Green, gray or black film on siding is usually algae, mildew or mold feeding on dirt and moisture, most often on shaded north and east walls. It is mainly cosmetic on vinyl, but persistent damp can point to drainage, gutter or landscaping problems and can affect wood. Clean with a low-pressure solution and a gentle rinse, and fix the moisture source so it does not return.
The short version
- Growth on siding needs moisture, shade and a thin layer of organic film. Remove any of the three and it slows.
- Vinyl, fiber cement, painted wood and stucco all clean differently. High pressure can damage each of them in its own way.
- Water behind the siding is the real hazard of aggressive cleaning, so low pressure and downward spray angles matter.
- Repeated growth in one spot usually points to overflowing gutters, sprinklers, tight plantings or poor airflow.
- Mold on siding is different from mold inside the home. Interior mold or a musty smell calls for a different kind of help.
What that green and black film is
Most of what homeowners call mold on siding is a mix of organisms: green algae, mildew, cyanobacteria and sometimes lichen, living on a thin film of pollen, dust and airborne organic matter. Each needs water, a food source and a place to settle. Siding supplies all of those on the wrong side of the house.
Color helps with guesswork, though it is not exact. Green tends to be algae. Gray, black or dark speckled patches are often mildew or dark-pigmented growth. Orange or rust-colored streaks may be something else entirely, such as iron-rich water from a well or sprinkler, or rust from a fastener. If a stain does not look like growth, suspect minerals.
Most of it stays on the surface. That is the reassuring part. It can look alarming on a white vinyl house, yet the siding underneath is usually fine.
Why it grows on some walls and not others
Look at where your house is dirty. Growth concentrates on the north and east sides, where the morning sun is weak and walls stay damp. It also gathers behind shrubs that block airflow, near the ground where splash-back from rain keeps the lower courses wet, and beneath overhanging gutters that leak or overflow.
Northern Illinois adds its own twists. Humid summers keep surfaces damp for long stretches, mature trees shade walls well into the afternoon, and lots near ponds and the Fox River or other waterways tend to hold humidity. Sprinkler systems that spray the siding feed the problem, particularly with well water that carries minerals.
Newer houses sealed tightly with vinyl or fiber cement can look worse sooner than you would expect, because the surface is smooth and shows every film.
Does it matter, or is it only cosmetic?
On vinyl and fiber cement, surface growth is mostly an appearance issue. Left alone for years, a heavy film can hold moisture against the surface and make later cleaning harder. On wood siding, constant dampness is a bigger concern, because wood that stays wet can eventually soften, peel its paint and rot. If you can push a screwdriver into a board with little resistance, stop cleaning and call a carpenter.
The bigger message is often about the cause. Persistent damp on one wall can mean a gutter overflowing above it, a downspout that discharges against the foundation, a hose bib that leaks, or landscaping that holds moisture. Cleaning without fixing those just resets the clock.
As for health, outdoor surface growth is generally not the same issue as mold inside a home. People with allergies or respiratory problems may be sensitive to spores, and anyone with health concerns should ask a physician. If you notice musty odors or visible growth indoors, that is a separate matter for a qualified remediation professional, not a siding wash.
Cleaning each siding type
Vinyl cleans easily with a mild solution and low pressure. The main hazards are water forced up under the laps and heat or force that warps panels. Washing from the top down with a downward spray angle helps. Fiber cement is durable but porous in places, and aggressive pressure can mark or roughen the surface or drive water into joints and edges.
Painted wood needs the gentlest approach. Force raises the grain, strips paint and drives moisture into the board. Stucco absorbs water and can hold stains, so pressure should be low and rinse thorough. Aluminum dents and can chalk, and brick and stone tolerate more but can lose mortar if blasted.
In general, siding manufacturers recommend gentle cleaning with a soft approach and warn that a pressure washer can force water behind the siding. Check your manufacturer's care instructions if you have them. For the method differences, see Soft Washing vs Pressure Washing: Which Does Your Surface Need?.
- Vinyl: low-pressure solution, downward angle, careful around seams
- Fiber cement: low pressure, avoid joints and edges, rinse thoroughly
- Painted wood: gentlest approach, inspect paint and boards first
- Stucco: low pressure and thorough rinse; stains may linger
- Brick and stone: more tolerant, but protect mortar and landscaping
A careful look at DIY
Plenty of homeowners clean siding themselves, and a hose, a soft brush and a mild cleaning solution can work on a small area. Test on an out-of-the-way spot first. Work from the bottom up when applying a cleaner so it does not streak, and from the top down when rinsing. Keep the spray angled down.
The risks are well known. Pressure washers used close to the wall force water behind laps, which can soak sheathing and insulation. Mixing household chemicals can produce hazardous fumes, and strong solutions can kill shrubs and stain concrete. Ladders on wet ground are a common source of falls.
For a two-story house with growth all around, or siding that is hard to reach, the time and risk start to outweigh the savings. Rob's house washing is quoted after seeing a photo, since the size of the house, height and amount of growth shape the work.
Stopping the regrowth
Growth will return if the conditions do. The most useful fixes are the dull ones. Clean gutters so they do not overflow onto the wall; see How Often Should You Clean Gutters in Northern Illinois?. Redirect sprinklers away from siding. Trim shrubs back from the wall so air can move, and keep mulch from piling against the lowest course. Prune trees to let in more morning sun.
A cleaning every year or two keeps films from becoming deeply stained. Shaded walls may need it more often, while sunny ones can go longer. Expect some return in about a season or two in the worst spots, as growth restarts from airborne cells.
Be skeptical of coatings and promises of years of protection. Some products help slow regrowth, and others add little. Ask what the product is, what it does to the finish, and what happens at re-application.
When to stop and call someone else
A cleaning contractor is the right call when the siding is sound and the problem is film. Call a carpenter or siding installer when boards feel soft, paint is peeling in sheets, panels are cracked or loose, or caulk has failed. Call a roofer or gutter installer when the moisture comes from leaking roofs or poorly pitched gutters.
Look for signs of water getting inside the wall: stained interior walls near the exterior, bubbling paint inside, or a damp smell. Those need a building professional, because cleaning the outside will not resolve them.
If you are unsure which category your house is in, text Rob a few photos at (815) 451-0106. He will tell you whether the job is a wash, a repair first, or both.