How to Fill in Foundation Block Mortar Joints

Wash the foundation with a high pressure water hose to remove any loose dirt or debris from the block surfaces and the mortar joints. Run a nylon brush over the mortar joints that need filling to make certain you get any embedded dirt from between them, then wash the surface a second time.

Remove any loose or cracked mortar from the joint lines using a brick pointing chisel. Remove enough mortar to expose a depth of at least three-quarters of an inch between the blocks to give the new mortar room to fill. Rinse away any small chips or dust remaining with a hose.

Mix one part Portland cement with one part lime and six parts sand in a 5-gallon bucket. Add water to the bucket while mixing the materials together to form a mortar mix with the same consistency as peanut butter.

Touch the walls to determine if they’re still damp from the last wash. If not, spray them lightly with the hose. The damp walls will prevent the blocks and existing mortar in the joints from drawing out the moisture from the new mortar, making it dry brittle.

Scoop up a moderate amount of mortar from the bucket onto a brick trowel. Place the trowel edge onto the side of the wall with the flat of the trowel parallel to the wall and even with the mortar lines you wish to fill. The mortar should lie on top of the trowel, so that you can push it into the joints.

Use a pointing trowel to push small amounts of mortar from the trowel into the joints, filling them to the height of the block surfaces. Move the trowel down the line of the joint to continue filling the joint with the mortar, pushing the mortar in place as you go along the foundation wall.

Remove any excess mortar from the joints with a rounded wooden stick. Wait an hour or two for the mortar to set firm and then drag a tooling jointer over the mortar to create a mortar shape within the joint that matches the other joints.