Q: The mortar joints surrounding the stones in my walkway are crumbling. Can they be patched? —Nellie Kurtzman, Mount Vernon, NY
Martin D'Arcy, owner, DNF Construction, replies: A stone walkway with cracked mortar is an eyesore and can become a tripping hazard if water gets underneath and loosens the stones.
The best way to repair these joints is with a sand-topping mortar mix, which is stronger than regular mortar and can handle wider joints. I mix it with an acrylic fortifier so that the joints will be less susceptible to water penetration.
The hardest part of this job is chipping out the crumbly old mortar without damaging the stones. Once the old stuff is out, filling the joints is as easy as decorating a cake. Just keep in mind that while the new mortar won't match the existing mortar color, it will lighten and blend in over time. The only way to get the same shade would be to replace all the old mortar, crumbling or not.
Tackle this job only when there's no danger of frost. A two-day dip into the 20s F can ruin fresh mortar.
Chisel Out the Mortar
Wearing gloves, safety glasses, and knee pads, break out the crumbling mortar with a cold chisel and 3-pound sledge. Chisel right down to the setting bed; be careful next to the stone or use a grinder with a diamond blade to carve out difficult spots. Clean out the joints with a stiff-bristled nylon brush.
Make the Topping Mix
Pour the dry mortar into a wheelbarrow, and stir in an acrylic fortifier according to the manufacturer's directions. Add just enough water to give the mortar a sticky, peanut-butter consistency. It should remain workable for about 45 minutes. Before applying the mortar, sponge the stones and joints with water.
Fill the Joints
Ladle the mortar into a grout bag or a zip-top bag that has a ½-inch-diameter hole you've snipped in a corner. Squeeze the mortar out in a smooth motion, forcing it to the bottom of the joints. Fill them up to 1 inch deep in one pass, leaving the mortar just proud of the stones. Deeper joints should be half filled, packed down with the trowel, then topped off. If any mortar lands on top of the stone, scrape it off immediately.
Trowel the Mortar
Push the mortar down into the joints using a brick trowel, then roughly tool it to the same height as the existing joints. Scrape off the excess mix and dump it back into the wheelbarrow. As you work, dip the trowel in water to keep it clean. Also, sponge-clean the stones with water to prevent mortar stains.
Pack it Down
Wait 60 to 90 minutes for the mortar to set up enough to leave a thumbprint when pressed. Using a tuck-pointing trowel, smooth and pack down the mortar so that it matches the height of the existing joints. Clean the stones as necessary.
Brush the Joints
Run a stiff, dry, natural-bristle brush over the mortar in one direction, smoothing away the bumps and ridges left by the tuck-pointing trowel. Stay off the walk overnight. The mortar will fully harden in about a week. If the weather during that time is very hot, windy, or dry, keep the mortar moist with wet burlap.