How to Stop a Leak in a Concrete & Stone Basement


Leaks in a concrete and stone basement can be some of the most tedious and difficult to repair because the concrete, acting as the mortar for the stone, often has rotted in areas you cannot access or see. Discovering the true nature of the leak is half the problem; trying to patch it without rebuilding the whole wall is another. However, there is a very old-fashioned and fun way to patch this kind of leak. It takes some practice and time, but the process is enjoyable.

Things You'll Need

  • Chisel
  • Hammer
  • Shop-Vac
  • Hydraulic cement
  • Bucket
  • Water
  • Gloves
  • Trowel
  • Wire brush
  • Muriatic acid (if needed)
  • Waterproof sealant
  • Brush
  • Determine the nature of the leak. If the leak is actually moisture leaching through the concrete and stone, the waterproofing on the exterior of the foundation has failed. This type of leak is more of a general dampness over the entire surface of the wall. If the leak originates at a specific point (usually forming a localized pool), the concrete has been eaten away.

  • Dry the wall as best you can. If the nature of your leak is a waterproofing failure, skip to the last step. For leaks from the concrete rotting, go on to the next step.

  • Using a hammer and chisel, begin to remove the concrete as best you can from between the stones in the area of the leak. As you do this, stop occasionally and push the chisel straight into the concrete and see if you can wiggle it around. If you reach a point where the concrete is solid and cannot remove anymore or move the chisel around, stop and go to step 7; your leak is coming from a fine wall crack and you are ready to repair it. If you eventually find a pocket behind the stone where the concrete will seem to have "disappeared," this is the major source of your leak; moisture gathers and pools in this cavity until it overflows through the cracked concrete in the wall. Open up access to this cavity as much as you can, but you needn't remove any stone.

  • Use a Shop-Vac to clean out the area you have exposed.

  • In a bucket, mix a small amount of hydraulic cement (a specialized cement designed to be waterproof and can even be used underwater) and water according to the instructions on the package. Put on a pair of gloves -- rubber-palmed and snug-fitting ones will work the best for the next step.

  • Scoop up a small amount of hydraulic cement from the bucket and shape it between your hands into a small ball, about the size of a golf ball. Step back from the wall, and throw it as hard as you can toward the opening you have created that leads to the internal cavity. This is an old-school technique called "throwing" or "pitching" and was used not only to repair inaccessible cavities in stone walls but in foundries to suffocate fires in furnaces before they exploded (using firing clay). The speed at which you throw is essential because it will allow the balls of hydraulic cement to bond together and slowly fill the cavity. Don't worry about being neat, just have fun with it and work fast, you don't want to let any of the hydraulic cement begin to set up while you are still trying to bond-fill the cavity. You'll clean up the wall in the next step when you have filled the cavity as best you can.

  • Use a trowel to apply the remainder of the hydraulic cement to the seam near the surface. Allow the cement to spread onto the beginnings of the stones as an extra sealant.

  • Scrub off any excess cement on the stone with water and a wire brush or pad. If you have to, use a litte muriatic acid but do so sparingly, immediately washing the acid off with water as well.

  • Paint the entire wall with a new coat of waterproof sealant. Let this dry and then paint it again. Clean the area and you are done.

Tips & Warnings

  • Before pitching the cement, paint a coat of waterproof sealant on the surrounding area and let it dry; this will make cleaning any cement off the stones easier.
  • Part of the nature of hydraulic cement is that it becomes very hot, very fast because it is designed to cure within minutes and not hours like normal cement. Hydraulic cement can cause serious burns to your skin, always wear gloves when handling it and wash any cement off your skin immediately.

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