How to Repair Mortar Joints

Use an Angle Grinder for Larger, Harder Repointing Brick Jobs

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Repairing brick mortar requires basic tools: hammer, flat utility chisel, safety glasses, dust mask and whisk broom. Filling the cleaned-out joints requires masonry tools: brick trowel, 3/8-in. pointing trowel, a special tool for contouring the joints and waterproof gloves.

If you do tackle larger jobs or encounter hard mortar that can’t be easily chiseled out, we recommend that you rent or buy an angle grinder fitted with a diamond blade. Select a grinder with a 4-1/2 in. blade diameter; larger grinders are harder to control and cut the mortar too deep. To begin, Cut grooves 3/4 to 1 in. deep in cracked or deteriorating mortar using a 4-1/2 in. angle grinder fitted with a diamond blade. Push the blade into the joint until the grinder head contacts the brick, and make a single pass along the center of the joints.

Chip Out Loose Mortar

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Break out old mortar using a hammer and cold chisel or a flat utility chisel that’s narrow enough to fit into the joints. Position a flat utility chisel at the edge of the brick and drive it toward the relief cut to fracture and remove the mortar. Wear safety glasses and a dust mask and remove 3/4 to 1 in. of old mortar (more if needed) until you reach a solid base for bonding the new mortar. If the mortar is so soft that the bricks are loosening up, you’ll have to remove and properly reset them. If the cracked mortar is harder, make a relief cut down the center of the mortar joint using the pointed edge of the chisel and then gently chip out the mortar (brick grout) that contacts the brick.

If the removal work is going really slowly, use an angle grinder to make the relief cuts. Exercise care here; the grinder can easily nick and chip the bricks, so don’t use it to clean out the mortar contacting the brick. To avoid nicking the bricks, cut the vertical joints before cutting the horizontal joints.

Clean the Mortar Joints

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Once the old mortar is removed, dust out the brick cavity joints using a whisk broom or compressed air. Prepare the joints to receive new mortar by misting them lightly with a garden hose sprayer.

Mix the Mortar

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Using only the amount of water specified by the manufacturer, gradually add in the water and mix the mortar in a cement boat until it’s the consistency of peanut butter and sticky enough to cling to an overturned trowel. It should be stiff but not crumbly. Allow the mortar to “rest” for 10 minutes as it absorbs the water, then remix it using your brick trowel. Don’t try to revive mortar that’s drying out by adding more water to it. Mix a fresh batch instead.

Fill the Joints with Mortar

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The basic steps for how to mortar brick start like this: Load mortar onto an overturned brick trowel, hold the trowel under the horizontal joint—tight to the brick—and sweep 1/4-in. slivers of mortar into the cavity using a 3/8-in. wide pointing trowel. Fill the horizontal joints first. Avoid getting mortar on the brick face.

Follow these additional tips for filling mortar joints:

• Pack the mortar tightly with no voids for the strongest, most water-resistant joints.
• Fill deeper joints (those greater than 3/4 in.) in two stages. Allow the first layer to partially harden (until a thumbprint barely leaves an indentation) before adding the second layer.
• In hot weather, work in shaded areas first (if possible) so the sun won’t dry the mortar too fast. Mix smaller batches of mortar.
• Don’t work in temperatures below 40 degrees F.

Fill the Vertical Joints Last

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Load smaller amounts of mortar onto the back of the brick trowel, hold the trowel tip along the vertical joints and above the horizontal joints—tight to the brick—then sweep and pack the mortar into the cavity using the pointing trowel.

Figure A: Common Mortar Joint Profiles

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Before finishing repairing the mortar joint, determine which joint matches your existing joints using Figure A above. Next, buy the mortar finishing tool you need to match the contour and depth of your existing mortar joints. We recommend that you repoint brick sills and other horizontal brick surfaces (ledges, wall tops, etc.) with flush joints to promote drainage—regardless of the type of mortar joint in your vertical walls. Allow the mortar to cure to “thumbprint” hardness before you finish the joint. Shape the vertical joints before working the long horizontal joints. These are the most common mortar joint profiles:

• Raked joint: Formed by removing mortar to 1/4 in. deep with a raking block.
• V-Joint: Formed by a brick jointer, it has a concave, “V” look.
• Flush joint: Formed by cutting off the mortar with the edge of a brick trowel.
• Concave joint: Formed by the curved end of a brick jointer.

Rake the Joints

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For this project, we used a raked joint mortar profile. To make your own raked joint tool, drive a 6d box nail into a short 1×2 board so that it matches the depth of the existing joints. To “rake” joints, hold the board perpendicular to the bricks and move it back and forth, first along the vertical joints and then the horizontal joints. Other joint profiles require other shaping tools.

Clean the Bricks

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Use a soft-bristle brush to remove mortar chunks on the brick face before they harden and to sweep loose mortar from the finished joints. The brush keeps the mortar from smearing. If you do smear mortar onto the brick, you’ll have to go back later and use a chemical cleaner. Prevent water from entering and damaging your brickwork by applying color-matched polyurethane caulk where stucco, wood and other materials meet brick. Mist the new mortar twice a day for two days using a hand pump sprayer or a light mist from a garden hose to help it harden.

Looking for ideas to revamp your plain brick house or fireplace? Consider German smear, a mortar wash technique that transforms the look of plain brick.