Why basements leak, and how to fix it

Most basement leaks start with water collecting against the foundation, usually because of grading, downspouts or drainage that has stopped working. Water then finds its way in through cracks, joints, window wells and pipe penetrations, or rises through the floor. The best fix is almost always to manage water outside first, then seal or drain what is left.

Where the water comes from

Before fixing a wet basement, figure out which kind of water you are dealing with.

Surface water

Rain and snowmelt that run toward the house instead of away from it. Signs include leaks that show up during or shortly after rain or a spring thaw, often at one wall or corner. This is the most common cause, and the cheapest to fix.

Groundwater

A high water table or saturated soil puts hydrostatic pressure on walls and floors. Water can come through the floor-wall joint or cracks in the slab, and the problem may last for days or weeks after wet weather. Weeping tile and a sump system are meant to relieve this pressure.

Plumbing and sewer backup

Leaking supply lines, failed water heaters, or sewer water backing up through floor drains during heavy storms. Sewer backup is a health concern and often needs a backwater valve, which many municipalities encourage or subsidize. Check with your city. See plumbing red flags for related issues.

Condensation

Not every damp basement is leaking. Warm humid air condenses on cool foundation walls and cold water pipes in summer. A simple test is to tape a square of plastic to the wall for a couple of days: moisture on the room side points to condensation, moisture behind it points to water coming through the wall.

Common entry points

• Cracks in foundation walls or the floor slab.
• The joint between the floor slab and the wall.
• Form tie holes in poured concrete walls.
• Mortar joints in block foundations.
• Window wells without drains, or with drains that are clogged.
• Pipe and cable penetrations through the wall.
• Where a garage, porch or addition meets the house.

Fixes, in the order to try them

1. Fix grading. Soil should slope away from the foundation. A common guideline is a noticeable fall over the first couple of metres. Check local requirements and keep soil and landscaping below siding.
2. Extend downspouts. Discharge them well away from the foundation, onto ground that slopes away, and keep eavestroughs clean. Check your municipality's rules on where downspouts may discharge.
3. Clear window wells and fit covers if they fill with water or snow.
4. Check the sump pump if you have one. Test it by pouring water into the pit, make sure the discharge line is clear and not frozen, and consider a battery backup.
5. Seal cracks. Non-structural cracks in poured walls can often be sealed by injection from inside. Cracks that are wide, growing or horizontal need assessment first. See foundation warning signs.
6. Exterior waterproofing and drainage. When outside fixes are not enough, excavating to apply a membrane and replace or repair weeping tile addresses the problem at its source. It is disruptive and costly, but often the most durable option.
7. Interior drainage. An interior drain channel along the footing leading to a sump can manage water that gets past the wall. It controls water rather than stopping it.

After a leak

Water damage needs prompt drying. Remove wet carpet, underpad, cardboard and stored items. Wet drywall and insulation that cannot be dried quickly often need to be cut out. Use fans and dehumidifiers, and keep an eye out for musty smells or visible growth in the following weeks. Our guide to moisture, mould and indoor air covers what to watch for. If the water came from a sewer backup, treat everything it touched as contaminated and use professional cleanup.

How ehZee can help

An ehZee home inspection checks grading, drainage, the foundation and the basement for signs of past and present water. You can walk through with the inspector, hear the findings on the day, and receive a written report within two days, so you know what to fix first.

Common questions