Why Does My Brick Exterior Have Dark Streaks?

Dark streaks on brick often trace back to water. Rain hits the wall, picks up dust, soot, or organic debris from the roof and gutters, then runs down the face of the brick. As the water evaporates, it leaves behind those dark lines. In a humid subtropical climate, frequent rain and high moisture can make the staining more noticeable.

Another common cause is efflorescence, a white or gray deposit left when water moves through the brick and evaporates. But dark streaks can also come from metal flashing or gutters that rust, or from organic growth like algae and mildew. If the streaks appear only under windows or near downspouts, the water source is likely right there.

• Check gutters and downspouts for blockages
• Look for cracked or missing mortar joints
• Inspect window sills and door thresholds for leaks
• Note if streaks follow a pattern from roof edges
• Feel the brick after rain to see if it stays wet longer in spots

You can try cleaning light surface streaks with a soft brush and plain water. Work from the bottom up to avoid making the stains run further down the wall. For tougher marks, a gentle detergent mixed with water may help, but avoid harsh pressure washing, which can drive water into the brick and worsen the problem.

If the streaks are from algae or mildew, a solution of water and a mild bleach alternative can work, but test a small, hidden area first. Always protect nearby plants and rinse thoroughly. If cleaning does not remove the stains, the issue may not be surface dirt, and a professional assessment could be useful.

Prevention starts with managing water flow. Keep gutters and downspouts clear, and make sure they direct water away from the brick. Fix any leaking roof edges or flashing. Sealing brick can help, but it is not always recommended because brick needs to breathe, so check with a professional before applying any coating.

Regular maintenance matters. Trim back tree branches that rub against the wall, and remove any vines that hold moisture against the brick. Staying ahead of the rainy season by checking your exterior after storms can reduce the chance of streaks turning into deeper moisture problems.