Green film on siding is usually algae, black or gray specks are usually mildew, and soft green clumps are moss. All three grow where siding stays damp: shaded walls, walls behind shrubs and trees, under eaves and porch ceilings, and anywhere a sprinkler or a leaking gutter keeps wetting the wall. The USDA Forest Products Laboratory says mildew cannot decay wood, so the growth itself is mostly cosmetic. What matters is the dampness it points to, and how you clean it off. Siding makers mostly want a garden hose, a soft brush and a mild cleaner. Several rule out pressure washers, and the rest limit them.

What is growing on your siding?

  • Mildew is a fungus. The Forest Products Laboratory’s Wood Handbook chapter on finishing says it cannot decay wood, but it can feed on some of the natural chemicals in wood and on natural oils such as linseed oil in finishes. It usually leaves black deposits.
  • Algae is usually green, and it often grows together with mildew.
  • Moss is a simple plant that grows on surfaces in shady areas. Oregon State University Extension names Dicranoweisia cirrata and Bryum capillare among the common mosses on structures in the Pacific Northwest.
  • Lichen is an alga and a fungus growing together. OSU Extension says mosses and lichens both grow readily on roofs in the Pacific Northwest.

Mildew or dirt? Try the bleach test

The Forest Products Laboratory’s test takes two minutes. Put a drop or two of household bleach (5% sodium hypochlorite) on the dark spot. Mildew usually bleaches out within 1 to 2 minutes. If the spot stays dark, it is probably dirt, bleeding from the wood’s natural chemicals, or an iron stain.

Two other stains are common on cedar, and neither is mildew:

  • Iron stains show up as blue-black streaks around nails, or as reddish-brown rust. Iron from steel nails, or from steel wool and wire brushes, reacts with the tannins in cedar. The Handbook says oxalic acid removes the blue-black stains, and extractive bleeding too, but does nothing for mildew. The Western Red Cedar Lumber Association recommends hot-dipped galvanized, aluminum or stainless steel nails to prevent them.
  • Extractive bleeding is the wood’s own natural chemicals coming to the surface. The association says moisture usually causes it, and mild staining often washes away with rain over a few weeks.

Why it grows on some walls and not others

  • Still air and shade. The Handbook says mildew is common on walls behind trees or shrubs, where air movement is restricted and walls stay damp. Rodda-Miller, a Pacific Northwest paint maker, says it is most likely on north-facing walls and in shaded, damp areas.
  • Dew. Eaves, soffits and porch ceilings cool quickly, collect dew, and grow mildew. CertainTeed notes that on vinyl, mildew often shows up where rain never reaches to wash it off, such as under eaves and inside porch enclosures.
  • Extra water. LP says to keep lawn sprinklers from spraying siding and gutters working, because ongoing contact with water can discolor siding, shorten paint life and encourage fungus and mildew. Benjamin Moore adds dripping air conditioners to the list.
  • The climate. The Handbook puts the moisture content of wood outdoors at roughly 12 to 14 percent in most of the country, and slightly higher, 12 to 16 percent, in the Pacific Northwest.

For moss, sun is the long-term fix. Oregon State University Extension’s advice for roofs is that moss depends on shade, and that trimming trees to let in more sunlight works better than any cleaning.

Is it hurting the siding?

Mildew and algae on their own, no. The Handbook is direct about it: if paint is sound but discolored with mildew, wash it; it does not need repainting.

The dampness is another matter. The Handbook also says paint is not a preservative, and will not prevent decay if conditions favor fungal growth. Moss adds to it: Oregon State University Extension says moss does not directly damage roof materials, but it traps soil and debris that hold moisture, so wood stays wet longer. Wood trim and the bottom courses of wood siding under moss are in the same position. Heavy growth that comes back quickly is a reason to probe the wood underneath, especially sills, trim and the lowest boards. What causes dry rot explains how wet wood has to be before it rots, and dry rot in siding and trim covers what to look for.

How to clean mildew and algae off siding

For painted wood siding without cleaning instructions of its own, the Forest Products Laboratory’s method is:

  1. Mix one part household bleach with three to five parts warm water, plus a little powdered household detergent: the Handbook’s version is 1 gallon of bleach, 3 to 5 gallons of water and 1/2 cup of detergent. It says to use as dilute a solution as possible, and that nothing stronger than one part bleach to three parts water should be needed.
  2. Use powdered detergent, not liquid. Liquid detergents may contain ingredients that react with bleach to give off toxic fumes. Never mix bleach with ammonia, or with any cleaner that contains it.
  3. Protect yourself and the plants. Avoid splashing yourself or the landscaping. James Hardie says to shield landscaping, and Allura says to cover shrubs with a plastic drop cloth.
  4. Scrub gently with a bristle brush or sponge.
  5. Rinse with a garden hose pointed down, so water does not flood the back side of the siding.
  6. Let it dry. If the finish needs renewing, use one that contains a mildewcide. Benjamin Moore warns never to paint over mildew, because it grows through the new paint.

Where the siding maker names its own cleaner, use that instead. The makers do not agree on the details, and their instructions are written for their own products.

What each siding maker says about cleaning

Fiber cement

  • James Hardie. Its cleaning bulletin calls for a soft cloth or soft brush and a garden hose, mild dish soap for dirt, and a commercial mildew cleaner, such as Jomax, Mildew Check or Mold Armor, for mold and mildew. It says to start at the top and work down. If a pressure washer is used, it calls for a wide fan tip, at least 6 feet from the wall, and under 1,500 psi. It rules out sandblasting and acid washing, and warns that damage from improper cleaning may not be covered by the warranty. Hardie’s homeowner cleaning tips go further: rinse the siding every 6 to 12 months, skip the high-pressure power washer, and avoid harsh chemicals such as bleach or acid.
  • Allura. Its cleaning bulletin sets the same pressure-washer limits: a wide fan tip, 6 feet away, no more than 1,500 psi, and no water driven behind the siding. It names Jomax or Mold Armor for mildew and says to wash and rinse from top to bottom.

Engineered wood (LP SmartSide)

  • LP says to avoid pressure washers on SmartSide, because high-pressure washing can cause moisture intrusion, discoloration and paint damage, and can force water behind the siding. Clean with mild detergent and water using a soft brush, cloth or sponge, and rinse with a low-pressure garden hose, working from the bottom up. For mildew, LP suggests 5 parts water to 1 part Simple Green concentrate, applied with a sponge.

Vinyl

  • The Polymeric Exterior Products Association, the trade group for vinyl siding, says a power washer can be used with caution, following the instructions for both the siding and the washer. Hold it straight at eye level and never aim it upward, because that can force water behind the panels. For larger areas of mildew it suggests 30% vinegar and 70% water, and it warns against undiluted chlorine bleach and cleaners with organic solvents. It says to work from the bottom up.
  • Brands differ. CertainTeed does not recommend power washing its vinyl siding, citing moisture intrusion, damage and discoloration. Ply Gem allows 1,500 to 2,400 psi with a 40-degree tip, held level. Check which one is on your house.

Cedar and other wood

  • The Western Red Cedar Lumber Association says power washers are not recommended for any cedar product, and that power washing risks seriously damaging the wood and its ability to hold a finish. For mildew, it suggests a mild bleach solution, oxygen bleach being its preference, or a commercial mildew remover, and says restaining alone does not solve a mildew problem.
  • The Handbook explains the damage: power washing erodes the softer earlywood in each growth ring faster than the denser latewood, leaving ridges that are hard to repaint. It also warns against steel wool and wire brushes, which leave iron behind.

Hardboard, T1-11 and older painted siding

Many of these products are no longer made, so there may be no maker’s instructions to follow. The Handbook’s general method applies. Keep water out of the laps and away from the edges, where hardboard swells and T1-11 comes apart.

Why high pressure causes the damage it is meant to prevent

  • Water gets behind the siding. LP, Allura, CertainTeed and the vinyl trade group all warn about it, and the Handbook’s advice to rinse with the stream pointed down exists for the same reason. Water behind the siding sits against the sheathing, where no one sees it.
  • Wood erodes. The Handbook says the pressure needed to strip tightly bonded paint can easily erode wood deeply, and that if high pressure is needed to remove paint, the paint probably does not need removing.
  • Warranties. James Hardie says damage from improper cleaning may not be covered, and Allura says it will not take responsibility for it.

Where the wash water goes in Salem

Salem’s storm drains are not a disposal route for cleaning chemicals. Under the city’s stormwater code:

  • Section 71.050 bars anyone from discharging any pollutant, directly or indirectly, into the public stormwater system, and makes a violation an infraction.
  • Section 71.055 allows building wash water, provided chemicals, soaps, detergents, steam or heated water are not used.
  • Section 71.065 requires anyone who owns or occupies property to use practices that keep pollutants from leaving it and entering a stormwater system.

So a plain-water rinse can run off, but water with bleach, soap or detergent in it should not reach the street. Oregon DEQ’s washing guidance says wash water that contains detergents, soaps or cleaners should soak into the ground or go to the sanitary sewer, and that detergents should be phosphate-free and quick to biodegrade. It also says paint chips knocked loose by washing must not enter storm sewers, and suggests lowering the pressure or changing the cleaner to avoid detaching them.

Moss killers need the same care. Oregon State University Extension says not to let runoff containing moss-control products reach storm drains or waterways, and not to apply them when rain is expected within 24 hours. It does not recommend zinc strips for roofs, noting that zinc and copper are harmful to fish and other aquatic life.

Houses built before 1978

Paint on an older house may contain lead. The Forest Products Laboratory says power washing is not suitable for paint containing lead. The EPA says washing walls before painting likely does not disturb painted surfaces, but pressure washing that does is subject to the same containment rules as other renovation work, with no dust or debris, including in the waste water, allowed to leave the work area. Landscaping fabric does not count as containment.

In Oregon, the Construction Contractors Board and the Oregon Health Authority run those rules. A contractor whose work disturbs 20 square feet or more of painted exterior on a pre-1978 home needs a CCB lead-based paint renovation license. The rule does not cover people working on their own homes, but the gentle method is the safer one on an older house either way.

Hiring someone to wash the house

The Construction Contractors Board lists power and pressure washing for the purpose of cleaning, siding included, among the work that does not require a CCB license. Siding and painting work do require one. So a cleaning-only company may have no license to look up, and it is worth asking what pressure and cleaner it will use on your siding, and where the wash water will go.

When cleaning turns up damage

Washing puts your eyes on every wall, which makes it a good time to look for trouble: boards that feel soft, lower edges that have swollen, grooved panels peeling apart, or paint letting go in the same spots. If you find any of it, request a siding quote and we will follow up.

This page is general information, not professional advice about your specific situation. Have it assessed by a qualified professional before making decisions.