How To Repair Flagstone Steps

Flagstone steps add natural beauty and durability to your home, but over time, they may loosen or wear down. Repairing flagstone steps can be a simple DIY project that restores both safety and style to your entryway. In the video above, Ask This Old House mason Mark McCullough shows how to repair your flagstone steps, addressing common issues like loose stones and deteriorating joints.

Preparing for Flagstone Repair

Before starting, gather the necessary tools and materials to make the process smoother and more efficient.

Pro Tip: Roger Cook, This Old House landscape contractor, explains why surface prep is critical: “We wanna get all the loose cement and dirt out of here. So just take and scrape it up. Just don’t chip away any of the edges.” He recommends using a wire brush first, then ensuring the surface is completely dry before applying any adhesive.

Tools

You need the following tools: 

• Caulking gun
• Cold chisel
• Large sponge
• Masonry hammer
• Masonry trowels
• Mortar trough
• Paintbrush
• Stiff bristle brush

Materials

You also need the following materials:

• Concrete bonding agent
• Construction adhesive
• Masonry sealer
• Paintable caulk
• Painter’s tape
• Type S mortar mix
• Water

Steps To Repair Flagstone

Follow these steps to repair your flagstone steps and restore their look and function.

1. Removing Loose Stones and Old Mortar

Begin by identifying and removing any loose flagstones. Use a hammer and cold chisel to break the mortar bond around the loose stone. Hold the chisel upright and strike with the hammer to crush the joint. Once the mortar is loose, slide the chisel under the flagstone and remove it.

Next, use the chisel and hammer to remove the existing mortar bed and any mortar around the bordering stone. Clear away any dust and loose grit with the masonry brush or stiff bristle brush to prepare for the new mortar.

Pay close attention to the condition of the old mortar bed as you remove it—it tells you a lot about what went wrong. During one This Old House project, mason Mark explained the root cause of a failed mortar bed: “Mortar is supposed to get very hard as it sets up, but when you have moisture damage like we have here, this is what happens.” If your mortar bed crumbles to dust rather than coming out in solid pieces, water infiltration was likely the culprit, and you’ll want to ensure proper drainage before resetting the stone.

2. Cleaning the Area

Make sure the area where you’ll be working is clean. Remove all debris, dust, and loose material to create a strong bond for the new mortar. A clean surface helps the new mortar hold better and last longer.

3. Applying Bonding Agent

Brush a concrete bonding agent onto the exposed concrete. This step keeps the concrete from pulling moisture out of the mortar too fast. Coat the surface and edges of the stone and mortar to help them stick together.

Sal, a plasterer featured on This Old House, explains the science behind this step: “If you don’t put a bonding agent on that joint, well, that adds so much suction, that eventually, when it dries, you’ll crack.” The old concrete acts like a sponge, drawing all the water out of the new mortar before it can properly cure.

4. Mixing and Applying New Mortar

Mix the Type S mortar in a mortar trough according to the instructions on the package. Add water slowly while stirring until the mortar is thick, like oatmeal.

Use a masonry trowel to spread the mortar onto the prepared area. Press down lightly with the edge of the trowel to create small ridges, which help the stone bond to the mortar. Spread the mortar to the edges of the surrounding stones.

5. Resetting the Flagstones

Place the flagstone into the bed of mortar and wiggle it into place so it’s aligned with the existing floor.
If excess mortar comes up around the edges, scrape it off carefully. For stones at the edge of a step, pack extra mortar under the stone for added support.

6. Filling Joints and Smoothing Surfaces

Use the sponge, a bit of water, and light pressure to remove the excess mortar from the stone surfaces and smooth the joints. This will pull the sand aggregate up from the joint, giving it a rougher look to match the existing floor. Keep working on the area until you’re happy with the result.

Fixing Gaps Between Flagstone and Wood

When flagstone meets wood structures, such as stair risers, you’ll need to make a flexible joint to allow for movement and avoid cracking.

Here’s how to create a “soft joint”:

1. Clean out old material from the gap.
2. Use painter’s tape to line the edges of the gap.
3. Fill the gap with paintable caulk.
4. Smooth the caulk using a bit of dish soap on your finger.
5. Remove the tape and allow the caulk to dry.

McCullough showcases this method by smoothing out the caulk seam, creating a seamless finish.

Finishing Touches of Flagstone Repair

After completing the repair work, there are a few final steps to guarantee the longevity and appearance of your flagstone steps:

• Applying masonry sealer: Once the repair has cured, apply a masonry sealer to the entire flagstone surface. This sealer protects the stone and mortar from moisture and adds a subtle shine. It also helps blend the new repair with the existing stone for a smooth, finished look.
• Curing time: Let the repaired area sit for about 28 days to fully dry and harden. This gives the mortar time to reach full strength.
• Routine maintenance: To keep your flagstone steps in good shape, clean them regularly. Sweep away debris to prevent buildup, and check the steps for any small cracks or loose stones. Fixing small problems early can help avoid bigger repairs later.

If you’ve patched a section of your flagstone walkway or steps, consider seal-coating the entire surface once the repair has dried. As one This Old House masonry expert advised during a walkway repair: “Wait a couple of weeks and let this dry out and then seal coat the whole walkway. No one will ever know you put a patch in there.” A full seal coat helps the new repair blend seamlessly with the surrounding stone, making the fix virtually invisible.