How to Waterproof a Loose Foundation Stone

Remove the mortar surrounding the loose foundation stone using a chisel and a hammer to chip the mortar from the joint surrounding the stone. Pull the stone out of the wall after removing the mortar.

Brush away any loose mortar from the foundation recess with a wire brush.

Mix a small batch of mortar in a bucket using an electric drill and a grout bit. Apply a layer of the mortar to the base of the foundation recess. Place the stone back into place with the surface of the stone even with the surface of the surrounding foundation stones. Fill in the spaces surrounding the stone with mortar using the tuckpointing trowel to push the mortar into the joints. Allow the mortar to firm for roughly 1 hour.

Run a jointing tool through the joints surrounding the stone over the mortar to form the mortar surface to match the surrounding joint lines in the rest of the surface. Allow the mortar to cure in place around the stone for 24 hours.

Spray a layer of concrete sealant over the block and mortar line to seal the stone and mortar. Allow the first sealant layer to dry for about 1 hour and then spray a second layer onto the stone. Allow the sealant to dry for 48 hours to waterproof the surface of the loose stone.