterproofing properly once, before the finishes go in, is the cheapest version of doing it at all.

Where the water comes from

Water enters a basement in three places. Through the wall, through the joint between the floor and the wall, and through the floor itself. The causes are usually a failed or missing exterior membrane, a blocked or undersized weeping tile, a missing or failed sump pump, or hydrostatic pressure from a high water table.

Diagnosing the cause is the first job. Sometimes the leak you can see is not the leak that matters most. A careful contractor will walk the property, look at the grading and the gutters, inspect the sump, and only then propose a solution.

Exterior waterproofing

Exterior waterproofing is the gold standard. The perimeter of the basement is excavated to the footing, the wall is cleaned and any cracks are repaired, a waterproof membrane is installed, a drainage board is placed against the membrane to relieve hydrostatic pressure, new weeping tile is laid in clean stone with a filter sock, and the perimeter is backfilled properly.

It is invasive, dirty, and expensive. It is also typically permanent on a properly built foundation.

Interior waterproofing

When exterior excavation is impossible, for example because the foundation is on a zero-lot-line or because mature landscaping cannot be sacrificed, interior waterproofing is the workable answer. The perimeter slab inside the basement is removed, a drainage channel is installed at the footing-wall joint, a dimpled drainage membrane is placed up the inside of the wall, the perimeter drain is routed to a new sump basin, and the slab is re-poured.

Interior waterproofing manages water that has already entered the wall rather than stopping it before it gets in. It is the right call when exterior work is not possible. It is not a true equivalent of exterior waterproofing, and honest contractors will tell you so.

The sump and the backup

Whichever drainage approach you use, the sump is the heart of the system. The basin and the pump need to be sized to the perimeter drainage area and the expected water flow. A backup pump on a separate float, often battery powered, protects the basement during the power outages that often coincide with the storms that need the pump most.

An alarm that notifies you if the primary pump runs more than expected or the backup engages is worth installing. Pumps fail quietly until you have a flood.

Backwater valves

Many GTA municipalities have subsidies for installing a backwater valve on the sanitary line. The valve protects against sewer backup during heavy rain events that overwhelm the city sewer. It is the cheapest insurance against one of the worst kinds of basement flood. Worth installing on every basement renovation that does not already have one. Check with your municipality for current rebate availability.

Grading and gutters

The cheapest waterproofing in any project happens above grade. Downspouts extended away from the foundation, gutters that actually flow, soil graded so water moves away from the house, and window wells that drain properly. Many basement leaks are solvable here, before any concrete needs to be touched.