Mildew is a flat surface fungus that wipes off; mold grows into what it feeds on. On most Wilmington siding the black streaking is actually algae - here is how to tell the three apart.
Short answer: mildew is a flat, powdery surface fungus that sits on top of a material and usually wipes off. Mold is fuzzy or slimy, grows into whatever it is feeding on, and needs a moisture source fixed rather than just cleaned. And the thing most Wilmington homeowners call "black mold" on their siding is usually neither - it is algae, which is a plant, not a fungus, and which is the single most common growth on a Cape Fear home.
Both are fungi, so the line between them is less scientific than most people expect. In everyday use the distinction that matters is how they grow:
The practical consequence is the whole reason to tell them apart. Mildew is a cleaning problem. Mold is a moisture problem that is also a cleaning problem, and cleaning it without fixing the water is how it comes back in a month.
This is the correction worth making before anything else, because it changes what you should be paying for. The dark streaking that runs down a wall or dirties a roof around Wilmington is very often algae. On roofs it is typically a blue-green algae that carries a dark protective pigment, which is why it reads as black stains running down a shingle slope. On siding, especially shaded north and east walls, you get a mix of that plus green algal film.
Algae is a plant. It photosynthesizes, it needs light and moisture, and it does not eat your house the way a fungus does. That makes it far less alarming than "black mold" sounds and it responds well to the right chemistry. It is also why scrubbing does so little: you can remove the visible layer and leave a living film that repopulates in weeks.
Standing at the wall, three checks get you most of the way:
Smell helps too. A persistent musty, earthy odor near an area, especially indoors or in a crawlspace, is more consistent with mold with an active moisture source than with surface mildew.
The Cape Fear coast supplies every input at once. Humidity stays high for most of the year, so surfaces spend long stretches damp even without rain. Warm nights mean dew sits on north-facing walls into the morning instead of burning off. Mature live oaks and pines across Forest Hills, Sunset Park, and the older Middle Sound streets hold shade and drop organic litter that feeds growth. Salt aerosol off the Atlantic and the river settles as a haze that holds moisture against the surface. Then spring pollen adds a fine organic film across everything.
The result is a straightforward pattern: on most homes here the shaded elevation greens or darkens noticeably faster than the sunny one, and it is normal for the difference between two walls of the same house to be dramatic.
Mildew favors surfaces that stay damp and shaded but are not soaked. Around a Wilmington property that usually means the underside of a covered porch ceiling, painted trim and soffits on the shaded side, screened enclosure framing, outdoor furniture cushions and umbrellas left out through the season, and the north face of a fence. These are also the surfaces most likely to come clean with modest effort, because the growth genuinely is sitting on top.
Mold on an exterior becomes a real concern when it is concentrated where water is being delivered repeatedly rather than spread across a whole elevation. Watch the underside of eaves below a gutter that overflows, the band of siding behind a downspout elbow, deck boards under a planter that never dries, window sills and the wood trim below a failed caulk joint, and crawlspace framing. In those places the growth is downstream of a defect. Cleaning it makes the wall look right for a while and does nothing about the water.
Pressure alone does not kill anything - it removes what is loose. Blasting a wall knocks the visible layer off, leaves living material in the pores, and drives water into places it should not go. That is why a purely mechanical wash regrows so quickly, and it is why the correct method on siding, roofs, and painted trim is soft washing: a properly diluted cleaning solution does the biological work while low pressure only rinses. The solution has to dwell long enough to kill the organism, then be rinsed thoroughly. We walk through the tradeoff in more detail in our guide to soft washing versus pressure washing, and the damage side in whether pressure washing damages vinyl siding.
Household bleach straight from the bottle has two problems on an exterior. Diluted wrong it either does not kill the growth or it is strong enough to damage plants and finishes. And with no surfactant it sheets off a vertical wall in seconds, so it never dwells long enough to work. People then compensate with pressure, which strips the surface and, on older vinyl, removes the chalky oxidized layer unevenly and leaves a blotchy wall.
The other half of the failure is that scrubbing is a surface action against an organism with depth. On porous concrete, unsealed wood, and grout, the living structure is below what a brush reaches.
A small patch of surface mildew on a porch ceiling, a section of fence, or outdoor furniture is a genuinely reasonable DIY job. Work in the shade so the solution does not flash off, wet the surrounding plants before and after, apply a properly diluted cleaner with a pump sprayer, let it dwell for several minutes without drying, agitate gently with a soft brush, and rinse thoroughly. Wear eye protection and gloves.
The job stops being reasonable at height, on a whole elevation, on a roof, and on anything where you would be spraying upward into siding laps from a ladder. Roofs in particular are a bad DIY target: walking a damp shingle slope is dangerous, and the wrong cleaner or too much pressure strips granules and can affect a shingle warranty.
Some things look like a cleaning problem and are not:
Exterior washing is an outside-surface service and does not address what is behind the sheathing or under the floor. If there is a musty smell indoors, staining on interior drywall or ceilings, condensation on ductwork, or visible growth on crawlspace framing, that is a different trade - a moisture or remediation specialist, and often a plumbing or crawlspace encapsulation question. Anyone who offers to solve an interior mold problem by washing the outside of the house is selling the wrong service.
Faster than inland, and it varies more by elevation than by house. A sunny, well-drained wall can hold up well for a good while after a proper soft wash. A north wall under oak canopy near Greenfield Lake, or a shaded elevation out toward Myrtle Grove, will show early regrowth sooner. The variables that actually drive it are shade hours, airflow, proximity to standing water and heavy canopy, and whether gutters are keeping water off the wall - not the brand of chemical used. A realistic expectation for most homes on this coast is annual attention to the shaded elevations, with the sunny sides going longer.
Any effective exterior cleaning solution is hard on foliage in concentrate. The routine that protects a yard is boring and it works: pre-wet all beds and foundation plantings thoroughly, keep them wet through the application, rinse them again afterward, and keep pets and children off treated areas until surfaces have been rinsed and dried. On lots that back onto tidal creeks or drainage toward the river, runoff control is not an optional courtesy - it is the difference between a responsible job and a problem downstream.
The useful question to ask about any quote is what is actually growing and what method is being matched to it. A wall of algae, a mildewed porch ceiling, and a rot-driven mold patch are three different jobs, and a single per-square-foot number applied sight-unseen to all three is a guess. If you want the surfaces most affected handled properly, start with house washing in Wilmington for siding and trim, or roof cleaning if the streaking is on the shingles - and get the gutter and flashing question answered before, not after.
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