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What Is That Dark Streak Running Down My Exterior Brick Wall?

Last updated October 25, 2026

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The short answer

Common Causes of Brick Water Stains

A leak from the roof, gutter, or flashing is a frequent culprit. Water escapes from a damaged component and runs down the wall behind the brick veneer or over its surface. Clogged gutters that overflow can also send a concentrated stream of water down your exterior wall, creating a visible dark trail over time.

Another source is condensation from an air conditioner or plumbing vent pipe. If your AC's condensate line or a vent is discharging too close to the wall, the constant moisture drip leads to staining. Inadequate sealing around windows, doors, or where decks attach to the house can similarly channel water down the brick.

The Dangers of Ignoring the Streak

This streak is a symptom, not just a cosmetic flaw. The water causing it can seep behind the brick, leading to hidden moisture problems. Over time, this moisture can damage the wall's wooden framing, insulation, and interior drywall, potentially causing structural issues or mold growth inside the wall cavity.

Persistent moisture also degrades mortar joints through freeze-thaw cycles or efflorescence, weakening the wall's integrity. In areas with frequent rain or high humidity, this degradation can accelerate. Ignoring the leak allows the problem to worsen and become far more expensive to repair.

How to Find the Source of the Leak

Start by inspecting the area directly above the top of the stain. Look on the roof for missing or damaged shingles, and check all gutters and downspouts for clogs or separation from the fascia. Examine roof flashing around chimneys, vents, and wall intersections, as these are common failure points for leaks.

Inside your home, check the attic or the interior wall corresponding to the exterior stain. Look for signs of moisture, such as damp insulation, water spots on wood, or mold. Sometimes, the leak's entry point is several feet away from where the water finally appears on the exterior brick, making a thorough inspection crucial.

When to Call a Water Damage Professional

If your investigation reveals active water entry or interior damage, it's time to contact local water damage pros. They have tools like moisture meters and thermal imaging cameras to pinpoint moisture you cannot see. They can assess the full extent of the damage behind the brick and within the wall structure.

The pros can develop a plan to stop the water, dry the affected materials properly, and prevent mold. They can also advise on necessary repairs to the exterior envelope of your home. Their assessment can be valuable for an insurance claim, though policies vary, and your insurer decides what is covered.

  • The stain is large, growing, or accompanied by interior water damage.
  • You cannot locate the exact source of the water intrusion after a careful inspection.
  • There is a musty odor inside your home near the affected wall, suggesting mold.
  • The brick or mortar appears to be crumbling or deteriorating.
  • You have reason to believe the leak is from a broken pipe inside the wall.
  • Recent heavy storms or a known plumbing issue preceded the stain's appearance.

Repairing the Damage and Preventing Recurrence

Repair starts with permanently fixing the leak source, which may require a roofer, plumber, or handyman. Once the water intrusion is stopped, any damaged interior materials like drywall or insulation may need removal and replacement by restoration professionals. The exterior brick may need cleaning or repointing of mortar joints after the wall assembly is completely dry.

To prevent a recurrence, maintain your gutters and downspouts, ensuring they direct water well away from your foundation. Seal any cracks in mortar or around penetrations. Consider having your roof and flashings inspected periodically, especially after severe weather. Proper maintenance is the best defense against water stains and the hidden damage they signal.

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Frequently asked questions

Can I just pressure wash the stain off my brick?

Pressure washing may remove surface dirt, but it won't fix the leak causing it. Using too much pressure can damage mortar and force water deeper into the wall. It's best to solve the water source first, then clean the brick gently after repairs are complete and the wall is dry.

Could the dark streak be mold on the brick?

It's possible, especially in shaded, damp areas. Mold or algae can grow on damp masonry. However, mold growth also indicates a chronic moisture problem. A mold professional can test a sample to confirm, but addressing the underlying water issue is necessary to stop any biological growth from returning.

Will my homeowners insurance cover repairs for this?

Coverage depends on the leak's cause. Sudden, accidental events like a storm-damaged roof may be covered. Damage from long-term neglect or lack of maintenance typically is not. You should document the damage and the source, then file a claim. Policies vary, and your insurer decides what is covered.

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