Repairing Basement Leaks

Basement leaks not only make life miserable, but they can weaken the foundation of your home. Basement walls should seal out a moderate amount of water, but few will withstand a great deal of water pressure. If the wall leaks only during heavy rains, you may be able to solve the problem by adding extensions to your gutter downspouts to direct rainwater away from the basement walls. You may find that your yard slopes in a way that causes rainwater to flow toward your home. If so, some corrective landscaping may be needed to solve your problem.

Tools: Stiff brush, cold chisel, baby sledgehammer.

1. For slow seepage or damp walls, brush on interior sealer. Clean away dirt, grease, and dust from the wall. If you are using a cement-based sealing product, wet the wall thoroughly with a fine mist from a garden hose. Mix the liquid and powder components of the sealer thoroughly and apply with a stiff brush.

2. Fill any cracks. As you brush, be sure to fill in all the pores in the wall. Go over cracks several times to fill them. If a crack is too large to fill in with sealer, use hydraulic cement (see below). With some sealers, you must keep the sealant wet for several days to ensure bonding. Apply a second coat if necessary.

1. To plug leaking holes or cracks, widen and "key" the spot. Enlarge the hole or crack with a cold , chisel and hammer. Undercut it to make a “key,” so the plug won’t come loose. Make the hole at least 1/2 inch deep. Whisk out fragments of concrete.

2. Mix the cement. In a bucket, add water to the dry hydraulic cement mix until it has a puttylike consistency. Then work it by hand. For a hole, roll it into the shape of a plug. Roll a long snake shape for a crack.

3. Apply the cement. Squeeze the material into the opening. Keep pushing and pushing to make sure it fills every tiny crevice. Any water leaking through the wall at the time of the repair should stop running. Hold the cement in place for several minutes to allow the patch to set.

Solving Basement Water Problems






Damp walls, dripping pipes, rusty hardware, mildew. To identify condensation, tape a mirror in the dampest spot and wait 24 hours. If it’s foggy or beaded with water, suspect condensation.

Excess humidity in the air, usually from an internal source, such as a basement shower, washing machine, unvented dryer, or from a significant difference between the wall temperature and inside air temperature.

Install a dehumidifier, improve ventilation, and seal interior walls.


Dampness on a section of a wall or floor, most often on a wall near floor level. As with condensation, tape a mirror to the wall. If moisture condenses behind it, seepage is the culprit.

Surface water is forcing its way through pores in the foundation or an expansion joint. The source may be poor drainage or a leaky window well.

Improve exterior drainage. If problem is minor, an interior sealer may work. If not, waterproof the outside of the foundation.


Localized wetness that seems to be oozing or even trickling from a wall or floor. It usually appears during heavy rain. Test by running a hose outside near the leak. Pay particular attention to mortar joints between blocks.

Cracks that may result from normal settling or improperly poured concrete. (If you see a cracklike line running horizontally around your basement wall, it may be that the builders poured part of the wall and allowed it to harden before pouring the rest.) Faulty roof drainage or a grade that slopes toward the wall exacerbates the problem.

Improve exterior drainage. You may be able to plug several holes. For widespread leakage, waterproof the entire foundation and install drain tile.

Subterranean water

A thin, barely noticeable film of water on the basement floor is often the first sign. Test by laying down plastic sheeting for two days. Penetrating moisture will dampen the concrete underneath.

Usually a spring or a high water table forces water up from below under high pressure, turning your basement into a well. This may happen only during rainy periods.

Install a sump pump. Drainage tile around the perimeter of the foundation may help, but only if it drains to a low spot or a storm sewer.


1. To seal a wall from the outside, excavate to the trouble area. If the problem is fairly high up on the foundation wall, you may be able to do the digging yourself. Otherwise, hire an excavating contractor to backhoe a trench wide enough for you to work in. Remove the dirt close to the wall by hand. Brush the wall clean.

2. Apply sealer. You can hire contractors who specialize in this type of coating. To do it yourself, wash the wall clean, allow it to dry, and apply two coats of tarlike bituminous sealer. Or, backplaster the wall with two coats of mortar and apply the sealer.

3. Install perforated drainpipe and a polyethylene barrier. Dig a trench along the footing and install a sloping, perforated drainpipe embedded in gravel. Stick heavy-gauge polyethylene sheeting to the wall sealer. Drape it over the footing, but not the gravel drain. Overlap all seams at least 6 inches.

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