Exterior foundation waterproofing: membrane, drain and free-draining backfill

Foundations7-minute readPublished September 14, 2026

Written by the Construction AIM team, a general contractor holding an RBQ licence

When water comes through basement walls, sealing from the inside often treats only the symptom. Exterior foundation waterproofing goes after the source: the walls are dug out and protected with a membrane, the drainage is redone, and the excavation is backfilled with materials that let water flow down to the drain. Here is the role of each layer of the system.

Why waterproof from the outside?

The water that seeps in comes from the soil around the building. Working from the outside stops it before it reaches the wall, instead of dealing with water that is already inside. The excavation also makes it possible to examine the full height of the wall and the footing, repair cracks from the outside and replace the French drain in the same job.

The type of foundation matters:

• Poured concreteIsolated cracks can sometimes be injected from the inside; a repair done from the outside must be completed with a membrane.
• Concrete blocksWater travels through the cores of the blocks. Exterior waterproofing is often the only realistic way to prevent leaks.
• StoneIn older buildings, working drainage and foundation waterproofing are essential, with mortars compatible with the original masonry.

Signs that your foundation needs waterproofing

• Recurring leaksWater that keeps coming in at the bottom of the walls or through cracks.
• Efflorescence and stainsEfflorescence (a white deposit) or damp stains on the basement walls.
• Worn-out drainA French drain at the end of its life or clogged.
• Finishing the basementA basement finishing project: better to deal with the water before closing up the walls.

Step 1: excavation and wall preparation

The job starts with a request to Info-Excavation to locate underground utilities and, depending on the municipality, a permit. In Montreal, foundation waterproofing is among the work for which an excavation permit may be required, depending on the borough and the distance from the public domain.

The perimeter is then excavated down to the footing. The Safety Code for the construction industry requires the walls of the excavation to be shored, with specific exceptions, and materials to be kept more than 1.2 m from the edge. The walls are cleaned, debris is removed and active cracks are repaired according to their type, with mortar or by epoxy injection.

Step 2: the Delta-MS membrane

The Delta-MS membrane, made by Dörken, is a dimpled sheet of high-density polyethylene (HDPE). Its dimples create an air gap: water that reaches the membrane drains down by gravity to the drain instead of pressing against the concrete. The membrane also acts as a capillary break, which means it stops soil moisture from migrating into the wall. According to the manufacturer, it keeps working even if the foundation moves or cracks.

A few installation points, taken from the manufacturer’s guide and from our own method:

• 15 cmminimum overlap at the vertical joints of the membrane
• 150 mmof crushed stone around the drain, under the Code
• 1.2 mminimum distance between the edge of the excavation and stockpiled materials

• mechanical fastening over the full height, from the base of the wall to above finished grade;
• an overlap of at least 15 cm (6 in.) at vertical joints;
• top of the membrane fastened and sealed against the wall with a termination bar that keeps debris from blocking the air gap.

The Delta-MS Drain version adds a geotextile heat-bonded to the dimples. This filter holds back fine soil particles and keeps the drainage space clear.

The membrane does not replace a structural repair: a crack that moves or a bulging wall is first a matter for a structural engineer.

Step 3: the French drain

The membrane guides water downward; the drain still has to carry it away. The new drain is laid at the base of the wall, at footing level. It can be a perforated pipe certified to a Bureau de normalisation du Québec (BNQ) standard, or a crushed stone drain wrapped in geotextile. The Quebec Construction Code requires the sides and top of drainage pipes to be covered with at least 150 mm of crushed stone.

The membrane guides water downward; the drain still has to carry it away.

Cleanouts make it possible to inspect and clean the drain later on. The drain is connected to a sump pit or to a discharge system, depending on site conditions and regulations. In Montreal, By-law 11-010 requires the exterior drainage pipe to be connected to the sump pit with a non-perforated pipe. To learn more, read our guide on replacing a French drain in Montreal.

Step 4: free-draining backfill and grading

Clean stone goes around the drain. Above it, the excavation is backfilled with free-draining granular material, compacted in layers; excavated soil can be partly reused if it is suitable, and the quote says so. Compacting in layers limits settlement that would leave a low spot where water flows back toward the wall.

At the surface, the ground must carry water away from the building. We aim for a slope of at least 5% over the first 1.5 metres. Downspouts complete the system: in Montreal, they may not be connected to the foundation drain and must discharge onto a permeable surface at least 1.5 m from the foundation.

Limits and unforeseen conditions

Some conditions cannot be seen during the visit: contaminated, frozen or waterlogged soil, rock or shale, unstable old foundations. A high water table may call for specialized pumping, and in a semi-detached or row plex, access may be limited to some sides of the building. Any additional work is approved in writing before it is done.

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