Remove Mildew from Siding

Mildew shows up on siding as black, green, or gray patches that spread slowly across the north side of houses, under eaves, and anywhere moisture lingers. Left alone, it degrades paint, works into porous materials, and broadcasts spores that colonize deck rails, window trim, and every other damp surface within reach. Removing it is straightforward work that takes an afternoon and basic supplies, but doing it right means understanding what kills mildew versus what just smears it around. A proper cleaning restores the siding's appearance and buys you years before the next treatment, especially if you address the drainage and shade issues that invited the growth in the first place.

1. Protect plants before spraying. Move furniture, planters, and decorations away from the siding. Cover foundation plantings with plastic sheeting or wet them thoroughly before you start — bleach solution kills foliage on contact. Disconnect outdoor electrical fixtures if you're working near outlets or lights.
2. Mix bleach solution correctly. In a pump sprayer or bucket, combine one part household bleach with three parts water. For vinyl or painted wood, add one tablespoon of trisodium phosphate per gallon if mildew is heavy. For aluminum siding, skip the TSP and use straight bleach-water — TSP can corrode aluminum.
3. Spray bottom to top. Spray or brush the solution onto the mildewed areas, starting low and working upward to avoid streaking. Saturate the affected sections thoroughly. If using a pump sprayer, hold the nozzle 12 inches from the siding and overlap each pass slightly.
4. Wait for bleach to work. Allow the bleach mixture to sit on the siding for 10-15 minutes. You'll see mildew stains lighten as the bleach works. Don't let the solution dry on the surface — if it starts to dry before 10 minutes, mist it lightly with more solution.
5. Brush away remaining mildew. Use a soft-bristle brush on a pole to scrub areas where mildew remains visible. Work in small circles with moderate pressure. For textured siding, angle the brush to reach into grooves and seams where mildew hides.
6. Flush away all bleach residue. Spray the siding with clean water from a garden hose, starting at the top and working down. Use a wide spray pattern and rinse until no soap or bleach smell remains. Pay extra attention to seams, corners, and trim where solution pools.
7. Spot-treat stubborn stains. Once the siding dries, walk the perimeter and check for remaining stains. Spot-treat any lingering mildew with straight bleach on a rag, let sit five minutes, and rinse. Make notes of areas that seem perpetually damp for later attention.
8. Stop mildew before it returns. Trim back branches that shade the siding and prevent airflow. Check gutters for clogs that cause overflow onto the wall. Redirect downspouts or extend them if water splashes back onto the foundation. Fixing the cause prevents mildew from returning next season.