Even the most beautiful brickwork is just so much rubble without a bed of mortar to seal and hold it together.
What Is Mortar Repointing?
When mortar starts cracking, receding, and falling out of the joints, it’s time to chisel out the old stuff and trowel in the new, a process called repointing. This is a simple job, but it has to be done with care, using the right tools and the right materials to avoid damaging the brick and permanently compromising the integrity and appearance of the wall. For me, that means using hand tools and lime mortars similar to the ones masons employed 100 years ago.
Can You Mortar Over Old Mortar?
If your brick is 50 years old or less, you can probably repoint it safely with modern, portland cement–based mortar (although the guy doing the next repointing, who'll have to grind it out, may curse your decision). But if your house was built before World War II, the mortar is likely a mix of lime putty and sand, and you should try to match it. Otherwise, over time, as the soft old brick swells and shrinks against the rock-hard mortar, the bond between them will break, moisture will get trapped in the wall, and the brick faces will start popping off. Traditional lime mortar acts like a cushion, flexing with the brick's movement while allowing moisture to migrate easily out of the wall.
A restoration mason can analyze old mortar and make a compatible mix, or you can send mortar samples to companies such as U.S. Heritage and get a custom blend with the same color and characteristics. You can choose between hydraulic lime, which comes in bags and hardens when it reacts with water, much like portland cement, and lime putty mortar, which comes in buckets and slowly hardens by reacting with carbon dioxide in the air.
How to Mortar Bricks in 6 Steps
Rake Out the Bed Joints
Scrape out the horizontal joints to a depth of at least ¾ inch. (Joints wider than ½ inch have to be deeper: 2½ times the joint's width.) A carbide-tipped grout saw works well on these ¼-inch-wide "butter joints." (On wider joints, use a cold chisel and an engineer's hammer.) Avoid electric angle grinders. Unless you're trained to use them, they're almost guaranteed to cut into and disfigure the brick.
Clean Out the Head Joints
After removing three or four courses of bed-joint mortar, dig out the vertical joints. Be careful not to hit the edges of the bricks above or below. Here, the old mortar is being chiseled out by tapping a 5-in-1 painter's tool with a hammer.
Wash Down the Wall
Whisk away all the crumbly debris with a stiff-bristled brush, then check that the blade of your tuck-pointing trowel will fit into the joint. If the blade is too wide, grind it down with an abrasive wheel. Mist the wall with water until the brick is thoroughly damp and starts to drip; this is a critical step because dry materials will suck the moisture out of the new mortar and prevent it from curing properly. Wait until the next day before filling the joints.
Mix the Mortar
Mist the brick again. Then, following the directions on the bag, stir water into the dry mix until it reaches the consistency of peanut butter and clings to an upended trowel. Wait until a film of water forms on the mix, about 15 minutes. Stir the water back in. The mortar is now ready to use and remains workable for 8 hours. If it gets too dry in that time, you can "retemper" it occasionally by adding water.
Fill the Joints
Scoop a dollop of mortar onto a brick trowel or hawk, hold it up even with a bed joint, and push the mortar against the back of the joint with the tuck-pointing trowel. Eliminate voids with a few slicing passes of the trowel's edge, then add more mortar until the joint is filled. Once you've finished three or four courses of bed joints, go back and fill the head joints. Finally, smooth and compact all the mortar with the trowel's flat face and scrape the excess mortar off the brick.
Brush the Wall
When the mortar is firm to the touch, brush diagonally across it to remove any dry mortar crumbs. (Brushing in line with the joints can pull out the fresh mortar.) Then carefully sponge mortar residue off the brick faces. For the next three days, use a tarp to protect the soft joints from the sun, wind, or hard rain, and give the wall a daily misting to keep the brick and mortar moist.