Many natural factors can damage stone walls. Erosion is the obvious choice, as is frost heave. And even though mortared walls are less subject to erosion because of their footing, but their mortar joints—and the surface of the stones themselves—can crack and take in water, then freeze and split. A damaged wall that doesn't receive timely attention poses a safety hazard.
You may be tempted to replace damaged stone with synthetic stone made from epoxy or cement-based materials. Such imitation stone is less expensive. Though the patches may look good at first, they will become conspicuous with further exposure to the elements and ultimately mar the appearance of the wall.
We'll walk you through common stone wall repairs, including replacing a popped stone, rebuilding a damaged wall, repairing stone mortar joints, and more.
How to Replace a Popped Stone
Step 1: Remove the Stone
To return a popped stone to its original position, drive wedges between stable stones to take the weight off the popped stone. Work the popped stone out of the wall carefully, without dislodging the other stones. Drive the wedges into the wall slightly further.
Step 2: Reinsert Stone
Reinsert the popped stone in its cavity and tap it home with a carpet-covered 2x4 and a small sledgehammer. Use a crowbar to take the weight off the wedges and remove them. You may have to work the wedges and the crowbar back out of the wall at the same time.
How to Rebuild a Damaged Wall
Step 1: Mark Area
A collapsed section of wall can often be repaired without total replacement. Inspect the wall and visualize a V-shape section that you'll need to remove. Mark the area on the wall with marking chalk—don't use spray paint. Then number the stones with chalk so you can replace them to their original positions. Take a photo of the wall to further help you replace the stones in the right order.
Step 2: Chisel Mortar
Working above the marked section, chisel away any mortar that anchors the capstones and take them off the wall. Dismantle the damaged section.
Step 3: Rebuild Wall
Using your snapshot as a guide, rebuild the wall, working from the leads toward the center. Set the stones using the same techniques used to build it. Be sure to replace any small stones used to keep larger ones in place. Fill in the center recess between the wythes with cracked rubble. Replace the capstones.
How to Repair Stone Mortar Joints
Step 1: Chip Out Mortar
Carefully chip out the damaged mortar with a thin cold chisel and a small sledgehammer. Clean the joint until you reach solid mortar.
Step 2: Mix New Mortar
Mix new mortar to a consistency for use with a mortar bag and fill the bag. Mist the joint with water from a spray bottle and squeeze mortar into the joints.
Editor's Tip: If you repair masonry with mortar that doesn't match the original, it will really stand out. Matching the tint of existing mortar takes some experimenting and patience. You can't tell what the new mortar will look like until it has dried. Mix up the mortar in several batches and spread out thin amounts to dry. Pick the dried color that best matches the original.
Step 3: Pack Each Joint
Pack each joint tightly with a pointing trowel, adding more mortar if necessary. Tool the joints to match the original and remove excess mortar with a stiff brush.
How to Replace a Mortared Stone
Step 1: Remove Damaged Stone
Remove the damaged stone by chiseling out the mortar around the stone. Angle the chisel in the direction of the damaged stone to avoid damaging others. Then pry out the stone. Using a wider cold chisel, remove as much mortar as possible from the cavity. Brush or blow out the cavity to remove loose mortar and dust (wear eye protection).
Step 2: Mark Cut Lines
Hold a new stone slightly larger than the recess against the wall and mark cut lines so you can cut it to fit properly.
Step 3: Cut Stone to Size
If the stone is thin enough, cut it with a circular saw and masonry blade. Then using a mason's hammer, carefully chip the replacement stone until it fits the recess with enough space for the mortar joints. Be careful not to cut too much off the stone; doing so makes the joints wider than those on the rest of the surface and calls attention to the repair.
Step 4: Insert Stone
Mist the cavity and spread mortar on the bottom of the cavity and the top and sides of the replacement stone. Insert the stone and push it into place using the pointing trowel. Pack mortar against all sides of the stone. When the mortar has set, tool it to match the original.
Tips for Maintaining Stonework
Maintenance of stonework requires attention to details. These tips that can help you keep your wall, walk, and patio in good repair.
- Climbing plants pose no danger to hardened mortar, but roots disturb footings and tendrils take hold in cracks. Plant life in the wrong places displaces stones or bricks, blocks rainwater runoff, and hides other damage. To remove plants, cut the main stem above ground level and treat it with a product recommended by your local extension service.
- Residues of some weed killers can cause spalling on some stone surfaces. If stone-faced steps are worn, you may be able to reuse them by turning them over.
- Choose the most gentle cleaning method that can handle the job. Start with water cleaning, the most gentle method, then turn to chemical-base cleaners or mechanical methods such as sandblasting only if you need to.
- Water increases damage to badly deteriorated mortar. Repair the mortar before cleaning with water.
- A pressure washer can be an effective cleaning method, but also may erode softer stones like sandstone.
- Steam cleaning is an effective method for removing embedded soil and poses less risk than sandblasting. Sandblasting is a method of last resort.
- A poultice of absorbent material mixed with a solvent can remove many chemical stains without damaging the stone. Keep the poultice on the stain as long as necessary to remove the stain.
- Always test your cleaning agent in an inconspicuous spot before using it on the entire surface.
- Soil erosion is often the cause of a toppled wall section. This is a problem that does not usually show up for many years. To repair the damage, remove stones from the damaged area and at least two stones wider. Dig a 6- to 8-inch trench where you have removed the stones. Fill the trench with gravel a little at a time and tamp it as you go. Rebuild the section of wall. The gravel will allow water to drain under the wall without washing out its support.