change text size:

Poor drainage causes most basement leaks, check these points before doing expensive work on foundation walls.

Many leaks into a basement come over the foundation wall or through cracks in the wall. Leaks over the top of the foundation are nearly always caused by the grade of either soil or walkways next to the wall being higher than the top of the wall, and possibly even sloping to the wall instead of away from it. Leakage from cracks is made worse by these factors.

Poor footing drainage will at times sit your basement in a bathtub of water, worsening the leakage potential. Make sure your roof downspouts don't discharge to the footing drain. If you live in an area where there is a high ground water table, you might need to replace your footing drain, and possibly even install a sump pump.

Improving poor drainage

A very large amount of water flows off the roof in a heavy rainstorm, which, if the downspouts are connected to the footing drains, can saturate the soil around the outside of the basement walls. This is like sitting your basement in a tub of water, making it likely that water will find a way in through any openings, cracks or weak spots.

If the downspouts are connected to the footing drains we recommend that they should be disconnected; put a cap on the drain pipe going into the ground, and extend the downspout about 3 ft (1 m) away from the house wall or provide a splash pad, to get the water away from house. This will ensure that the roof water is discharged a sufficient distance from the house walls.

Check that your window wells are adequately drained, and that the soil in them is 4 to 6 inches (about 100 to 150 cm) below the top of the foundation wall at the bottom of the window. If the soil is higher than the top of the foundation wall, you run a serious risk of water getting over the top of the foundation and into your basement.

Check the grade of the soil and walkways or patios around your foundation walls. It's important that the soil not be closer to the top of the foundation wall than about 4 to 6 inches (about 100 to 150 cm). If the soil is higher than the top of the foundation wall, you again run a serious risk of water getting over the top of the foundation and into your basement.

The grade of the property should be sloping away from the house so that the rainwater can run to the edges of the lot, and then to its outlet points, usually to the roadway, and to a rear lot drainage swale.

Improving basement water resistance

Basements usually leak from below the outside soil grade, because water under pressure (about 3 psi at the bottom of say an 8 ft or 2.5 m high basement) forces ground water through cracks in poured concrete or masonry blocks, or though joints or wall penetrations. Concrete is not waterproof of itself, it will allow the passage of moisture and water vapour. This why cardboard boxes stored directly on basement concrete floors always smell after a while, and make the contents smell too.

The first essential step is to answer "When, and from where, does the basement leak?". Consider the basement construction in your answer. A poured concrete, block on poured concrete, full block, brick, and stone or rubble "concrete" all need to be described in relation to the leak. E.g. "The basement leaks through the cracks in the joints of the block section on top of the poured concrete wall, which doesn't leak. It leaks during long rainfall, or soon after a heavy rain shower." Once you know when and where the leaks occur, you're ready for the next step.

Ensure the outside drainage is correct

Check where the roof drainage discharges and the grading around the house to reduce the water pressure trying to get into the basement, and correcting any grading and drainage issues is where you start. Discharge the roof rain water as far from the house wall as is reasonably possible; 3 ft (1 m) is a minimum. Roll-out flexible plastic drains work, as do permanently connected plastic diverters or extenders. Divert the rainwater to a rain barrel connected to a soaker hose to water specific areas of your garden, but be prepared to handle the overflow as the barrel will likely fill up quickly from a heavy rain.

Ensure that the soil grade is well below the top of the foundation wall all around the house, paying particular attention to the area of any leaks. Make sure that the soil slopes away from the house for as far as possible, about 10 ft (3 m) is good, more is better. This step alone is often enough to correct many leakage problems. If it does not eliminate the leaks, the choices become those of working on the outside and/or inside of the basement wall, below the outside grade level.

Outside the basement wall

Working on the outside of the basement wall involves excavating down to the footing and footing drain. This is done most easily using a little backhoe, operated by a contractor. It can be done by hand, but it’s a lot of work! However this method is the most certain of success.

When the foundation wall is exposed it can be coated with a waterproofing compound of some kind ranging from parging to a rubber membrane (see http://www.advancedcoatings.on.ca/rub-r-wall-airtight.html). For a belt-and-braces approach, apply a protective drainage layer such as a dimpled plastic drainage sheeting on top of the membrane (see http://www.deltams.ca/pdf/DELTA-MS.pdf). Either can be used alone; used together, with a good footing drain, they effectively ensure a dry basement.

While the footing is exposed, check the footing drain. If it is clay pipe (common on older homes) it has likely cracked and collapsed; it will be ineffective. Now is the time to replace it with a perforated plastic drainage pipe (see http://www.bigodrain.ca/index.php), proper gravel cover, and both a “sock” to prevent the gravel getting into the pipe and filter cloths to prevent soil particles from filling up the spaces in the gravel and sealing it to water flow. Ensure that the pipe has good slope (1 in 12) from the highest point of the footing to the outlet of the drain.

If you can’t run the drain to a surface discharge at least 10 ft (3 m) from the house walls, you’ll need a sump pump, usually on the inside of the basement. The footing drain is connected to the sump at the low point, under the footing. Take care when this is done to avoid cracking the footing. Choose a good quality, submersible sump pump, brace the discharge piping securely, and use a GFI receptacle to protect yourself in case of a short circuit to the sump. Discharge the pump outlet outside to the surface as far away from the house walls as possible, at least 10 ft (3 m) from the house walls and onto ground that is not sloping back to the house.

Inside the basement

Any excavation to be done inside the basement is far less than working outside the basement walls; for many this is the preferred approach after ensuring that the soil grading outside is correct and roof runoff properly discharged.

The first step is to install a perforated drainage pipe around the base of the walls, below the level of the floor slab, to drain the ground water outside the walls. The water will flow though the stone sub-grade below the footing, unless the footing was poured directly on clay. Then you have no option but to improve the outside footing drain.

The drain is installed with 1 in 12 slope, in a "sock", with stone bedding and surround, and the concrete floor slab is re-poured on top of it. Outlet the drain to a sump inside the basement.

If you have a block wall, in part or completely, you'll need to relieve the pressure of the water that percolates into the block. The cavities in the block fill with water over time because there is no outlet other than by leakage. If you provide a controlled outlet, you eliminate the pressure and greatly reduce or even eliminate any leakage through the block.

To relieve the pressure, you need to locate the cavities in the block; usually there are two. You'll drill a hole into the third points of each block, to get one hole in each cavity, as close to the level of the floor slab as you can. Be prepared for water to spurt out when you penetrate the cavity! The wall thickness of the block is only an inch or so (25 mm) and you will need a short piece of copper pipe for each hole to drive into the block, just far enough to get into the cavity.

There are three alternatives to discharge the water. The first is to extend the copper pipe through the floor slab (before it is poured) to the stone surrounding the footing drain you've installed. The many copper pipes will make pouring and finishing the floor slab edge "fiddly".

The second method is to run a header pipe around the walls on the floor slab, outletting to the sump. To the end of the copper pipe attach plastic piping, and connect to a tee in the header pipe. Make this header pipe a good size, about 1 inch (25 mm) diameter.

The third method is to parge the inside face of the basement walls and then cover with the dimpled plastic sheeting. Water from behind the sheeting needs a drainage outlet. A simple outlet could be open pipes through the floor slab at the base of the wall, inside the edge of the sheeting, at about every 2 ft (or 500 mm). Then the sheeting could be sealed to the floor along its entire bottom edge.

Working inside the basement is easier, and should result in a dry basement, but it is not as certain as working outside the basement.

Whichever method you choose, the inside of the basement wall can afterwards be patched and parged after it has had time to dry out, as it has likely deteriorated from the long term effect of leakage.