The way to repair a leaking foundation wall depends on the type of material the foundation is built from. Masonry and concrete in most cases can be repaired quickly and easily. Stone foundations often require extensive repair or replacement. Small cracks in masonry foundations often occur due to settlement or earth movement, too fast drying of the mortar between courses of blocks when installed, and even excess groundwater can exert enough pressure on the outside to crack a wall. .

First examine the inside and outside of the foundation wall to see if there is a physical reason the wall is cracking. If the earth along the foundation has subsided due to missing downspout leaders that added tons of water against the foundation or perhaps the grade was changed without someone noticing that the rainwater is now flowing into the building.

Standing water can supersaturate soils, causing them to sag and exert thousands of pounds of pressure on adjacent objects, such as a foundation wall. Since a basement is filled in only on one side and is an outdoor space inside, this hydraulic pressure on the ground can easily crack a masonry wall and, over time, can also crack a solid concrete foundation. Your first action should be to correct this problem by regrading or other necessary work to ensure that all water escapes from the foundation. Once this is done, turn your attention to the base itself. Minor cracks occur in all walls over time and if inward bowing of the wall is not evident, it indicates that a simple shrinkage crack has occurred in most cases.

Using a small four-inch electric grinder with a masonry blade (always wear eye and ear protection, as well as a dust mask) carefully grind the masonry joints between the blocks or cut into the concrete crack perhaps one good inch or more. Do not cut more than two inches deep to avoid cutting any rebar into the concrete wall. Once you have finished grinding and the dust has settled, remove all traces of dust with your shop vac making sure the crevices are clean on the inside as well.

If you’re working on a masonry base, mix up some new mortar so it’s fairly loose and pliable. Using a brush or spray bottle, thoroughly wet the blocks with running water along the entire length of the crack. If the blocks are too dry, they will absorb all the water from the new mortar and dry it out too quickly, causing the mortar to fail.

The mortar will simply separate from the blocks, leaving a new crack. You want the new mortar to dry as slowly as possible. Concrete foundations are handled differently. There are a number of concrete crack sealants on the market today that are used to inject into a concrete crack and seal it. Most can be installed with the use of a standard caulk gun. When you inject the caulk, fill the new void completely, but try to leave it just a hair below the surface of the wall when finished. The sealer will air dry and requires no additional work.

Once all cracks have been repaired and completely dried, a good masonry sealer can be applied to the interior surface to further waterproof the foundation. Always be sure to paint the wall higher than the exterior grade elevation and paint up to and down the floor at least three inches to ensure a good seal at the floor-to-wall joint. Masonry sealers work on both masonry and concrete walls and come in various colors. The application is done with simple brushes and thick-haired rollers. No special tools are required.