As a waterproofing contractor here in Toronto, I’m often asked what is the most common cause of a leaking basement. But first, I want you to conduct a small experiment for me.
Take a used tin can (baked beans, peas, whatever) and remove the bottom end. Then, place the open-ended can in a cooking pot that’s a little taller than it. Hold it down as you slowly fill the pot with water to just below the top end of the can. While you watch:
- Water seeps into the can past the bottom end
- The rate increases as the outside water-level rises
- Finally, the water levels are the same on the inside and the outside
Two things are relevant here. First, there was no seal between the bottom of the tin can and the cooking pot. Secondly, the water found a way inside the can because of what’s called hydrostatic pressure.
A basement is comprised of two separate structures which are by definition underground. These are:
- A concrete floor set in an excavated space (the “cooking pot”)
- A rectangle of four walls standing on it (the “tin can”)
If you were to fill the excavated space with water, this would place hydrostatic pressure on the structure. In turn, this would seek relief by entering the basement through the concrete floor and walls. These may be well waterproofed; however, as the structure settles, the joint between them weakens and gives way to water pressure.
And that, my friend, is the most common cause of leaking basements. In fact, it’s almost built into the basic design. Just like your camping tent in a storm, the leak is usually at the seam.
How to Avoid the Problem
Do you remember how the flow of water into the tin can increase as the water level rose? The same is equally true of a leaky basement. The correct solution is installing an outside drain at floor level. This leads the underground water away as it arrives, so it never builds up enough pressure to become a problem.
In the basement water-proofing industry we call this a weeping tile or french drain system. They’re simple to install during initial construction, but a messy job to repair and install after construction. Getting a certified and insured waterproofing contractor is important if you think you may need this type of work.
In my decades in the basement repair business, I’ve come across countless botched repair jobs involving water-proofing interior basement walls. While waterproofing your foundation can be effective if done properly, if there is an outside water source placing constant pressure on your foundation, it probably won’t last. The hydrostatic pressure invariably dislodges the material, especially since the area was inevitably damp when the seal was applied.
The only solution that’s effective is an interior water-catchment system that feeds a sump pump to an outside drainage point. It’s expensive though. However, it’s also better than abandoning a working basement, which could otherwise add good value to your home.
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