Diagnosis

Dark Streaks and Staining on Treasure Coast Stucco

Most stucco staining is a drainage problem wearing a disguise. Read where the streak starts and what shape it takes, and the cause usually names itself.

Staining is a drainage story

Almost every stain on a stucco wall was drawn there by water. Something concentrated the flow, the water ran down the face carrying dirt or minerals or spores, and the wall kept a record. That is why the shape of a stain tells you more than its color.

Look at a streaked wall and ask three things. Where does the mark begin, what sits directly above that point, and does it spread or stay narrow as it runs down. A stain with a clear starting point almost always has hardware, a joint, or a roof edge above it doing the concentrating. Most staining on Treasure Coast homes is a drainage issue rather than a failure of the stucco, but a few patterns do mean water is getting into the wall, and those are worth separating out before you erase the evidence with a pressure washer.

Reading the pattern before you clean anything

Take photos first, on a dry day, in flat morning light. Once a wall is washed you lose the map, and the map is the useful part. Photograph each elevation whole, then close up on anything with a defined starting point. A handful of patterns come up again and again on homes here, and each points somewhere specific.

• A narrow streak running down from a single point: a fixture, fastener, bracket, or hose bib is shedding water and whatever it carries
• Twin streaks from the bottom corners of a window: water is running off the sill and over the edges instead of dripping clear
• A wide even band under the eave: roof runoff washing dirt down the fascia and onto the stucco
• Repeating vertical stripes along a whole elevation: tile roof runoff concentrating at the same points every storm
• A stained band about two feet up from grade all the way around: rain splashing soil and mulch back onto the wall
• A fan spreading downward out of a crack: water is leaving the wall through that crack rather than entering it
• Growth confined to one shaded wall with no defined source: biological, not a leak

Green and black growth, and why the shaded wall always loses

Our climate is close to ideal for algae, mildew, and lichen. Humidity stays high most of the year, and shade from live oaks, deep overhangs, or a neighboring house means some surfaces never see enough sun to dry properly. The north side of a house in Palm City or Stuart goes green before any other wall on the property, every time.

Algae reads green to nearly black and grows in soft, spreading patches. Mildew looks like fine dark speckling and is easy to confirm: put a drop of diluted bleach on it and wait a minute, and mildew lightens quickly while dirt does not. Lichen is the crusty gray disc that sticks hard and needs real dwell time to release. None of it is eating your stucco. Growth becomes a diagnostic signal only when it concentrates in one patch on a wall that gets the same sun as everything around it, because that patch is staying wetter for a reason.

Rust bleed and metal staining

Orange and brown staining means metal, and on stucco that metal is one of three things. It is a fastener or accessory on the surface, it is corner bead or lath inside the wall, or it is iron in the water hitting the wall.

Surface sources are the easy version. A rusting light fixture screw, a shutter bracket, a bolt on a railing. The stain starts exactly at the hardware and runs straight down in a narrow line. Replace the fastener with stainless, seal the penetration, clean the wall.

Rust from inside the wall is the version that matters. It shows up along a crack rather than under a fixture, most often on an outside corner where the corner bead runs, and usually with a raised ridge you can feel with a flat palm. That is corroding metal expanding and pushing the stucco off the wall. Salt air off the ocean and the Indian River Lagoon makes it far more common in Jensen Beach, Hobe Sound, Port Salerno, Vero Beach, and Sebastian than inland. Cleaning that stain does nothing. The metal has to come out.

Sprinklers, well water, and the splash zone

Irrigation is the most common staining cause we see on Port St. Lucie homes, and it leaves two marks. The first is a rust colored band where heads throw water onto the wall, because iron in well water and in much of the local irrigation supply oxidizes on contact with stucco. The second is a chalky white band from mineral content in hard water drying on the surface.

The fix is aim, not chemistry. Walk the yard while the zones are running, at the hour they actually run, and watch what hits the house. Heads settle and get bumped by mowers, so one that was fine at install can be soaking the same eight feet of wall twice a day for years.

Then there is the splash zone at the base. Rain off the roof hits mulch, sand, or bare soil and bounces dirt back onto the stucco, which is why so many houses carry a dirty band about knee high. That part is cosmetic. What is not cosmetic is a wall base kept wet because grade slopes toward the house or mulch is piled over the weep screed.

Cleaning without wrecking the wall

Stucco does not need high pressure, and high pressure is how walls get damaged. A pressure washer in the wrong hands cuts lines into the finish, blows water past the sealant at windows, and strips the texture off a sand finish in one pass. We have repaired more walls damaged by cleaning than by dirt.

The right approach is low pressure with chemistry doing the work, which is what soft washing is. Wet the wall and the plants below it, apply a diluted sodium hypochlorite solution with a surfactant, give it dwell time, agitate stubborn areas with a soft brush, then rinse at hose pressure. Work early or in shade, because our sun dries the solution before it can work. If the wall carries an elastomeric coating, stay well back, because a lifted edge turns a cleaning day into a repair.

• Low pressure and dwell time instead of high pressure
• Pre-wet and post-rinse all plants, hard surfaces, and screen enclosures
• Never aim up under a weep screed, a soffit vent, or a window head
• Soft brush the lichen rather than trying to blast it off
• Test any solution on a small, low, out of sight area first
• Do not clean a wall you already suspect of holding water until it has been looked at

When a stain is telling you about water inside the wall

A handful of stains are worth acting on rather than washing. The clearest is one that returns in the same shape within weeks of a proper cleaning, because something keeps delivering water to that exact spot.

Rust bleeding out of a crack, especially with a ridge you can feel, means corroding metal in the wall. White crusty deposits mean water is moving through the masonry and carrying salts out with it, which is a moisture story rather than a surface one. A patch that stays dark two or three days after rain while the wall around it dries is holding water. In each case, sound the wall around the stain with a knuckle, listen for the change from a hard ring to a dull thud, then look up. Something above it is putting water where it should not be.