Waterproofing for Brick Walls: Sealers That Let Brick Breathe

The fastest way to damage an old brick wall is to seal it with the wrong product. Glossy, film-forming sealers trap moisture inside the masonry, and when that water freezes, the brick face pops off in flakes, a failure called spalling. Effective waterproofing for brick walls does the opposite: it keeps liquid rain out while letting water vapor escape from inside.

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Here is the short version. For exterior brick, the right choice is almost always a penetrating silane, siloxane or silane/siloxane blend water repellent. These soak into the pores, line them with a water-shedding layer, and leave the surface looking unchanged. Before applying anything, fix cracked or crumbling mortar joints by repointing, because water gets in through failed mortar far more than through the brick itself. Interior basement brick is a separate problem that usually needs drainage fixes on the outside, not just a coating on the inside.

Below you will find how the products differ, how to apply them, and how to tell when a wall needs more than a sealer.

How Waterproofing for Brick Walls Actually Works

Brick and mortar are porous. They absorb rainwater and then release it as vapor when the weather dries. That wetting and drying cycle is normal. Problems start when water gets in faster than it can leave, or when it cannot leave at all.

Water repellents and sealers fall into two broad families:

For most homeowners, a siloxane masonry water repellent or a silane/siloxane blend is the safest bet. It preserves the natural look of the brick, allows vapor transmission, and typically lasts many years before reapplication. Film-forming acrylics have their place on pavers or certain decorative surfaces, but on a load-bearing or historic brick wall they carry real risk.

Repointing Comes First

A water repellent cannot bridge open joints or cracks wider than a hairline. If your mortar is sandy, recessed, cracked, or missing, water will keep entering regardless of what you spray on the face. Check your wall by scratching joints with a key or screwdriver. If mortar crumbles easily or joints are recessed more than about a quarter inch, plan to repoint first.

Repointing means raking out deteriorated mortar to a depth of roughly two to two and a half times the joint width and packing in fresh mortar. Matching the mortar matters. Older brick, especially pre-1930s, often requires a softer lime-based mortar. Modern hard Portland cement mortar on soft historic brick forces moisture and stress into the brick, causing the brick to crack instead of the joint. Let new mortar cure for several weeks, following the product and label guidance, before applying any repellent.

Safety When Working With Masonry Sealers

• Ventilation and respirators. Many silane and siloxane products are solvent-based. Wear an organic vapor respirator, especially when spraying, and keep people and pets away from overspray.
• Gloves and eye protection. Use chemical-resistant gloves and goggles. Masonry cleaners and efflorescence removers can be acidic and harsh on skin.
• Never mix chemicals. Do not combine acid cleaners with bleach or other products, and rinse thoroughly before applying sealer.
• Follow each label. Coverage rates, temperature ranges and recoat windows vary widely between masonry sealers. The manufacturer’s instructions take precedence over general advice.
• Protect surroundings. Cover plants, windows, trim and vehicles. Solvent-based repellents can damage vegetation and leave haze on glass.

Tools and Materials

• Penetrating silane, siloxane or blended water repellent labeled for clay brick
• Low-pressure pump sprayer (garden-style) with a fan tip, or a large masonry brush or roller
• Stiff nylon brush and masonry cleaner or efflorescence remover
• Garden hose; avoid high-pressure washing on old brick
• Pointing trowel, joint raker or grinder, and matching mortar for repairs
• Plastic sheeting, painter’s tape and drop cloths
• Respirator, goggles and chemical-resistant gloves

Step-by-Step: Applying a Breathable Water Repellent

1. Diagnose the water source. Look for clogged gutters, overflowing downspouts, missing flashing, failed window sills, or sprinklers hitting the wall. Fix these first. Sealer does not stop water from a leaking gutter pouring onto brick.
2. Repair mortar and cracks. Repoint failed joints and let them cure.
3. Clean the wall. Scrub off dirt, moss, mildew and efflorescence with a masonry cleaner and nylon brush, then rinse with low pressure. Efflorescence (white salt deposits) indicates moisture movement; clean it, but also find the cause.
4. Let the brick dry thoroughly. Most repellents need a surface-dry wall, often two to three days of dry weather after rain or washing. Check the label; some water-based silane products tolerate slightly damp masonry.
5. Test a small area. Apply to an inconspicuous patch and let it cure. Confirm the look does not change and that water beads off the next day.
6. Apply from the bottom up. Using low pressure, flood the surface until it runs down about 6 to 8 inches below the spray pattern. Work in manageable sections, keeping a wet edge.
7. Apply a second coat wet-on-wet if directed. Many products call for a second application before the first dries, especially on porous brick.
8. Protect from rain while it cures. Most need several hours to a day without rain. Clean glass and trim overspray right away.

Interior Basement Brick Moisture

Damp brick in a basement or crawlspace behaves differently. Water is being pushed through from the soil side by rain, a high water table, or poor drainage. Coating the inside face with a film sealer often traps moisture in the brick, causing peeling, efflorescence and crumbling mortar behind the coating.

Start outside. Extend downspouts at least 4 to 6 feet from the foundation, regrade soil so it slopes away from the house, and keep gutters clean. Inside, run a dehumidifier to keep relative humidity around 50 percent or lower. Repoint deteriorated joints with a compatible mortar. For persistent seepage, an interior drainage system with a sump pump, or exterior excavation with a membrane and footing drain, are the durable solutions. Breathable mineral coatings can help on the inside face, but they manage appearance and minor dampness, not active leaks.

Troubleshooting Brick Waterproofing Problems

Water still soaks in after sealing

Coverage may have been too light, or the wall was damp during application. Mortar joints may also be the entry point. Retest with a spray bottle; if joints darken but brick does not, repointing is needed.

Brick face flaking or spalling

Often the result of a non-breathable coating or paint trapping moisture, or hard cement mortar on soft brick. Stop the trapping source; removing film coatings may require a masonry restoration pro.

White haze after application

Overapplication or applying in direct hot sun can leave residue on dense brick. Some haze weathers off; stubborn residue may need a solvent wipe per the manufacturer.

White salt deposits keep returning

Efflorescence means water is migrating through the masonry. Clean it, then track down the moisture source, whether rising damp, a leak, or poor drainage.

Cost Ranges

DIY water repellent is one of the more affordable masonry projects. Coverage varies with brick porosity, often roughly 75 to 200 square feet per gallon per coat. Silane/siloxane products are priced higher than basic acrylic sealers but last significantly longer. For a typical house wall, DIY materials might run from under a hundred dollars to several hundred dollars.

Professional application generally costs a modest amount per square foot, while repointing is more labor-intensive and priced higher per square foot, with historic lime mortar work costing the most. Basement waterproofing systems with interior drains and sumps usually run into the thousands, and exterior excavation more still.

When to Call a Pro

Hire a masonry contractor or waterproofing specialist when:

• Large sections of mortar need repointing, especially on historic or multistory walls
• Brick is cracking, bulging, leaning or spalling extensively
• You need a film coating or paint removed from brick
• Basement walls leak actively, show bowing, or have horizontal cracks
• The work requires scaffolding or high ladder access

A qualified mason can match mortar properly, diagnose moisture paths, and recommend the right product for your specific brick. That knowledge pays off, because the right approach to waterproofing for brick walls protects the masonry for decades, while the wrong one can shorten its life.