Interior vs exterior French drain in Montreal: how to choose

Comparison by Pietro Di Benedetto, President of Sous-Sols Experts Inc. RBQ 5668-8765-01, SOPREMA-certified applicator.

Sous-Sols Experts’ recommendation. For most Greater Montreal residential homes, an exterior French drain is the permanent solution when access allows. Interior is the right choice when exterior is impossible (party wall, attached garage, unrecoverable landscaping, municipal restriction). The exterior’s higher cost ($8,000 to $30,000 vs $8,000 to $15,000) is offset by 30 to 50 years of service and addressing the cause rather than the symptom.

What exactly is a French drain?

A French drain is a perforated pipe installed at the foot of a foundation to collect groundwater and channel it away from the wall, either to a municipal collector or to a sump pit with discharge pump. The name comes from French drainage practices imported in the 19th century; today in Quebec, it has been mandatory around any new foundation since the 1950s under the National Building Code. The drain can be installed outside the foundation (classic configuration, placed during construction) or inside (interior perimeter floor drain, retrofit when exterior is impossible).

Exterior French drain: how it works

The exterior French drain is installed in a trench dug along the foundation wall, at footing level (typically 7 to 9 feet below grade). The perforated pipe is laid on a bed of clean crushed stone, covered with geotextile to prevent fines migration, and the trench is backfilled with more crushed stone for 2 to 3 feet up, then topsoil. During excavation, we apply a new exterior waterproofing membrane to the foundation wall (typically SOPREMA modified bitumen membrane) and repair any visible cracks. The result: groundwater is intercepted BEFORE it reaches the wall, and the wall itself is protected by a membrane under 25-year material warranty with a certified applicator.

Interior French drain: how it works

The interior French drain (or interior perimeter floor drain) is installed inside the basement, at the joint between the concrete floor and the foundation wall. We cut a 12 to 18-inch-wide trench in the concrete floor around the basement perimeter, install a perforated pipe on crushed stone, connect the pipe to a sump pit with discharge pump, and re-pour the slab over top. A drainage membrane (DELTA-MS or equivalent) is often applied to the interior face of the foundation wall to channel water passing through the wall to the floor drain. The result: water entering through the wall or wall-floor joint is collected and discharged rather than accumulating in the basement.

Side-by-side comparison

Direct comparison across seven key dimensions:

• Typical cost: Exterior $8,000 to $30,000 (including membrane) | Interior $8,000 to $15,000.
• Service life: Exterior 30 to 50 years (SOPREMA membrane warranted 25 years) | Interior 25 to 40 years (drain), pump replaced every 10-15 years.
• Action on cause: Exterior prevents water from reaching the wall (treats source) | Interior manages water that’s already entering (treats symptom).
• Landscaping impact: Exterior excavation and landscape restoration | Interior partial concrete floor demolition, sometimes basement finish to redo.
• Work season: Exterior April to November (frozen ground impossible to excavate in winter) | Interior year-round.
• Access required: Exterior requires space around the home (excavator entry, backfill zone) | Interior requires basement perimeter cleared of contents.
• Best fit when… Exterior: detached home, accessible lot, replaceable landscaping, full budget; Interior: party wall, attached garage, valuable landscaping, finished basement to protect, reduced budget.

When exterior is clearly the better option

Exterior is the right solution in five typical Greater Montreal situations:

1. Detached home with open lot. Access allows clean excavation and quick backfill. This is the standard case on the North Shore, South Shore, and West Island suburbs.
2. Complete rehabilitation over a long horizon. If you keep the home 10 years or more, the amortized cost of exterior over 30-50 years is below interior over 25-40 years.
3. Finished basement with quality build-out. Demolishing a finished floor and redoing 12 feet of drywall and baseboards costs more than redoing the lawn.
4. Multiple cracks distributed across the wall. Exterior treats all cracks and the membrane in one pass; interior leaves the cracks in place, water continues entering the wall.
5. End-of-life original waterproofing membrane. On a pre-1980 Greater Montreal home that often has only basic asphalt coating, exterior is the chance to install a real SOPREMA membrane under 25-year warranty.

When interior is the right choice

Interior is the right solution when exterior is impossible or disproportionate:

1. Party wall with neighbour. Postwar 1960s-1970s row houses in several Montreal neighbourhoods (Rosemont, Villeray, Saint-Léonard) have shared walls where exterior excavation is not possible.
2. Attached garage. When one side of the foundation is under an attached garage, you cannot excavate from outside without demolishing the garage.
3. Unrecoverable landscaping. Mature hedges, established gardens, paver terraces on concrete slab, inground pools close to the wall: restoration cost exceeds the exterior-over-interior savings.
4. Municipal or condo restriction. Certain municipal bylaws or condo agreements prohibit excavation beyond a certain distance from the neighbour.
5. Short-horizon budget. If you’re selling the home within 5 years, interior solves the buyer-disclosure problem at lower cost.

Myths to dispel

Myth 1: “Interior doesn’t stop the water, it hides it.” False. A properly installed perimeter floor drain discharges water as efficiently as an exterior drain. The difference isn’t discharge efficiency, it’s the point of interception (before the wall vs after the wall).

Myth 2: “Exterior lasts forever.” False. SOPREMA membrane is warranted 25 years; perforated PVC drains last 30 to 50 years depending on soil. No solution is eternal.

Myth 3: “Interior causes mould in the wall.” False when done properly. A DELTA-MS drainage membrane installed correctly with a watertight joint between membrane and floor drain channels water to the drain without stagnating in the wall. The wall stays damp (normal), but water doesn’t accumulate in the finish.

Myth 4: “All companies prefer exterior because it’s more expensive.” Mixed. Some companies push exterior by default because it’s more profitable; others push interior because they aren’t equipped to excavate. The right contractor recommends the solution matching your situation, not their equipment.

How Sous-Sols Experts decides

Our decision process at inspection: (1) diagnose the water entry point: visible crack, wall-floor joint, or floor seepage. (2) Evaluate exterior access: measurements, landscaping, party wall, depth. (3) Evaluate condition of existing exterior membrane via probing (coring if needed). (4) Evaluate interior basement: existing finish, space for interior trench, ceiling height. (5) Discuss with the homeowner the budget constraints, ownership horizon (5 years vs 20 years), and sensitivity to construction work. The recommendation we deliver in writing identifies a preferred option and the alternative with the cost and rationale for each. The choice belongs to the homeowner, not to us.

Free inspection

If you’re hesitating between interior and exterior, the on-site visit and diagnosis are free. We typically arrive within 48 hours in the Greater Montreal area and deliver a written quote within 24 hours with both options priced. Call Pietro at (514) 979-5425 or use the contact form. For the technical details of each method, see our exterior French drain and interior foundation drain pages.