Is that mold on the outside of your house?
Probably not, is the short answer. Most of what grows on the outside of a South Florida house is algae or lichen rather than mold, and most of it is a landscaping problem rather than a remediation one.
Planting hard against stucco on a shaded elevation. The most common exterior call we get, and usually the cheapest to solve.
- AlgaeWhat the black streaking on Florida roofs almost always is — a cyanobacteria, not mold.
- 18–24 inClearance to keep between planting and the wall. The single most effective thing on this page.
- $0What a lot of this costs to fix, because the answer is usually a pruning saw and a sprinkler head.
We get called out to exterior growth constantly, and a good share of those visits end with us telling somebody they do not need us. That is a slightly odd thing to publish, so here is the whole explanation instead.
The important distinction is not what it looks like. It is whether what you are seeing is a surface organism living on the outside of a building, or a sign that part of your wall assembly is staying wet.
What is actually growing out there
Four things get called mold. Only one of them usually is.
- Black streaking on the roof — algaeThose dark runs down a roof slope are almost always Gloeocapsa magma, a cyanobacteria that feeds on moisture and the limestone filler in roofing material. It is unsightly, it holds heat, and on tile it is nearly universal in this climate. It is not mold and it is not a building-health problem.
- Green or gray film on stucco — algaeA thin uniform film on a shaded elevation, often heaviest near the base and under eaves. Also algae. It tells you the surface stays damp, which is worth knowing, but the film itself is not damaging the stucco.
- Crusty gray-green patches — lichenRaised, circular, and much harder to remove than algae because it is anchored into the surface. Slow-growing and mostly cosmetic on stucco, though on tile it can lift and hold moisture.
- Genuine mold and mildewDarker, patchier, and typically concentrated where something is repeatedly wetted — under a leaking gutter joint, beneath a window that sheds water onto the wall, at a hose bib. Outdoors this is far less common than the other three, and where it turns up it is usually pointing at a specific water source.
When exterior growth is actually telling you something
The organism is rarely the problem. What matters is whether it is a symptom of a wall that no longer dries.
- Growth at the base of the wallA band in the lowest foot or two suggests splash-back, irrigation, or leaf litter holding moisture against the slab edge. That one is worth acting on, because it is the part of the assembly closest to the interior floor.
- Growth around a crack or a penetrationConcentrated growth at a stucco crack, a hose bib, a light fixture or a window return means water is being repeatedly delivered there — and stucco cracks let water into the wall, not just onto it.
- The same location, insideThe one finding that changes the conversation. Exterior growth plus a damp or stained interior surface at the same spot means it is no longer an exterior question.
- An elevation that never driesA north or east wall under mature canopy, or a preserve-edge lot with vegetation hard against the building. This is chronic rather than acute, and it is the situation most likely to eventually become a real problem.
- Nothing elseA uniform film on a shaded wall with a dry interior and no cracking is, genuinely, a cleaning job. We would rather tell you that than sell you an inspection.
The fixes, cheapest first
- Cut planting back from the wall Eighteen to twenty-four inches of clear air between vegetation and stucco. This is the highest-value item on the page by a distance: it restores air movement at the wall, lets sun reach the surface, and stops foliage holding water against the building after every rain. How to checkIf you can no longer walk between the shrub and the house, it is too close.
- Point the irrigation at the ground Heads set years ago and never revisited routinely spray stucco two or three times a week, indefinitely. Walk the zones once while they are running and watch where the water actually lands. How to checkDo this at the hour the system runs. Overspray looks completely different at 5am than it does at noon.
- Clear leaf litter from the slab edge A packed band of leaves and mulch against the wall holds water exactly where you least want it. Mulch should sit below the stucco termination, not against it.
- Fix what is dripping A gutter joint, a downspout discharging at the foundation, an air conditioning condensate line ending beside the wall. Concentrated exterior growth usually has a specific source directly above it.
- Thin the canopy rather than removing the tree Raising and thinning a mature tree gets air and light back onto the elevation without losing the shade that keeps your cooling bill down. An arborist is the right call here, not a remediation company.
- Soft wash — do not pressure wash A soft wash: low pressure, a solution that kills the organism, then a rinse. Worth knowing that it removes what is there and does nothing to stop it returning — on a shaded north wall or the north slope of a tile roof in this climate, plan on repeating it every few years rather than treating it as solved. How to checkIf a contractor proposes pressure washing your tile roof, get a second quote.
What we would tell you on the phone
If you describe uniform green film on a shaded wall with a dry interior, we will tell you it is algae, point you at the list above, and not schedule anything. That is not us being generous — an exterior cleaning is not our trade, and sending a remediation crew to a landscaping problem wastes your money and our day.
What we do want to hear about is exterior growth paired with anything happening inside: a damp baseboard, a stained wall base, a musty smell in the room behind that elevation. That combination is worth a moisture reading, and it is the case where an exterior symptom is genuinely worth acting on.
What people ask about this
What are the black streaks on my roof?
Gloeocapsa magma — the organism described above. It reads as streaks because it travels down-slope with rainwater, which is also why it shows worst below vents, in valleys, and anywhere runoff concentrates. The practical point is that this is a roof-cleaning job and not a remediation one. No mold contractor should be quoting you for it, ourselves included.
Will pressure washing get rid of it?
It will remove it and cause other problems. High pressure drives water into stucco and behind roof tile, abrades the surface so the growth returns faster, and on barrel tile it can crack tiles outright. Soft washing — low pressure plus a solution that kills the organism — is the standard for a reason. Be especially careful about anyone offering to pressure wash a tile roof.
Does exterior mold get inside?
Not by growing through a sound wall. Concrete block and intact stucco are not a food source and not a route. What does get inside is water — through a stucco crack, a failed window seal, or a penetration — and where that happens you will usually find growth on the interior surface at the same location. The exterior film is a clue about moisture, not a colonization route.
Should I use bleach on it?
For exterior algae on a hard surface, a diluted solution applied at low pressure is roughly what a soft wash is, so yes in principle. Be careful of the runoff — it will damage planting under the wall, which is the same planting you may want to keep. And it does nothing about the shade and moisture that let it grow, so it will come back on the same schedule.
How often does it come back?
On a shaded elevation with vegetation against it, within a year or two. On a wall that gets air movement and some sun, considerably longer. That difference is the entire argument for fixing the conditions rather than just cleaning the surface, and it is why the pruning saw does more for you than the pressure washer.
The communities this comes up in
Shaded elevations and mature landscaping are what drive this, so it turns up hardest in the communities built around preserves, golf courses and old canopy.
Not sure whether what you are looking at is algae or a wall problem?
Send a photograph and say which elevation it is on and what the room behind it is doing. If it is algae we will tell you so and point you at a pressure-washing contractor rather than booking anything.