Algae growth on vinyl siding (C) Daniel Friedman Algae & Mold Stains on Building Exteriors

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Building exterior walls, siding, trim algae or mold stain cause diagnosis & cure:

This article describes algae or mold stains on wood, vinyl and other building exterior surfaces, explaining the common cause as well as methods of cleaning and prevention.

This article series catalogs the different types of stains or discolorations found on building exterior surfaces: walls, roof, trim, walks, patios, decks, etc. Our page top photo shows algae growing on the shaded side of a vinyl-clad building

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Cause & Cure for Algae & Mold Stains on Building Exterior Surfaces

How to Diagnose & Prevent Mold or Dark Algae on building Exterior Walls, Siding, Trim, Roofs

Algae growth on painted wood siding (C) Daniel Friedman

Both mold and algae can show up as dark (or other colored) stains on building ecterior surfaces. These growths may appear on natural wood, stained wood, paitned wood, vinyl, or other plastics, aluminum, and many other materials, even masonry or stone.

Because by visual inspection it is not always obvious if we are seeing an algae or a mold growth on building surfaces, we note that either algae or mold stains on suilding surfaces are principally due to a combination of shade, moisture, and a building surface that uses a material or coating that is particularly conducive to algae or mold growth.

[Click to enlarge any image]

Our photo (above) shows black staining on painted wood clapboards on an older building in New York state.

Without closer examination and maybe a lab test, we're not sure if the black stains are algae or a mold, but in either case the causes of these unsightly stains are the same: moisture, shade, and from the stain pattern, possibly the absence of insulation and a moisture barrier in the wall structure.


Algae tends to produce stains that are amorphous in shape with larger fairly uniform patterns that sometimes follow water or moisture tracking. I think the black stains above are probably an algae, perhaps the same algae genera/species we discuss at ALGAE STAINS on ROOFS. Conversely, the spotty stains shown in the reader-photo below, those spots are likely to be a fungus: mold.

Mold spots on soffit and wood beams (C)

From upper right in this photograph we see a half-round, almost certainly under-sized rain gutter (risking spill-over and water damage), then a solid, un-perforated eave or soffit cover that sports round mold spots, then a natural-colored wood beam that also has a few mold spots.

It also appears that this home is designed with no under-roof ventilation system - which might work if it's a "hot roof" design.

Several conditions might invite this mold growth and staining

Watch out: don't raise a health panic about exterior building mold; mold on building exteriors, regardless of the mold genera/species present, is rarely going to be other than a cosmetic concern; that is, it's not likely to be a health concern for building occupants. Don't allow fear-of-mold to lead to inapparopriately-expensive "mold remediation" work on a building exterior.

On the other hand, if a building is suffering leaks or other water problems at the building exterior or envelope, those conditions can sometimes also add to the risk of harmful mold growth in the building interior or in building cavities (ceilings, walls, crawlspaces). And wet conditions may also translate into wood rot damage to any building component.

Crawl space mold (C) Daniel Friedman

Above: in contrast to more-even gray weathering or perhaps black algae and mold, white, green and gray mold growth may also show up on wet wood framing and on the underside of subflooring over a wet basement or crawl space. If the interior of your building exhibits mold growth such as in our photo, that's likely to be a risk to building occupants.

Weathering to Gray on Un-Painted Wood Outdoors

Weathering, graying, and perhaps some mold on exposed wood timbers (C)

What about the graying stains on the exposed wood beamsof the home shown above. The reader submitting the photo wrote:

... we have just converted a bungalow to a house (completed summer 2016). Already there is mild growing on the soffits and external oak frame.

I am very concerned as to the cause (as this is a very recent build) and would be very grateful if you could assist with a diagnosis. The front of the property is north west facing. Photos of the mold. attached.

Wood naturally weathers to gray

First let's note that wood framing left exposed to the weather will normally change over time to a gray or silver-gray color. During that weathering period that can take a few years, the color of the wood will be uneven and sometimes not totally beautiful. Weathering of exposed, un-painted wood to gray will occur fastest where there is more sun exposure and is the natural result of photo-oxidation of the wood surface.

Wood weathering vs mold  on exterior (C) InspectApedia JP

Above is another image of the home under discussion. The pink arrow points ot the mold-spotted un-vented roof eave. At left in the photo a green arrow points to what is at least normal gray weathering of exposed wood post and beam additions to the home, though mold growth could also be present on that wood.

Our red arrow [Click to enlarge any image] points out a helpful diagnostic clue relating wood weathering, graying, mold growth? and water: it's no coincidence that the post at the red arrow shows more darkening close to its base: that's where rain splash-up will wet the post bottom.

Mold also stains exposed wood

However mold growth, more likely to occur on un-sealed, un-painted wood, will add its own two-cents to weathering-casued color change and variation in wood appearance on the building excterior, and in part because of its less homogenious pattern, it may be less attractive. You can clean off such mold growth on exposed wood but it will of course recur unless the wood is treated or sealed.

All mold genera/species are everywhere all the time in outdoor air - though at varying, ususally low concentrations.

When mold grows on a building surface it's because of a combination of conditions: some material that is particularly good food for a particular mold genera/species (or usually more than one) combined with moisture and often as in your photos perhaps shade.

A short answer: clean the surfaces, let them dry, seal with sealant or paint that includes a fungicide, correct site conditions or building conditions that help reduce future mold or algae growth.

Note that when you see stains on a building exterior that are biological in nature (that is not running paint oxides, road dirt, sap, etc.) they are often algae, not mold, though the causes and cures remain the same for both exterior algae stains and exterior mold stains. Algae stains are often black, brown, green, sometimes red or even yellow; mold stains can be in those colors as well as others.

Removing or Preventing Mold on Exteriors

Various species of outdoor mold will grow on just about any surface with sufficient moisture and heat. In new construction, it can be minimized by storing wood off the ground and providing adequate ventilation. Although sealers and stains contain a mildewcide, any mildew should be removed before finishing or refinishing, or it will continue to grow through the new finish. As detailed in Best Practices Guide to Residential Construction:

To remove mold ("mildew") from building surfaces, use a sodium hypochlorite solution, which can be made with household chlorine bleach. Depending on the severity of the problem, the solution should range from 1 to 8 parts bleach to 1 part water.

Spray the solution onto the siding (avoid sprayers with aluminum parts), starting at the top and working down. If two applications do not remove the stains, you may need to scrub in the solution with a brush. Thoroughly rinse everything with water.

Bleach can harm plants, discolor the finishes on trim, and corrode aluminum, brass, and copper. It is best to cover plants with tarps and protect any stained or painted surfaces.

When the cleaned wood surfaces are dry (below 18% moisture in wood) you might choose to apply a clear sealant or a paint, either of which will want to include a fungicide to slow down further mold or algae growth. Note I say "slow down" since 100% prevention is difficult.

Watch out: can you rely on clear sealant to stop mold or algae growth on un-painted wood? Not completely. First, if you paint or apply a clear sealant over mold, the mold may simply grow through the new coating, or it may continue to grow underneath a clear coating - uglier than ever and more difficult than ever to remove. So before coating, such surfaces need to be cleaned of mold or algae.

Mildew? ... Really? Speaking accurately, which would be refreshing in politics as well as forensic building diagnosis, there is no mildew found on building surfaces even though people use that term.

Mildew, which is comprised of two groups of fungi within the larger class of molds, grows onlyOn living plants: mildew is an obligate parasite that is found, for example, on grapes.

The mildew sub-group of molds includes itself two families: Oidium-Erysiphe (powdery mildew) and Peronosporacae (downy mildew). But in both cases, these grow only on living plants.

So if you see what you think is mildewOn a building exterior, unless the exterior of your building is made of grape leaves, it's mold, but it's not not mildew.

However because the terms mildew and mildewcide are used very widely among the building trades we have permitted that topic name in this article. To learn more about mildew and to see photos of what real mildew looks like

Also see our photos of mold that is often mistaken for mildew
and finally,

For complete information about recognizing and dealing with mold growth in or on building surfaces

Mossy roof (C) Daniel Friedman

Related articles on algae & mold stains on building exteriors


See STAINS on BRICK SURFACES where we discuss all sorts of stains and paint removal from brick exterior or interior surfaces; these methods also help clean stone surfaces.




Continue reading at STAIN DIAGNOSIS on BUILDING EXTERIORS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see STAINS on & in BUILDINGS, CAUSES & CURES - home,



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