How to Repoint Brick Mortar Joints (Tuckpointing)

Drag the corner of a key or an old screwdriver firmly along a mortar joint. Sound mortar resists and barely marks; failed mortar crumbles into sand, gouges easily, or falls out in chunks. Look also for joints that have receded more than about 1/4 inch behind the brick face, hairline cracks running the length of a joint, and any spot where you can see daylight or push a blade in past the depth of the brick. Map the failed areas with chalk before you start so you are working to a plan rather than chasing crumbling mortar across the whole wall.

Mortar is the sacrificial part of a masonry wall. It must be softer than the brick so that movement and freeze-thaw stress crack the joint, which you can repoint, rather than the brick face, which you cannot repair. Type N mortar at roughly 750 psi is the correct default for above-grade house brick. Type O at roughly 350 psi is the choice for soft, older, or handmade brick. Scratch a hidden brick face and a sound piece of the original mortar with the same key: if the brick scratches as easily as the mortar, you have soft brick and want Type O.

Repointing is temperature-dependent, which is why it has a season. You want roughly a week of mild weather: no colder than 40 degrees Fahrenheit overnight and no warmer than 90 degrees during the day. Mortar that freezes before it cures turns brittle and crumbles out, and mortar that bakes loses its water before the cement hydrates and cures weak and chalky. Overcast, still days are ideal. In cold climates that makes late summer and early fall the last dependable window before frost, and a wall in direct afternoon sun needs shading or a later start.

Cut the old mortar back to two to two and a half times the joint width. On a standard 3/8-inch joint that is 3/4 inch to a full inch of depth. Hold the plugging chisel at a shallow angle, roughly in line with the joint, and drive it with light taps rather than heavy blows, letting the chisel track along the mortar instead of levering against the brick edges. Do the horizontal bed joints first, then the vertical head joints. Keep the back of the cut square rather than tapered, and stop when you reach sound, hard mortar.

If the existing mortar is modern portland cement and refuses to yield to a chisel, a thin diamond blade in an angle grinder speeds the work considerably. Run it down the center of horizontal bed joints only, staying well clear of the brick edges, and finish the corners and all vertical head joints by hand. A grinder in a head joint almost always overshoots into the brick above or below. On soft, old, or handmade brick, skip the grinder entirely, because the vibration alone cracks brick faces.

Sweep every raked joint out with a stiff-bristle masonry brush to clear loose grit and dust, working from the top of the section down. Dust left in the joint acts as a release agent and is one of the main reasons fresh mortar fails to bond. Then wet the wall with a pump sprayer until the brick is damp but not running. Dry brick pulls the water straight out of fresh mortar and leaves it powdery and weak. Aim for a surface that looks darkened and feels damp, with no standing droplets.

Add water gradually to the dry mix and stop while the mortar is still stiff enough to hold its shape. The target consistency is like thick peanut butter: a ridge drawn through it with a trowel should stand up without slumping, and a trowel-load held sideways should cling rather than drop. Mix only what you can use in about 30 minutes. Let it rest for five minutes after mixing, then remix briefly before use, which improves workability without adding water.

Hold the mortar hawk tight against the wall just below the joint you are filling. Scoop a thin sausage of mortar onto the tuck pointing trowel and push it into the joint in layers no more than about 1/4 inch deep, compacting each layer firmly before adding the next. Packing in thin, well-compressed lifts is what makes the joint dense and waterproof; one thick shove leaves voids that will hold water and fail. Do all the horizontal bed joints across the section first, then come back and fill the vertical head joints.

Wait until the mortar is thumbprint hard, meaning a firm press leaves an impression but no mortar on your thumb. This usually takes 30 to 60 minutes depending on weather. Then run the jointer along each joint with steady pressure to compress the surface and shape it to match the rest of the wall, doing the head joints before the bed joints so the horizontals finish clean. Tooling is not cosmetic: compressing the surface closes the pores and is a large part of what keeps water out.