The green, gray and black growth on South Florida walls, roofs and driveways is mostly algae and mildew, not the indoor mold that triggers health advisories. Surface growth can be washed off safely with the right cleaner, but it returns unless the moisture feeding it is reduced. Mold inside the house, a musty smell, or any sign of water intrusion belongs in a different category and should not be treated with a pressure washer.
Why growth is so persistent here
Three conditions drive exterior growth, and the region has all of them in abundance: warmth, moisture and organic food. Annual average relative humidity in Miami sits around the low 70s percent, based on long-term climate summaries, and the National Weather Service describes a rainy season from roughly mid-May to mid-October that delivers close to 70 percent of the year's rain. Add salt-laden coastal air, pollen, leaf litter and dust, and exterior surfaces never really dry out for months at a time.
That is why a white house can look streaked after one wet summer, and why many owners find the same wall dirty again a year later. Cleaning removes the symptom. Reducing moisture is what slows its return.
What you are looking at
| Appearance | Usually | Where it shows up |
|---|---|---|
| Green fuzz or slime | Algae | North-facing walls, shaded concrete, fences |
| Black or dark gray specks and streaks | Mildew or cyanobacteria | Stucco under eaves, roof slopes, grout lines |
| Orange or rust-colored film | Iron deposits, sometimes algae | Walls hit by sprinklers |
| Raised, crusty patches | Lichen | Tile roofs, walls, wood |
| Soft green carpet | Moss | Roof valleys, damp ground-level surfaces |
Mold outside versus mold inside
The U.S. Environmental Protection Agency separates routine surface cleanup from true mold remediation. Its homeowner guidance on indoor mold says small areas, roughly ten square feet or less, can often be handled by the household, and that moisture control is the key to preventing a return. It also states that using a chemical biocide such as chlorine bleach is not recommended as a routine practice during mold cleanup, because dead mold can still cause reactions and must be physically removed.
That guidance is aimed at interiors. Outdoors, diluted bleach and other sodium hypochlorite solutions are widely used by professionals on algae and mildew, because rain and runoff dilute them and the surfaces are not breathed in daily. Even so, the same precautions apply: ventilate near open windows, keep it off plants, never mix it with ammonia or acids, and rinse well.
In Florida, mold assessment and remediation are licensed professions regulated by the Department of Business and Professional Regulation. Interior mold work is a separate job from exterior mildew cleaning, and a pressure washing company is not a substitute for a licensed mold professional if there is mold in walls or air conditioning systems.
How exterior growth is removed
- Dry brush or sweep. Remove loose debris first.
- Protect landscaping. Wet plants thoroughly and cover delicate ones.
- Apply a mildew-killing solution at low pressure. This is the soft wash described in the comparison of methods.
- Let it dwell. The product needs time to work, but should not dry on the surface.
- Rinse gently. Work from the top down so dirty water does not run over clean areas.
- Treat stubborn areas again. Heavy growth may need a second application.
On roofs, the same logic holds, but with stricter limits because of slope and surface fragility. See the roof cleaning guide for details.
Cutting down the moisture
Most of what keeps mildew coming back is fixable:
- Sprinklers. Adjust heads that spray walls, fences or driveways. Constant wetting grows algae and, with well water, leaves rust.
- Gutters and downspouts. Overflow and splash-back at the base of a wall keep stucco wet. Extend downspouts away from the foundation.
- Vegetation. Trim shrubs back from walls so air circulates and sun can reach damp spots.
- Shade and canopy. Thinning branches over a roof or driveway helps both dry faster.
- Drainage. Standing water on pool decks, patios and walkways is a permanent algae nursery.
- Sealing and paint. Mildew-resistant paint and breathable sealers can slow regrowth, but they do not replace cleaning.
Warning signs that it is more than surface growth
Call a professional, and do not simply wash, when you see any of the following:
- Peeling paint with bubbling underneath, which can indicate moisture in the wall.
- Stains that appear on interior walls below a stained exterior section.
- Soft, spongy wood on fascia, soffits or trim.
- Cracked stucco with dark staining along the crack line.
- A persistent musty smell indoors.
Washing over unresolved water intrusion can push more water into walls. A building inspector, a roofer or a licensed mold assessor may be the right first call.
Where to start
For a house with moderate growth, a soft wash followed by a moisture checklist is a sensible plan. Homes near water or with heavy tree cover may need a recurring schedule. Ask any provider about their method and how they manage runoff, using the checklist in the guide to hiring a company. Pool owners can combine cleaning with the pool deck guide.
Frequently Asked Questions
Is the black stuff on my house toxic mold?
Most of the dark growth on exterior stucco and roofs is mildew or algae. Whether any given patch poses a health risk cannot be determined by looking, but exterior growth is a different issue from mold inside the home.
Can I clean mildew with vinegar or soap?
Mild solutions work on light growth, though they may need repeated application and may not kill the organism at the root. Heavy growth often needs a stronger cleaner.
Why does mildew come back so fast?
Humidity, rain, shade and sprinkler overspray keep surfaces wet. Cleaning removes the growth, but unless moisture is reduced it can return within months.
Should I paint after cleaning?
Often yes if paint is chalky or peeling. Let the surface dry fully first, and follow the paint maker's instructions about drying time and mildew-resistant primers.
Do I need a licensed mold professional to clean the outside of my house?
Typically not. Florida's mold licensing covers mold assessment and remediation, while routine washing of mildew off exterior surfaces is generally treated as a different job. Check with the state regulator if you are unsure.