Water flowing from a downspout must be directed away from the foundation, onto a surface that absorbs it or channels it toward the street—never at the base of the foundation. For a house with a footprint of 100 m² in the Greater Montreal area, nearly 98,000 liters of rainwater and snowmelt flow through your gutters and downspouts each year.
Key points
- Rivière-des-Prairies, the reference station for the Island of Montreal, receives 988.6 mm of total precipitation per year, consisting of 825.8 mm of rain and 158.9 cm of snow.
- A 100 m² roof therefore collects approximately 98,860 liters per year—nearly 100 cubic meters—which flows down a few downspouts.
- The rule of thumb: direct the water at least 1.8 m away from the foundation, on a slope with a gradient of about 2% over the first 3 meters.
- You should allow for one downspout per 10 to 12 meters of gutter, or more on a steeply sloped or large roof.
- July is the rainiest month of the year, with 94.3 mm of rainfall, and it’s also the month when short, intense thunderstorms can overwhelm an undersized downspout.
A rigid extension pipe diverts water away from the foundation of a Montreal duplex.
Where should water from a downspout drain?
The correct answer is simple: on a surface that absorbs or drains water, at a safe distance from the building. An extension to the downspout that ends 30 cm from the footing directs water exactly where it causes the most damage—into the loose backfill surrounding the foundation. This backfill is more permeable than the surrounding natural soil, so water seeps down quickly and presses against the concrete.
There are three effective solutions in the Greater Montreal area. A sloped lawn or flower bed, which absorbs runoff. A paved surface sloping toward the street, which directs water to the gutter. And a rainwater collection barrel equipped with an overflow that also directs water away from the building. The U.S. Environmental Protection Agency points out that this type of collection directly reduces the volume of water leaving your property.
Distance, slope, and annual volume to be drained for a 100-square-meter roof.
Calculating Volume: What Your Roof Actually Releases onto the Ground
Volume is calculated by multiplying the horizontal projected roof area by the annual precipitation. The Ministry of the Environment’s climate norms for the Rivière-des-Prairies station indicate 988.6 mm per year.
A 100 m² roof therefore collects 98.9 m³, or nearly 98,860 liters. A 150 m² roof collects 148,000 liters. And during a single 20-mm downpour, that same 150-square-meter roof sends 3,000 liters down its downspouts in less than an hour. This is the true order of magnitude you need to keep in mind before deciding that a single aluminum downspout is sufficient.
How Many Downspouts Does Your House Need?
Allow for one downspout for every 10 to 12 meters of continuous gutter, and one for each section of the roof that cannot drain into another. A 20-meter gutter fed by a single outlet overflows during every summer storm—not because it’s clogged, but because the flow exceeds the capacity of the single outlet.
Two other factors increase the number required: a steep roof slope, which accelerates runoff toward the downspout, and a valley side, where two slopes drain into the same point. In these cases, the solution is not a larger gutter, but additional outlets, or upgrading from a 5-inch to a 6-inch size with matching downspouts. Our teams often address these types of situations during a complete gutter installation rather than by repairing the section that’s overflowing.
Distance, Grade, and Extensions: The Numbers That Matter
Aim for a minimum of 1.8 m between the outlet and the foundation, and 3 m if the ground is level or if the lower level is finished. The ground itself should slope downward at a rate of about 2% over the first three meters, which is a 6-centimeter drop. Without this slope, an extension will simply shift the problem a few feet.
Rigid aluminum extensions hold up better than retractable models, which get crushed under the snowblower and fill with ice. If mowing or snow removal is a problem, an underground pipe sloping continuously toward a visible discharge point remains the best option, provided the outlet is never blocked by accumulated snow. The issue of freezing isn’t just theoretical: the 158.9 cm of annual snowfall explains why so many outlets get clogged in February—a topic we cover in our article on ice buildup in gutters.
What Happens When Water Stays at the Foot of the Wall
The first effect is hydraulic: the pressure of the water against the footing widens existing cracks and fills the French drain faster than it can drain. The second is structural: the freeze-thaw cycles, which are very frequent during the Montreal spring, cause saturated soil to heave and put stress on the foundation.
The third issue is related to sanitation. Chronic moisture in the basement creates conditions conducive to mold growth, and the Quebec National Institute of Public Health states that controlling water sources is the first step to take in a home. A poorly oriented downspout is one of the easiest sources to correct. The building code requirements for drainage and wastewater disposal are set forth in the Canada Codes publications, and work involving the building falls under the jurisdiction of the Régie du bâtiment du Québec.
Disconnection and Municipal Regulations
Several municipalities in Greater Montreal now prohibit connecting a rainwater downspout to a sanitary drain or to a combined sewer system. The rationale is based on stormwater management at the source: keeping rainwater on-site rather than sending 98,000 liters per household to a wastewater treatment plant.
Before modifying a connection, check the regulations of your borough or city. Disconnection is normally done by cutting the pipe above ground level, sealing the old inlet, and installing a surface outlet leading to a rain garden, a flower bed, or a rain barrel. A rain chain can replace the downspout on a small, decorative section, but it cannot handle the flow from a large roof.
Materials, Sizes, and Available Options
The most commonly used alloy in Quebec remains aluminum, which is available in white, black, brown, and about thirty other shades, because it doesn’t rust and can be bent to custom sizes on-site. PVC is less expensive but becomes brittle in our cold weather, and copper is primarily an aesthetic choice for historic buildings.
Two pipe sizes are commonly installed: the 2-by-3-inch size, which is standard for single-family homes, and the 3-by-4-inch size, which has a cross-sectional area that drains about twice as much water and is required whenever a downspout serves a large roof area. Corrugated models are more resistant to crushing than smooth pipes.
The rest of the system comes down to the details: properly angled elbows at the top and bottom of the pipe, a removable end piece for clearing debris, fasteners every 1.8 meters, and a watertight connection to the gutter trough. These parts are available in the same color as the installed gutters, which prevents a white downspout from standing out against a dark facade. Always ask what the warranty covers in terms of labor and finish, not just the material.
On soil with poor drainage, a 60-cm layer of crushed stone placed beneath the outlet breaks up the force of the stream and prevents the surface from eroding. If the outlet runs under a wooden deck, extend the pipe beyond the beams rather than letting the water soak into the joists. Square-shaped sections are easier to conceal along a column, and municipalities bordering a stream or ditch often require that the discharge flow into the waterway via the ground rather than through a pipe. During snowmelt, this surface outlet must remain clear; otherwise, the water will flow back toward the foundation.
In dense urban areas, where permeable soil is scarce, two options stand out: a cistern or barrel connected to the downspout, and a rain garden dug into a basin. Both reduce peak runoff, which, when added up from one street to the next, causes sewer backups and local flooding after a storm. Our custom gutters are formed on-site, allowing us to adjust the number and position of the outlets to accommodate these constraints.
A bed of crushed stone absorbs the water flow and protects the flower bed.
Maintenance of downspouts, twice a year
A downspout almost always gets clogged at the upper elbow, where the pipe leaves the gutter. Two annual inspections are usually sufficient: one in the spring, after the samaras and buds have fallen, and one in late fall, once the leaves have all fallen.
The most reliable test takes two minutes. Run a garden hose into the gutter and watch the outlet: a thin trickle or gurgling sound indicates a clog, while water overflowing at the seam indicates a reverse slope. A gutter guard greatly reduces the frequency of clogs without eliminating them entirely, especially under maple trees. Professional gutter cleaning includes flushing the downspouts, which manual cleaning of the gutter alone does not do.
In summary, there are three categories of options for managing stormwater runoff from a residence: surface discharge onto the property, retention in a barrel or cistern, and infiltration through a vegetated basin. The right solution depends on the available clearance and absorption capacity. Our experts first verify that the aluminum gutters and downspouts are properly sized, then recommend the appropriate maintenance option, because even a well-designed system that is never flushed will eventually overflow just like any other.
Frequently Asked Questions
How far from the house should a downspout end?
At least 1.8 m from the foundation wall, and up to 3 m on flat ground or when the basement is finished. Distance is less important than the slope of the ground: if the ground slopes back toward the house, even 3 m won’t provide any protection.
Can I direct the runoff toward the paved driveway?
Yes, if the asphalt slopes toward the street and not toward the garage. In winter, this water freezes and creates a sheet of ice, so draining onto the lawn is preferable when the terrain allows it.
How many downspouts does a bungalow need?
Most bungalows in Greater Montreal work well with two to four downspouts, spaced at the ends of each roof slope. As a general rule, there should be one downspout for every 10 to 12 meters of gutter.
Why does my gutter overflow even after I’ve cleaned it?
There are three common causes: a clogged downspout below the elbow, a single outlet serving too long a run, or a gutter with insufficient slope. If it overflows in the same spot every time it rains, the problem is likely related to capacity, not leaves.
Is one rain barrel enough to handle the runoff from a downspout?
No, a standard 200-liter barrel fills up in just a few minutes during a 20-mm downpour over a large roof area. It must always be equipped with an overflow drain directed away from the foundation.
Have your downspouts inspected
An inspection of downspouts takes very little time and can easily be combined with seasonal maintenance or a scheduled gutter repair. Our team serves Montreal, Laval, Longueuil, and the northern suburbs, and measures the actual slope of the property before recommending an extension or an additional outlet.
Sources: Ministry of the Environment, Climate Change, Wildlife, and Parks, 1981–2010 climate norms, Rivière-des-Prairies station; National Research Council Canada, Canada Codes publications; Régie du bâtiment du Québec; Quebec National Institute of Public Health, Mold in Your Home; U.S. Environmental Protection Agency, Soak Up the Rain.
