MIKE HOLMES: How to keep your basement dry

Keeping water out of basement starts on the ground

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The simplest thing you can do to keep water out of your basement is to control it at the surface. Surface water, like rain or melting snow, or water from irrigation, is pretty easy to control, compared to water below grade. There’s not much you can do about underground streams, or the water table in your area and the resulting hydrostatic pressure on your foundation. But there’s a lot you can do on the surface.

Most of the problems homeowners have with wet basements can be prevented if people correct the grade around their home. Make sure that surface water doesn’t become groundwater and add to the pressure that’s already down there.

The first step is to make sure that surface water drains away from your house. Ideally, you want the soil level right next to your foundation to slope away. Since water flows downhill, surface water, like rain and snowmelt, will flow away from your basement walls. If the soil around your home has either a flat or a negative grade, surface water will naturally flow towards your home or pool around the foundation. This will lead to trouble.

I’m going to hope your home is built right, and that the top of the foundation is at least six inches above the soil. That’s minimum. But the higher the better. Also, the soil level needs to fall away — on all sides — in a slope away from your house. Bad grading can result in water pooling next to the foundation, which can lead to wet basements.

You might live in an area where the general drainage comes towards your house — maybe you live at the bottom of a hill, below other houses. In this case, the grade needs to be altered to direct surface water away. Usually a swale is used to intercept and collect the water that’s running downhill towards your house and take it around and release it lower down the slope. (If that sounds like your situation, you will definitely need to have a professional assess your lot’s drainage.)

When a house is built, there’s always an excavation for the basement and foundation. That is eventually backfilled, of course, but the soil right around the basement’s exterior walls is never as firmly compacted as undisturbed soil. It’s more porous and will always contain more air — which will allow water from rain or melted snow or irrigation from plantings — to collect and seep down to your foundation level.

That’s one reason I don’t like plants around a house’s foundation. It’s not because I don’t like plants, but the irrigation into disturbed soil brings and retains too much moisture near basement walls.

When that backfill soil does eventually settle, after many years, there’s a depression right around your home that will fill up like a bathtub with surface water. And most homeowners by then will have landscaped the area and won’t even notice that the soil several feet out from their house is lower than further out into the yard.

If you don’t re-grade that soil, the water will continue to pool and gather in the lower spots and eventually make its way down to your footings.

An easy fix is to make sure the downspout from your eavestroughs comes down, and expels water as far away from house as possible — at least two metres (six or eight feet). If you don’t make sure the downspouts expel water far enough away from your foundation — beyond the area of backfilled, uncompacted soil — you are pouring water down to your footings.

In older houses, the downspouts were often connected to the main stack and the sanitary line. Now they are tied into a storm line in the street. In older homes it’s a good idea to disconnect the downspouts and have them empty above the surface.

After a number of years, it’s likely the weeping tile has shifted or broken or is full of tree roots, and every time rain comes down those downspouts it has nowhere to go but up against your foundation, putting lateral hydrostatic pressure against those walls, which could one day crack the foundation. Or it could cause a sewage backup into your home, which you definitely do not want.

You may find you need to direct water away beyond the end of the downspout, or from a low-lying area next to your house. The best fix is to put in a French drain, which is basically a shallow trench filled with gravel that will carry excess groundwater away from your house.

A French drain relies on gravity and on the grade to divert surface water. It needs to be as deep as the lowest point of the pooling water at its start, and it slopes down to where you need it to empty. Simple. Line the trench with landscape fabric, fill it with gravel and put more fabric on the top. Then you can sod over it to camouflage it as part of your lawn. You can also use a length of plastic weeping tile with a sock buried in the gravel if you like.

Before you dig a French drain, make sure you aren’t letting the water out onto a neighbour’s property. And make sure your downspouts don’t empty on or over the property line. Ideally they’ll empty onto your lot, or into a drainage swale between two adjoining lots.

It’s not supposed to happen, but over time homeowners do landscaping projects and renovations, and little by little they can significantly alter natural surface drainage patterns until they create big problems for people next door. Be aware of how your plans might affect surface water drainage.

For more information on home renovations go to makeitright.ca

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