Causes of Basement Flooding
Flooding of basements can occur any time. It can happen to anyone who has a basement, even if never
flooded before. While most often flooding occurs during big rains or rapid snowmelts in the spring,
it can occur even during dry weather.
There are a number of reasons why basements flood. Flooding can occur by seepage or flow through
the walls or foundation floor, from surface water sources, or by a sanitary or storm sewer backup.
Basements are inherently prone to flooding. They are, by definition, the lowest level of a building,
typically built partly or entirely below ground level.
Groundwater is water that is naturally located below the ground’s surface. The groundwater level
can be, at times, above the level of the basement floor. In some locations, groundwater can be above
the level of the floor at all times.
Sewers are also located in the ground. This includes all varieties – storm, sanitary, and combined.
While in most cases, sewers are below the level of the basement, the water level in the sewers can
be, at times, above the level of the basement floor.
Gravity does its best to move water from high to low. If either the groundwater level or sewer
level around your home is above the basement floor, gravity will try to move that water into your
basement. A crack in the foundation floor, for example, provides gravity with a perfect path for
water to be pushed into the basement. Sanitary sewers always have a path to the home, by design,
and it is called the sanitary sewer lateral. While under normal conditions, the lateral allows
water to flow from your home to the sewer, there is the potential for water to move from the sewer
toward your home.
The circumstances described are shown in Figures 2 and 3, which are discussed in more detail below.
In order to understand why a basement might flood, it is important to show the more common pathways,
intentional or not, that permit water to flow into or around your basement. Figure 1 indicates a
typical home in the City of Kingston, and how it is serviced during normal conditions. Not all
homes in Kingston have a storm sewer lateral (as shown in the figure), but most just have a single
sanitary sewer lateral.
Figure 1 highlights common problems (in red) that all might contribute to a higher risk of flooding.
Most are on private property, or are things that are within a home-owner’s ability to change -
whether by regular maintenance or by specific projects to eliminate or reduce potential flooding
sources. Some of these problems include:
Flooding During Dry Weather
Most flood events do happen during wet weather, but it is quite possible for a flood to occur during
dry weather too. Three of the most common reasons are as follows:
A blocked or failed sanitary lateral. The sanitary sewer lateral, just like the shingles on your
roof, or your paved driveway, is a feature that will degrade over time. As a lateral degrades,
several things can happen. For example, tree roots might penetrate and the lateral might collapse
because of gradual deterioration. These scenarios can block the lateral, resulting in a sewage
In this case, it will be your own home’s domestic wastewater that floods your basement. The only
way for the wastewater to drain becomes the lowest fixture in the home – usually the floor drain or
a basement level shower stall, sink or toilet. Your lateral, just like your roof, your driveway or
windows, needs maintenance, and ultimately needs to be replaced or rehabilitated. Consider having
your lateral inspected every 5 to 10 years depending on the age and condition you find it in. Call
either Utilities Kingston or a qualified and licensed plumber for this service.
Another reason for blockage of a sanitary sewer is simply due to what is being flushed down the
Our Toilets and Drain page has a list of things not to put down your drain, or flush
down your toilet.
Toilets are for human waste and toilet paper, and that is pretty much it!
Foundation drainage failure. Subdivisions are sometimes constructed in lower-lying areas
that are generally wetter than others. In such cases, the foundation drainage system, whether by
gravity or by pump, must work continuously to keep the ground water level around the foundation
lower than the basement floor.
Just as with sewer laterals, gravity foundation systems, often called weeping tiles, may
degrade over time or get plugged by fine sediments. As a result, the ground around the foundation
will cease to drain itself by gravity.
In other cases, sump holes in the foundation are constructed to accommodate a sump pump.
These devices pump out the water around the foundation and either discharge it to the lawn, storm
sewer, or illegally to the sanitary sewer. Discharging a sump pump to the sanitary sewer is
illegal, by way of City By-law No. 2008-192.
It is possible for these pumps to fail, or simply be unable
to keep up with the incoming water, or get plugged.
This flood type will be discussed further in the wet weather section.
Water supply-line break or hot-water tank failure. Sometimes, a flood is due to a break in
the home’s internal water supply plumbing or failure of the hot-water tank. This can result from
aging plumbing or equipment, a puncture of a pipe during construction, or freezing-induced splitting
of a pipe.
Flooding During Wet Weather
Flooding during wet weather is far more common that flooding during dry-weather. Rain, ground-thaw
and snowmelt put a heavy load on drainage systems, including the storm and sanitary sewers found
underground. With the additional water on the surface and underground, there are a number of
reasons why a basement might flood.
Surface inflow, or overland flooding. During periods of heavy rain or rapid snowmelt, surface
water may pool around the house, or accumulate in hard surface depressions such as driveways or
roads adjacent to a home. During extreme weather events, this water can flow into the home. Close
proximity to a natural stream or road-side ditch can also present a risk.
grading on the property will reduce the risk of surface water getting into your home. This is
illustrated in Figure 2.
Foundation drainage failure. Homes usually have some form of a drainage system built around
them. This safeguard promotes the movement of water away from the basement and blocks the entry of
water into the building.
For the purposes of our discussion, the waterproofing aspects of a
foundation are considered part of the foundation drainage system. Within this category, there are
three main causes for basement flooding, all generally a result of excessive groundwater around the
foundation, as illustrated in Figure 2:
- Seepage. If the water table rises, water can enter the basement via cracks, holes
and other unintended flow paths. This is generally considered to be part of the aging process of
the home and the materials used to build it.
Regardless of the condition of the drainage
materials and pipe work around the foundation, if water can enter the foundation floor or walls via
cracks and holes or other defects, it likely will do so during heavy rains, ground-thaw or snow-melt
periods, when there’s lots of water in the ground.
Settlement of the lot grading around the
building and downspouts discharging run-off water too close to the home can increase the quantity of
water around the foundation and increase the risk of water entering via cracks.
- Sump pump failure. If your basement is equipped with a sump and sump pump(s), it can
mean that the foundation drainage system of your home requires some assistance to keep up with the
groundwater around it, or it simply cannot drain adequately by gravity to the surrounding ground or
storm sewer. New homes are required to have sump pumps in these situations.
Sump pumps, when
working properly and adequately maintained, can safely pump excess water above the foundation and
away from it. Ideally, this water should be routed to the lawn or storm sewer. If the pumps cannot
keep up, or fails to operate (perhaps due to a power failure, or malfunction), the groundwater level
around the foundation can rise to the point that it flows up and out of the sump onto the basement
- Weeping tile failure. Over time, the foundation drainage system can deteriorate.
As a result, the weeping tile system can fail. This may be, due to a partially- or fully-collapsed
pipe, or due to sediments plugging the pipes. If the weeping tile fails, the drainage of water
around the foundation is either impeded or blocked altogether.
As a result, the groundwater
level around the foundation gets too high and it may spill into the basement via the sump, if one
exists, or via leaks in the foundation. In situations where there are leaks in the sewer lateral or
plumbing beneath the foundation, groundwater can inundate the sanitary lateral and restrict the flow
of sanitary wastewater. This could result in both groundwater and/or wastewater entering the
basement by way of the floor drain or lowest sanitary fixture.
Flooding due to failure of foundation drainage system (Source: Handbook for Reducing Basement Flooding, ICLR, 2009)
- Sewer backup. Most homes in the City of Kingston only have one connection to the
sewer system, and that is the sanitary sewer lateral. However, newer homes built in the last 20
years, as well as some other older neighbourhoods, have storm sewer laterals as well, for the
purpose of foundation and downspout drainage (See Figure 1.)
These laterals form either one or two intentional direct connections to the municipal
storm and/or sanitary collection systems. The municipal storm and sanitary systems operate well and
are maintained through a variety of municipal maintenance programs.
However, when a blockage occurs, or when the systems are overloaded during heavy rains, a
sewage backup can occur into a building, as per Figure 3. Here are a few reasons why:
- Sewers are full.
When the sewers are full, this is called a “surcharged condition”. It means the
pipe system is full and the water level in the manholes may rise well above the top of the pipe. If
the sewage level in the system exceeds that of your basement, flows can be blocked, or worse, sewage
can flow towards your home (see Figure 3). When this occurs, the wastewater may enter your basement
by way of the lowest fixture, which is usually a floor drain, shower drain, sink, washbasin or
The underlying cause of this is excess water in the sewer system, which
ultimately overloads the sewer with more water than it was designed for. Excess water generally
comes from leaks in sewer-mains and sewer laterals, inflow from surface features as well as
illegally-connected, private-side sources including foundation drains, sump pumps and downspouts.
The technical term for this is “extraneous flow” or “inflow and infiltration”.
For more detail, visit our Extraneous Flows page.
Flooding caused by sewer backup (Source: Handbook for Reducing Basement Flooding, ICLR, 2009)
- Flow Restriction.
Any situation that puts additional flow into the sewer
lateral may make the problems of a partially- or fully-blocked sewer lateral worse. If flow is
restricted, there is more chance a sewer backup can happen.
In summary, there are several different circumstances that can result in basement flooding.
If you have experienced floods and wish to learn more about why it happens and how to reduce the
chance of it happening again, consider reviewing the following videos,
produced by the ICLR:
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