The right drain depends on two things: what is wrong with your current drain, and whether crews can dig around the house. According to CAA-Québec, a faulty exterior drain is normally handled by an excavation contractor, while an interior drain is considered when excavating outside is difficult or impossible. In Quebec, the term “drain français” (French drain) means the drain that runs around the footing, so this guide compares the exterior drain with its interior alternative.

The facts below come from CAA-Québec’s guide Réparer un drain résidentiel ou une fondation, the APCHQ Performance Guide (edition consulted: 2006), the City of Québec’s page on sump pump systems and the City of Montréal’s RénoPlex rate table. All are in French.

What Is the Difference Between an Exterior and an Interior French Drain?

The exterior drain sits outside the foundation, at the footing, and carries groundwater away from the wall. The interior drain sits inside the basement, under the slab edge, and sends the water it collects to a sump pit where a pump removes it.

Question	Exterior drain	Interior drain
Where is it?	Around the footing, outside the wall	Inside the basement, along the slab perimeter
How is it done?	Cleaning, partial repair or full replacement, which requires excavation	The slab is cut over about 12 in. along the perimeter, a drain and gravel are laid, the walls get a drainage membrane and the slab is poured back
When is it chosen?	When the exterior drain is faulty and the ground can be excavated	When excavating outside is difficult or impossible
Pump needed?	Depends on the house	Yes, the trench connects to an interior sump pit with a pump

When Is Exterior Drain Work the Right Route?

When the existing drain is crushed, clogged or sagging and the ground around the house can be opened. CAA-Québec advises hiring an excavation and landscaping contractor, who can diagnose the problem with an optical camera and then choose between three levels of work: hydro-jet unclogging, repair of one section, or full replacement. A full replacement means excavation, several weeks of work and, according to the guide, waiting until the following year for the final landscaping.

The APCHQ sets the standard for a new build: the French drain must meet the code in force and work so that no water gets into the basement. It adds that a camera can assess a drain’s general performance, and that some water sitting in a drain is perfectly normal and does not mean it is failing. CAA-Québec puts the normal lifespan of a drain at about 40 years.

When Does an Interior Drain Make Sense?

When excavating around the house is difficult or impossible, CAA-Québec says an interior drain can be considered. The trade-off is that it depends on a pump. The guide asks for an annual check: test the pump, remove debris and test the backwater valve. It also warns about radon and recommends an airtight lid on the sump pit, which also helps against odours and insects.

Two points to settle before choosing this route:

Where the pumped water goes. CAA-Québec asks you to check local rules on discharge. In Québec City, for example, pumped water may not go to the sewer except where impossible, and gutters must never be connected to the foundation drain.
What happens in a power outage. In Québec City, sump pumps must be fed by a generator or a backup battery. Ask what backup the system includes, wherever you live.

How Much Do These Options Cost?

Prices depend on the house, so get quotes from two or three contractors, as CAA-Québec advises. For reference, the City of Montréal’s RénoPlex rate table, used to calculate the subsidy, values excavation at 190 $/m, waterproofing at 80 $/m, a French drain at 60 $/m and a sump pump at 1 600 $. These are calculation bases, not price quotes. Our article How to finance foundation repairs in Quebec explains how the program works.

What Should You Check Before Deciding?

Find out the real state of the drain. A camera inspection tells you whether it is crushed, clogged or sagging, which decides whether cleaning is enough.
Rule out surface water first. Gutters, downspouts and grading can flood a basement with a perfectly good drain. See our articles on gutter systems and on the 4 kinds of water damage that threaten a foundation.
Understand the pressure. Read about hydrostatic pressure to see why water at the footing matters.
Check access to the outside. Ask whether crews can actually reach the outside of the wall: a shared wall or tight access can make excavation difficult.

Marco Réparation de Fondation repairs, maintains and cleans French drains, installs interior drains and applies waterproofing membranes. It visits in person for every estimate and every job site inspection, holds a valid license from the Régie du bâtiment du Québec (RBQ No. 5775-3246-01) and serves Montreal, Laval, the South Shore (including Longueuil and Brossard), the West Island and Salaberry-de-Valleyfield. See our French drain and waterproofing page.

Water in your basement and not sure which drain you need?Contact Marco Réparation de Fondation for a free inspection and an honest opinion on the cause.

Frequently Asked Questions

Does an interior drain replace an exterior drain?

It can take over the job when excavating outside is difficult or impossible, according to CAA-Québec. It works differently, since it collects water inside and needs a pump, so the choice should follow a diagnosis of why water is getting in.

Do I need a sump pump with an interior drain?

Yes. CAA-Québec describes the trench as connected to an interior sump pit fitted with a pump, which keeps the water level under the slab.

How long does a French drain last?

CAA-Québec gives about 40 years as the normal lifespan. A drain that is clogged earlier can sometimes be cleaned by hydro-jet instead of replaced, once a camera shows its condition.