Understanding High Water Tables

Did you know that the city of Toronto sits on top of a labyrinth like maze of ancient rivers and sewer systems that have long been forgotten about?  While there are many reasons your Toronto basement might be leaking water, in some areas, rising water tables could be the problem.  While the water table below your home may be several meters below your foundation, that doesn’t mean that it won’t cause water problems in your basement.  Below explains what a water table is and how it can effect your home.

A water table – sometimes also referred to as a groundwater table – is the upper level of accumulated water that has collected on the saturated or impermeable surface below it. Groundwater tends to level out just like the water in your bath or swimming pool. That’s why it’s called a table by the experts, because the tops of tables are usually smooth too.

High water table in Toronto from underground rivers and streams

Two Types of Water Tables

Geologists speak of two kinds of water tables, namely perched ones and true ones. True tables are those prevailing in an area. They represent the average water height, plus / minus the prevailing season in terms of wet or dry. However water doesn’t always seep down consistently. Factors that affect water penetration include:

  • The degree of existing saturation
  • The permeability of the strata
  • The presence of impermeable rock.

Groundwater may perch higher at some points than others as a result of this phenomenon. Think of this as a step-change, or as if there’s a thick book laying on your desk or table.

Implications for Building Basements

Homeowners watching their new basement being excavated should not necessarily be content when the earth comes out dry. That’s because water could rise up during the rainy season if their home is in a low-lying area, and exert hydrostatic pressure on the walls resulting in leaks and cracks in your home’s foundation.

This is especially probable if their house is built on clay, or if they have to excavate through hard rock or shale to create space for the basement. That’s because excessive rain could cause water to perch around it temporarily, inevitably increasing the risk of mould-inducing dampness.

Artificial Causes

Basement contractors dig out a larger area than they require, so they can work on both sides of the walls as they damp or waterproof them. When they are finished, they backfill the spaces on the outside of the basement, usually with the same material they previously excavated.

This creates a leaky fill especially if they’re using rock or shale that water filters through more rapidly that the original underlying surface can absorb. This in turn creates a dam of wet material right around the outside basement walls. This can stand for days or even weeks in winter. Just imagine the hydrostatic pressure if you filled the space with water, and you’ll have a good idea of the consequences.

Prevention is Better Than Cure

If you are looking for a home and are concerned about water problems, or you currently have water problems in your basement, you need to consider a four-pronged strategy to knock this problem on the head. Ignore this advice at your peril. You could be inviting a high repair bill in the future.

  • Avoid building/buying your house – or add a basement – in a low-lying area that’s surrounded by high ground or where water drains to naturally.  Water will use gravity and  find the path of least resistance when searching for a low lying area, which could be that new home you’re eyeing.


  • Do install french drains / weeping tiles around the outside walls at the lowest point, to drain groundwater away before it causes damage.


  • Do not allow the contractor to backfill with loose material.  Be sure the backfill is replaced properly or you could be creating a swimming pool around your foundation. If necessary, import something more suitable.


  • Do everything you can to prevent water from finding its way from the surface to the bottom of your outside basement walls in the first place.

In these ways it is usually possible to manage groundwater effectively. Ignoring a water leaks in your basement for too long can lead to expensive repairs and health concerns from mold later on down the line if left untreated.

image courtesy of Environment Canada



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NuSite Group is a Toronto based basement waterproofing and foundation repair company that specializes in Basement Waterproofing, Basement Lowering, and Foundation Repairs. Call us today at 416-622-7000 or 905-731-1228 for a free in-home estimate and inspection.

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Had several cracks in my basement wall and they would leak whenever we had a heavy rain. It started about a year and a half ago and seems to have gotten worse over the past 6 months. I tried having my neighbor seal it, but it started leaking again within a few weeks. Called several waterproofing companies and got several quotes. Finally went with Nusite contractors for the job. They gave a reasonable quote and I liked the guy that came by for the estimate. They stayed within budget and cleaned up before they left. It's been almost 3 months now and it seems to be working great.