Most new basements today feature poured concrete walls that form a solid barrier against ground moisture. Some older homes, however, rest upon stone basement foundations that may shift and separate as a result of repeated soil expansion and contraction. Typical damage to stone walls includes uneven stones, cracked and crumbling mortar and subsequent leaking. In addition to surface repairs, you must make provisions to direct water away from the stone foundation.
Stone foundations do not rest upon concrete footings, making the stones more likely to shift at their weakest points, which are the mortared joints. In addition, when builders were constructing these homes, they did not install perimeter drain tile, which carries water away from the bottom exterior of the basement. When it rains, the wet soil surrounding the basement swells, putting pressure on the stone foundation. When the soil dries, it contracts, often leaving gaps between the soil and the stone foundation, which leads to more problems because, the next time it rains, water has a direct path into the gap and then into any cracks between the basement stones.
Interior Stone Repairs
Repointing is the standard method for repairing interior stone basement walls. The process involves using a chisel to scrape away the old crumbling mortar and then installing new mortar in the cracks. While repointing offers an immediate fix, if you do not address the cause of the cracking, the stones will shift again and new cracks will appear.
Exterior Foundation and Grade
The visible exterior of the stone basement will also benefit from repointing and a coat of waterproofing sealant, but additional steps are necessary to prevent further shifting and damage to the stones. Grade the yard away from the basement by adding soil to the yard next to the basement or by digging a slope away from the basement. The idea is to slope the yard down and away from the stones to reroute the flow of ground water.
Extensive Stone Damage
In some cases, the basement stones may shift precariously, no longer supporting the weight of the home safely. If this occurs, a foundation contractor can excavate around the old basement and reset the stones. At the same time, he can install drain tile to prevent further damage from water, which is an expensive repair and usually a last resort.
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