Symptoms
- Dark vertical streaks below windows, gutters, or the roofline
- Greenish or black film in shaded, damp areas
- General dinginess compared to a recently cleaned section
Likely causes
- CommonMildew or algae growth in shaded, damp areasNorth-facing or tree-shaded walls stay damp longer, encouraging growth that looks like staining.
- CommonDirty gutter overflow running down the wallOverflowing or leaking gutters carry dirt and debris residue down the siding below.
- PossibleMineral or rust staining from roofing or hardwareRusting fasteners or certain roofing materials can leave reddish-brown streaks.
What to do
- Identify the pattern: streaks below a gutter suggest overflow, general shaded staining suggests mildew.
- Clean the gutters if overflow is the cause, since new staining will keep appearing otherwise.
- Wash the wall with a soft brush and a mild detergent or a siding-safe cleaner made for mildew.
- Rinse thoroughly with a garden hose; use a pressure washer cautiously and only on a low setting appropriate for the material.
- For rust stains, a specialized rust-stain remover may be needed since regular cleaners won't lift it.
- Trim vegetation to increase sun and airflow on chronically shaded, stained walls.
- Repeat cleaning on a seasonal basis in problem areas rather than waiting for heavy buildup.
Tools and parts
- Soft brush
- Garden hose
- Pressure washer (optional, low setting)
Siding or mildew cleanerSearch: “exterior siding mildew cleaner”
When to call a pro
Call a house-washing or pressure-washing service for a full exterior cleaning.
Safety
Use caution
Use a stable ladder for high areas and keep pressure washers on a low setting to avoid damaging siding.