Stucco cleaning is a soft wash. A cleaning solution, applied at low pressure, kills the green algae and black mildew on the wall, and then a low-pressure rinse carries it off. Do not use a high-pressure spray on stucco or EIFS. It forces water behind the finish coat, into the wall, and it etches the texture. The same low-pressure method cleans vinyl siding, which cracks and takes water behind the laps under high pressure.
Why does stucco turn green or black?
Green or black stucco is almost always biological growth, not dirt and not age. Green is algae. Black is usually mildew or mold, sometimes with algae mixed in. Both live on the moisture that the wall holds.
Three conditions make stucco grow it faster than smooth siding:
- Moisture. Stucco is porous. It absorbs rain and holds it for longer than vinyl or painted wood. Clogged gutters, a short downspout, or a sprinkler that hits the wall keep the supply constant.
- Shade. A north wall, or a wall under trees, dries slowly after rain. On the Main Line the mature oak and tulip poplar canopy shades many walls all summer.
- Texture. The rough finish gives spores and pollen a large surface to hold, and small pockets keep water after the face of the wall looks dry.
Where all three meet, the growth is heaviest and comes back first. In Paoli and the other towns along the rail line, many older houses have stucco walls under big trees. The north side of those houses turns green first.
Neither color is a sign of failed stucco. It is a surface condition. But growth left on the wall spreads, holds moisture against the wall for longer, and can work into hairline cracks. That is a separate problem from the stain, and it is worth a look before any wash.
Why high pressure damages stucco, EIFS and vinyl siding
Stucco, EIFS and vinyl siding are cladding systems, not solid stone. Each one fails in its own way under a high-pressure spray.
Traditional stucco is a cement plaster over lath. High pressure etches and pits the finish coat, opens hairline cracks, and pushes water through those cracks to the sheathing and framing. It also drives the growth deeper into the pores, so it comes back sooner.
EIFS (Exterior Insulation and Finish System, also called synthetic stucco) is a thin finish coat over foam insulation board. It depends on a sealed surface to keep water out. A high-pressure spray can gouge the foam and force water past the finish. Trapped water is the most common way EIFS fails, and the damage is inside the wall where you cannot see it.
Vinyl siding takes gouges from a close nozzle, cracks in cold weather, and lets a spray from below drive water up behind the laps. Our guide to vinyl siding cleaning mistakes to avoid covers that surface in detail.
Before any wash, read the care instructions from the cladding maker if you have them. Most say low pressure only.
How to clean stucco, step by step
Soft washing stucco works because the solution kills the growth and the rinse only has to carry it away. For a small, low, sound section of traditional stucco, this is the method:
- Examine the wall first. Mark any cracks, chips, or soft or stained areas around windows. Do not wash over open cracks.
- Wet the plants and beds under the wall with clean water. Cover delicate plants.
- Apply the cleaning solution at low pressure with a pump sprayer or a soft-wash system. Start at the bottom and work up, so the solution does not run down dry stucco in streaks. Most mixes are diluted sodium hypochlorite with a surfactant.
- Let the solution dwell for the time on the label. Do not let it dry on the wall. On a hot or sunny wall, work in small sections.
- Rinse at low pressure from the top down, with a garden hose or a soft-wash rinse tip. Do not hold a tip near the finish.
- Rinse the plants again, and rinse any windows and trim that the solution touched.
Black mildew usually fades during the dwell. Heavy growth can take a second application. Stubborn stains that do not change are frequently not growth at all. Rust, dirt from a gutter overflow, or water staining from inside the wall each need a different fix.
Where stucco cleaning is the wrong job
Do not wash stucco with open cracks or failed patches. The water goes into the wall and stays there. Repair the cracks first, then clean.
Do not do your own wash on EIFS. The finish coat is thin and the foam behind it is soft. One pass with the wrong nozzle can cause damage that costs more to repair than the wash.
Do not clean from a ladder above the first floor with a sprayer in one hand. A wet wall, a wet ladder and a pump sprayer in one hand cause falls. Above that height, use a soft-wash system from the ground or hire the work out.
Do not wash a wall that stays wet from inside. Look for a stain that is darker at the bottom of the wall, near the windows, or after dry weather. That can be water behind the stucco. A wash hides that sign for a few weeks. Ask a stucco contractor to examine the wall first.
What a soft washing assessment covers
A good assessment looks at the wall, not only the stain.
- Substrate. Traditional stucco and EIFS take different solution strength and dwell time. Tap the wall: EIFS sounds hollow and feels soft under a firm press.
- Growth type and extent. Algae, mildew and lichen respond differently. Heavy growth can need a longer dwell or a second pass.
- Damage check. Cracks, chips and signs of old water entry get marked before the wash, not found after it.
- Moisture sources. Sprinkler heads, downspouts and grading that feed the growth go on the list, so the cause gets fixed as well as the stain.
- Shade and plants. Trees and shrubs against the wall keep it damp. They matter even when they are outside the scope of the wash.
On stucco, the most green and black growth is always on the north side and on the highly shaded walls. We set the mix for those walls, and we soak the plants under them before the wash and rinse them again after.
WashLabs does soft washing on stucco, EIFS and vinyl siding at $0.35-$0.70 per sq ft. For the difference between the two methods, see soft washing vs pressure washing.
How to slow the regrowth
A wash removes the growth on the wall. The conditions that grew it stay there until something changes. Four changes slow it down:
- Cut back trees and shrubs that shade the wall and keep it damp.
- Turn sprinkler heads away from the stucco.
- Clean the gutters and make sure the downspouts carry water away from the walls and the foundation.
- Check the grade at the base of the wall, so water does not pool against it.
None of these stops algae for good. It is part of the outdoor environment here. Plan a wash at least once every two years. Clean sooner when the green comes back, and expect the shaded north wall to need it first. Spring and early summer, before the growth has a full season, are good times for a first wash. A soft wash also works in any other season when the wall is above freezing.
Text a photo of the wall to (610) 665-3995. We reply within 24 hours.