How to Seal a Concrete Block Foundation

Things You'll Need

Roll on tar
Roller
Rolling pan
Extension pole
Concrete sealer
Stiff bristled brush
Pressure washer

These cement blocks will absorb water if they are not sealed.

Concrete blocks are naturally porous, and when they are laid into a block foundation, they tend to absorb water from the surrounding soil. Luckily, there are several ways to seal your block foundation and prevent the infiltration of water. Over the years, the construction industry has developed various methods that can be easily implemented by the average do-it-yourselfer. You can expect to complete this project in one to two days, depending on the size and scope of the project.

Step 1

Clean the outside of the block with a pressure washer. Because pressure washers have color-coded tips that distinguish water pressure, insert the yellow tip into the end of the washer first. This will supply a 30-degree fan pattern that is gentle on the block surface. Using a red tip in the pressure washer will damage the block. Clean the surface thoroughly and allow the water to dry for 2 to 3 hours.

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Step 2

Clean the interior of the block using a stiff bristled brush. Sweep the surface of the block as though you were cleaning the block but without chemicals. Remove dust and debris with the brush.

Step 3

Roll on foundation tar coating to the exterior of the block, going no higher on the foundation than the landscaping dirt's eventual height; going higher will cause the tar to be visible. The tar can be purchased at any hardware store in a 5 gallon bucket. Dip a roller directly into the tar and roll onto the foundation surface. Distribute the tar evenly across the wall, leaving no more than a 1/16-inch thick coat. If the conditions outdoors are cool, allow the bucket contents to sit in the sun for an hour to loosen the contents.

Step 4

Paint on two coats of concrete block sealer to the interior of the block. This sealer will penetrate the block pours, sealing them off from any invading water. Using a roller, roll on the sealer, allowing 30 minutes of drying time between each coat. Recoat the surface every two years.

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