5 Steps to Fix a Gap in Brick Mortar Before Winter Damages It

I noticed the first gap in my brick mortar on a chilly November afternoon, when a gust of wind rattled the window frame and I saw a thin streak of light through the wall. That quarter-inch opening between two bricks didn't look dramatic—just a narrow, dusty crevice. But I knew what would happen if I ignored it. By February, that same spot would be a running crack, the mortar crumbling like stale bread, and water seeping into the basement every time the snow melted. The freeze-thaw cycle is brutal: water seeps into the gap, freezes, expands, and shoves the bricks apart. One winter of neglect can turn a simple fix into a costly rebuild. Here’s the thing—you can patch that gap yourself in a weekend, with under $50 in tools and materials, and stop the damage cold. These five steps have kept my own brick wall tight through three Minnesota winters. Let’s get into it.

Step 1: Assess the Damage and Gather the Right Tools and Materials

Before you buy anything, take a close look at the gap. Run your finger along the mortar—if it crumbles or flakes off easily, you’ve got more than a surface crack. Use a screwdriver to probe the joint: push it in gently. If it goes deeper than 1/2 inch, you’re looking at a full repointing job, not just a cosmetic patch. If the gap is hairline thin (less than 1/8 inch) and the surrounding mortar is solid, a high-quality masonry caulk might hold for a season—but for anything wider, mortar is the only permanent fix.

Here’s what you’ll need for the job:

• Safety gear: safety glasses, a dust mask (N95 or better), and work gloves. Silica dust is no joke.
• Tools: a hammer, a cold chisel (or a masonry chisel), a wire brush, a pointing trowel (the narrow, diamond-shaped one), and a small hawk or a piece of plywood to hold the mortar.
• Mortar mix: For most exterior brick walls, Type N mortar is your best bet. It’s strong enough for load-bearing but breathable, so moisture doesn’t get trapped inside the brick. If your house is older than 1950—especially with soft, handmade brick—use Type O or a lime-based mix. Matching the color matters too: grab a bag of mortar dye or a pre-colored mix if your existing mortar has a tan or gray tint. Test it on a hidden spot first.
• Optional but recommended: a latex bonding agent (like a masonry adhesive additive) and a spray bottle for misting.

One trip to the home center will set you back about $30–$40 for the basics. Don’t skimp on the dust mask—trust me, you don’t want that fine powder in your lungs.

Step 2: Remove the Loose and Damaged Mortar Safely

This step is where most DIYers rush and mess it up. You need to chisel out the old mortar to a clean depth of at least 1 inch—or until you hit solid, unstirred mortar beneath. If you only scrape the surface, the new mortar won’t bond and will pop out in the first freeze.

Here’s my method: Put on your safety glasses and dust mask. Position the cold chisel at a shallow angle (about 30 degrees) against the mortar joint, not straight into the brick face. Tap the chisel with the hammer, working along the joint in small, controlled strikes. The goal is to loosen the crumbly mortar without chipping the brick edges. I’ve dinged a few bricks in my time—now I always keep the chisel pointed into the joint, not the brick.

For deep or stubborn areas, you can use an angle grinder with a masonry wheel, but be warned: it creates a massive cloud of dust. I only use the grinder outdoors and with a helper holding a shop-vac hose right next to the blade. If you’re not comfortable with power tools, stick to the chisel. Go slow, and check your depth with a ruler every few inches. The mortar should be cleanly removed, leaving a slightly rough surface inside the joint—that roughness helps the new mortar grip.

Step 3: Clean the Joint and Apply Bonding Agent (Optional but Recommended)

Once the old mortar is out, you’ll see a dusty, debris-filled groove. Blow it out with compressed air (a can of keyboard duster works in a pinch, or an air compressor if you have one). Then scrub the joint with a stiff wire brush to knock off any remaining loose particles. Finally, rinse it with a spray bottle of water—just enough to dampen the surface, not soak it. The joint should be clean and slightly damp before you apply mortar. Dry dust acts like a barrier and prevents adhesion.

Here’s a trick I learned from a mason friend: for older brick walls where the existing mortar is very hard or the brick is highly absorbent (common with soft, vintage brick), brush a thin coat of latex bonding agent into the joint. Let it sit for 10–15 minutes until it becomes tacky. This dramatically improves the bond between old and new mortar. I’ve done repairs both with and without it, and the bonded joints have stayed crack-free for years, while the non-bonded ones sometimes showed hairline separation within a season. It’s cheap insurance—a quart costs about $10 and covers many joints.

Step 4: Mix and Apply New Mortar Using a Pointing Trowel

This is the make-or-break step. Mix your mortar according to the manufacturer’s directions—usually, you add water slowly and stir until it reaches a consistency like peanut butter. Not too runny, not too stiff. If it’s too wet, it’ll slump out of the joint; too dry, it won’t pack tight. I mix in small batches (enough for 30 minutes of work) because mortar starts to set quickly, especially in cool, windy weather.

Load your pointing trowel with a dollop of mortar. Press it into the joint firmly, starting at the back and working forward. The key is to pack it in—don’t just smear it on the surface. I use the trowel’s edge to push the mortar deep into the groove, then scrape off the excess flush with the brick face. If the joint is deeper than 1/2 inch, fill it in two or three layers, allowing each layer to firm up for about 20 minutes before adding the next. This prevents shrinkage cracks as the mortar dries.

After the joint is filled, “strike” it to match the existing profile. Most brick walls have a concave or V-shaped joint. Use a striking tool (a rounded piece of metal or even a short length of copper pipe) to press the mortar into a smooth curve. This compacts the surface and sheds water. Let it sit for 15 minutes, then brush away any crumbs with a soft brush.

Step 5: Cure and Protect Your Repair from Freezing Temperatures

This is the step most people skip—and it’s why their repairs fail. Fresh mortar needs moisture to cure properly, not to dry out. If it dries too fast (from sun, wind, or low humidity), it’ll crack. If it freezes before it sets, it’ll turn to crumbly dust. The golden rule: keep the mortar damp for at least 24 hours, and ideally 48–72 hours, in temperatures above 40°F.

Here’s my routine: After finishing the repair, I mist the joints lightly with a spray bottle every 4–6 hours for the first day. If the weather is warm and breezy, I cover the area with a damp burlap sack or an old towel, then plastic sheeting over that to trap moisture. If a frost is forecast (anything below 32°F), I add an insulating blanket on top—an old moving blanket or a fiberglass insulation roll works. Check the repair after 48 hours: it should feel hard and sound solid when tapped. Avoid heavy rain or snow hitting it directly for at least 24 hours.

I once skipped the covering step on a chilly October night, and the next morning the mortar had a network of hairline cracks. I had to chip it all out and start over. That lesson stuck: don’t trust a weather forecast that says “above freezing”—always protect the repair if there’s any chance of frost within the first 48 hours.

Conclusion: Why a Little Effort Now Saves Big Headaches Later

Those five steps—assess, remove, clean, fill, cure—took me about four hours total for a dozen linear feet of mortar joints. The cost was under $50, and the result is a wall that’s shed snow and rain for three winters without a single new crack. Compare that to the $1,500 I was quoted for a mason to repoint the whole wall. This is one of those repairs where doing it right pays off instantly: you prevent water damage, stop drafts, and keep your brick structure solid for decades.

Before you close this tab, take a quick walk around your house. Look at the mortar near windows, corners, and where the wall meets the foundation. If you spot even a small gap, mark it with a piece of tape and plan to fix it this weekend. Your winter self will thank you. And if you want to go deeper, check out our guide on how to repoint a brick wall step by step for larger projects. For choosing the right mix, see best mortar mix for outdoor brick repair.