Why Does My Siding Have Dark Streaks Running Down It?

Dark streaks on siding are typically the result of water following a path down the wall and leaving behind whatever it picked up along the way. That could be dust and grime from the roof, rust from old gutters, or tannins from nearby trees. The streaks are not mold or rot by themselves, but they show you exactly where water travels.

In a hot, dry climate, most of the year stays dry, so streaks tend to show up after the wet season or after heavy irrigation. Check if your gutters overflow during rain, or if sprinklers hit the siding directly. Both are common sources of the dirty water that creates those lines.

• Gutter overflow carrying roof debris
• Sprinkler spray hitting the wall
• Rusty gutter or downspout residue
• Tree sap or pollen washed down
• Condensation from AC lines
• Improperly sloped window sills

Cleaning streaks usually involves a soft brush or a low-pressure rinse with a mild detergent. Avoid using a pressure washer on high settings, as that can force water behind the siding and cause new damage. Work from the bottom up to prevent water from getting under the edges.

For stubborn mineral stains, a solution of water and white vinegar often helps. Test it on a hidden area first. If the streaks come back quickly after cleaning, focus on fixing the water source rather than just scrubbing the symptom.

A water damage inspection can tell you whether the streaks are only cosmetic or a sign of moisture intrusion. The pros use moisture meters and visual checks to find hidden dampness behind siding. They can also spot issues with flashing, caulking, or drainage that you might miss.

If water has already reached the drywall or framing, you may need drywall water damage repair or other restoration work. The pros can assess the extent and recommend the right steps, which often involve drying out the affected areas and replacing damaged materials.