If the north side of your house has a green haze or black streaks — especially under trees or on walls that don't get much sun — you're looking at algae, mildew, or mold growing on your siding. It's common across Northern Kentucky, where humid summers and mature tree cover keep shaded walls damp for long stretches.
What's Growing on Your Siding
Vinyl itself isn't food for anything. The growth lives on the thin film of dirt, pollen, and organic debris that settles on the surface, and shade and moisture let it spread.
- Green film is usually algae.
- Black spots and streaks are usually mildew, or a mix of mold and algae.
- Orange or rust-colored stains usually aren't growth at all. They come from iron in irrigation water or runoff from metal fixtures, and need a rust remover instead of a mold treatment.
Why High Pressure Alone Doesn't Last
A high-pressure wand knocks off the visible growth, but it tends to leave live growth behind in seams, laps, and textured surfaces, so the green comes back sooner. On vinyl, high pressure also causes problems of its own:
- It can drive water up behind the panels and into the wall.
- It can crack or chip older, brittle vinyl and loosen panels.
- It can strip chalky, oxidized color unevenly and leave visible wand marks.
- It can damage window seals, vents, light fixtures, and outlets.
How a Pre-Treatment Soft Wash Works
- Prep the area. Close windows and doors, cover outlets and light fixtures where needed, move furniture and grills away from the wall, and wet down plants and shrubs near the house so they don't soak up the solution.
- Apply the cleaning solution. A diluted sodium hypochlorite solution with a surfactant is applied at low pressure. The surfactant helps it cling to vertical siding instead of running straight off. Applying from the bottom up avoids clean streaks running through dirty areas.
- Let it dwell. The solution needs several minutes — commonly 5 to 15 — to work. Green growth often fades or turns brown as it dies. Don't let it dry on the wall in hot sun; mist it with water if needed.
- Rinse at low pressure. Rinse from the top down with plenty of water. Because the solution has already broken down the growth, it rinses off without high pressure.
- Rinse the landscaping again. Plants, mulch, and grass near the house get another thorough rinse once the siding is done.
Mix strength matters. Too weak and the growth isn't killed; too strong and you risk discoloring trim, harming plants, and corroding metal. Follow the product label and the siding manufacturer's recommendations, and test a small, out-of-sight spot first.
How Long Siding Stays Clean
Mostly it comes down to shade and moisture. Open, sunny walls stay clean noticeably longer than north-facing walls under trees or walls next to irrigated beds. Many homes do well with a yearly wash; heavily shaded ones may need it every 6 to 9 months. Trimming branches back from the house, keeping gutters clear, and aiming sprinklers away from the siding all slow regrowth.
When to Pre-Treat vs. Just Rinse
Pre-treatment makes sense when:
- The siding has visible green, black, or brown growth.
- You have north-facing walls or areas in constant shade.
- Growth came back quickly after a previous pressure wash.
- Trees, shrubs, or irrigation keep a wall damp.
A plain low-pressure wash is usually enough when:
- You just need to remove dust, pollen, or cobwebs, with no growth.
- The siding is new or was cleaned recently.