A stone wall will stand for a long time if you build it correctly. But even the best-built walls will need repairs. Weather, time, earthquakes, frost heave, and other incidents can cause a wall or a stone in the wall, to crumble, shift, or fail. Depending on your patience and stone chipping skills, you can do most stone wall repairs yourself. The following will provide you with some building tips.
Step 1 - Assess the Damage
Determine exactly how many stones in your wall are damaged to the point of needing to be replaced. Hairline cracks and minor damage may not need to be replaced. Crumbling, cracked stone in a load-bearing location should be replaced. You may be able to cut some of the damaged sections out and replace them with smaller stones. Don't repair any more stones than is absolutely necessary to correct the damage. Often, most repairs are to the patching and spalling failure of an earlier repair, or to a stone split due to freezing and cracking or impact damage.
Step 2 - Remove Damaged Stones
Unless the stone is severely cracked or totally crumbling you may be able to remove only the damaged portion. If the entire stone must be removed, use a diamond blade saw to score the stone first. Then use a hammer and flat chisel to split open the score marks and remove the damaged stone. Use your hammer and flat chisel to remove the surrounding mortar and clean out the recession to a depth of 4 to 6 inches. Chisel inside corners to a 90-degree angle, or as close to that as possible.
Step 3 - Find Replacement Stone
If you have a historical stone wall, chances are the stone came from a local quarry. Check with your local masonry or stone supplier to find a matching stone. Take a chunk of the stone with you so you can match type and color with available stones.
Step 4 - Cut the Stone
Once you find an appropriate stone, have your local masonry supply cut it for you, or rent a diamond blade cutter and cut it yourself. Work the surface so it matches the surrounding stones. Once you've worked the surface, use the diamond blade cutter to cut the outer dimensions to fit the wall cavity where you removed the damaged stone. Allow at least 3/8-inch on all sides for your mortar. If your mortar joints on the existing wall are larger than that, adjust accordingly.
Step 5 - Set the Stone
Once your stone is cut, mix your mortar. In a historic stone wall, use a lime and sand mixture with a 3 to 1 ratio without Portland cement, so you match the original mortar. In a newer wall, use the same mortar used to build the wall. If you're not sure what the mortar mix should be, ask your local masonry supplier to test it for you. Dampen the stone and the stones surrounding the cavity so the mortar will cure slowly. Butter the joints and put your stone into the new cavity. Once the stone is settled (use wood shims if necessary to level the stone in the cavity), tuckpoint the stone. Tool the mortar profile and brush away excess mortar after it sets for a few hours.