Repairing concrete block wall mortar joints requires little time and few tools giving opportunity for beginners and seasoned DIY’ers to effect this repair. Please note that this article discusses mortar joint repair for concrete block walls. Mortar joint repair for older structures that employed brick and lime mortar may not be suitable. Modern mortar mix cures harder and under some conditions may cause the brick face to sheer.
Step 1 – Chisel Out the Old Mortar
Remove everything that’s loose that can be cleared by hand. Employ your safety glasses, cold chisel and hammer to remove any mortar still adhering to the repair area. Use a wire brush to thoroughly clean the area. Ensure that any dust or mortar is clear so that you have a clean joint for the mortar or mortar caulking to adhere to.
Step 2 – Small Repairs
For mortar joints up to 3/8” tall but not over 10 feet long use a mortar repair caulk. Mortar caulk is non-structural so if your repair involves replacing portions of concrete block or if they are greater than 10 feet do not use caulk. Proceed to the next step: Larger fixes. Read and follow the caulk's label directions.
Fill the joint with a caulk backer rod. Choose a backer rod that is non-absorbent, closed-cell polyethylene or similar material and slightly larger than the repair area. The backer rod is round and flexible like a rope with a spongy foam texture. To fill a 3/8 inch tall mortar joint select a 1/2 inch backer rod so that it fits tight to seal the repair against moisture.
Squeeze the caulk from your caulk gun so that the caulk fills the joint top to bottom and backer rod to the surface of the concrete block. Run the entire bead and then using your spray bottle filled with solvent or water, depending upon the type of caulk you selected, wet your finger and draw it down the joint to smooth and clean any excess.
Next wet the tip of your trowel to form “V” shaped joints or a mortar jointer / small diameter pipe for “U” shapes. Shape the joint to match the existing mortar.
Consult the caulk's label before applying paint.
Step 3 – Larger Fixes
Follow the same procedures listed in steps 1 and 2. Using a bucket, blend water and mortar mix to peanut butter consistency. Wet the mortar joints then us a pointed trowel to work the mortar into the crevasse. Pack the mortar tightly into the joint so that no gaps or air holes exist. This may take some time, be patient. Fill verticals first then horizontals.
Clean any excess while the mortar is wet. Use a small pipe, mortar jointer or the tip of your trowel as discussed above to shape the mortar.
Allow the mortar to thoroughly dry (consult label directions) before painting.