How to Fix an Uneven Sandstone Foundation

Drill a series of 1 1/2-inch holes in the sunken sandstone slab using a hammer drill. Typically, holes are space three to eight feet apart, depending upon the size of the foundation, although the holes should be evenly spaced from one another in a triangular pattern.

Mix a combination of 1 part Portland cement, 1 part fly ash and 2 parts sand with enough water to make a slurry mixture using an electric concrete mixer. Attach a 2-inch hose with a 1 1/2-inch nozzle to the mixer to pump the cement mixture into the holes, setting the pressure to 10 psi.

Pump the slurry into the hole where the foundation has caved the most. Continue pumping until the sandstone foundation rises about an inch, and then move to the other holes and repeat the process. Only allow the foundation to rise about an inch at a time so it remains fairly level.

Stop pumping the slurry when the foundation is raised to the proper height. Remove as much slurry from the holes as you can using a small bailer. Drop the bailer down into the hole and allow it to fill with the slurry, and then pull it up and empty. Repeat until the hole is as empty as possible.

Fill the holes with a premixed concrete patch material. Push the patch material down into the holes using a steel trowel until the entire hole is filled. Once the hole is filled, pull the trowel across the surface of the foundation to smooth the patch material. Allow the patch material to cure for 24 hours before allowing traffic in the area.