Algae growth on vinyl siding (C) Daniel Friedman Encyclopedia of Stains on Building Exteriors

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Building exterior stain cause diagnosis & cure:

This article provides a catalog of the different types of stains or discolorations found on building exterior surfaces: walls, roof, trim, walks, patios, decks, etc. We discuss each stain by color, type, cause, and cure. Diagnostic & Photo Guide to Stains on Chimneys, Foundations, Masonry Surfaces, Roofs, Walls, Floors, Concrete.

We include links to references useful in the identification of algae, moss, lichens, pollen and mold. Our page top photo shows algae growing on the shaded side of a vinyl-clad building

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Cause & Cure for Stains or Marks on Building Exterior Surfaces: roofs, walls, decks, siding, trim

Vertical stains on siding, probably algae (C) InspectApedia  Hugh Cairns RHI British Columbia

In the following guide we list types of stains by stain color & appearance, by building location or material, and by stain cause.

[Click to enlarge any image]

We distinguish among the following stuff that may stain or be found growing building roofs, walls, or other surfaces, with extra focus on asphalt shingle roofs as well as other roofing materials such as wood shingles, wood shakes, roll roofing, and even slate or tile roofs.

Our photo (left) contributed by professional home inspector Hugh Cairns in British Columbia is a classic example of the combination of thermal bypass temperature variations marking the locations of studs in a wood framed exterior wall.

Temperature variations across a building surface can often explain the pattern of algae growth and in some cases mold growth on building surfaces as well as the more harmless accumulation of dust and debris. [Click to enlarge any image at InspectApedia] Mr. Cairns writes:

The attached photo shows vertical staining on vinyl siding on a home in central British Columbia.

The repeating vertical pattern of the organic material is consistent with the placement of repeating exterior wall studs. The material is untested and as such its organic composition is unverified, however the appearance of the substance is consistent with algae or mold.

The patterned staining may be a result of surface condensation due thermal bridging, where localized surface temperatures from each piece of framing lumber act as a conduit for heat transfer allowing moisture and dust particles to gather and create favorable conditions for organic growth. [1]

Some types of building stains or discoloration are only cosmetic in nature, while others may indicate growths that are likely to reduce the life of the material such as roofing, siding, or trim.

Algae stains on concrete (C) Daniel Friedman

Algae often causes black stains on roof surfaces and green stains on other building exterior surfaces such as concrete (photo above left), decks, and building siding (photo below).

Green Algae stains on siding (C) Daniel Friedman

See ALGAE, FUNGUS, LICHENS, MOSS COMPARED for additional photos (including algae in the microscope) and details.

Below the black streaks on light colored asphalt roofing (photo below left) are usually identified as a specific algae.

More photos of black algae stains on roofs are at BLACK ALGAE STAINS On Asphalt Shingles.

Photograph of possible asphalt shingle bleed-through or extractive bleeding on asphalt roof shingles  (C) Daniel Friedman

For details about why roof algae forms and what can be done about it see ALGAE STAINS ON ASPHALT ROOF SHINGLES.

Black mold growth on building exterior wall siding (C) Daniel Friedman

Our photo at above right is a magnified view of tiny black dots on building siding that will probably be identified as a fungus, but algae is more common than mold on most exterior siding, particularly vinyl siding where it may appear as a green, brown, or black "stain".

Algae growth on painted wood siding (C) Daniel Friedman

Siding stain clean-off recommendations

From 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.

Black algae stains on exterior wall, Corsica E.R. 2016 (C)

Above: black stains on the exterior of a church wall on the island of Corsica in 2016, courtesy of E.R. Typically stains like those shown above are found to be either a dark algae or sometimes a combination of algae and fungus.


A reader asked about black streaks appearing down the building siding and originating from tarred top tile covering of a roof parapet.

If that's the stain source on your building, ask the installing roofer to either replace the tar (more-likely flashing cement) with something non-staining (an ugly job) or you might experiment with a sealant or coating, even silicone, atop the existing material.

The problem is getting any cover-over to bond. So ultimately you may need to cover the tarred joints with aluminum flashing that's siliconed in place to stop the weathering of what looks like poorly-formulated bleeding-out flashing cement.

Photograph of chimney soot stains on a roof. Cosmetic mold (C) Daniel Friedman

Mold on a building exterior surface indicates damp or shaded conditions, is not normally itself a hazard to building occupants, but it might indicate conditions that have also produced an indoor mold hazard.

Photograph of reddish brown chimney rust stains on an asphalt shingle roof.

Effloresence white stains on a chimney (C) Daniel Friedman

Reader Question: what are these black stains on our concrete porch?

Dark stains along concrete slab exterior porch perimeter (C) InspectAPedia Rhonda

4/1/2014 AUTHOR:Rhonda (no email)

COMMENT: I have a problem with my front porch, it runs the whole length of the porch, tried pressure washing it, it didn't come off and seemed to make it worse.


Rhonda, the black marks in your photo (shown here so that others may comment) may be due to moisture variations in the slab, moisture from below, or moisture entering the house-slab juncture, possibly bearing staining contaminants.

But most likely we're looking at a combination of variation in concrete hardness and moisture level.

The observation of those round-ish blot stains on the concrete slab surface in front of the entry door suggest that a door mat was placed there previously, holding moisture in the slab (possibly during curing). Does the stain pattern vary with dry weather?

This comment was originally posted at MOLD APPEARANCE - STUFF THAT IS NOT MOLD

At CONCRETE STAINS we categorize concrete stains and discolorations into diagnostic groups and we also discuss the causes of concrete stains or discoloration and methods for removing concrete stains. That article also describes products for deliberate coloring or staining of concrete floors, walls, or ceilings.

Roof stain or lichens or moss caused by trees (C) Daniel Friedman

Stained clapboards (C) Daniel Friedman

Excess moisture in wood species such as cedar, redwood, Douglas fir, and mahogany can dissolve the natural tannins in the wood and cause them to migrate to the surface, leaving a reddish-brown stain on the finish.


Stained clapboards (C) Daniel Friedman

Extractive bleeding roof stains (C) Daniel Friedman

Lichens are not the same growths as moss or algae, and are difficult to remove without damaging the roof surface. Also see ALGAE, FUNGUS, LICHENS, MOSS COMPARED.

Photograph of lichens growth on asphalt shingles (C) Daniel Friedman

Algae growth on painted wood siding (C) Daniel FriedmanBoth 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.

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.

Details about mold and algae stains on building exteriors including wood siding, trim, beams, vinyl siding, and other building surfaces are at ALGAE & MOLD STAINS on EXTERIORS

Mossy roof (C) Daniel Friedman

Moss tends to hold more water and moisture on the roof surface. So in our opinion moss is more likely to damage a roof surface than lichens or algae. Also see ALGAE, FUNGUS, LICHENS, MOSS COMPARED.

Photograph of reddish brown chimney rust stains on an asphalt shingle roof.

Thermal tracking stains (C) Daniel Friedman

Thermal tracking snow melt on a roof (C) Daniel Friedman

Roof stain or lichens or moss caused by trees (C) Daniel Friedman

What Causes White Blotches or Blooms in Paint on Exterior Stucco?

Effloresence on painted stucco (C) Daniel Friedman

White stucco blotches or stains may be caused by painting over high pH surfaces (over pH of 11), or over moisture, both conditions found when new stucco is insufficiently cured can lead to white blooms or efflorescence on the painted surface as well as a short life of the paint job as our photo shows (left).

See PAINT on STUCCO, FAILURES for an explanation of white blotches or white bloom that appears on painted stucco building surfaces.

Also see PAINT FALURE, DIAGNOSIS, CURE, PREVENTION for a complete inventory of causes and signs of types of paint failure on buildings.

Effloresence (C) Daniel Friedman

Readers should see STAIN DIAGNOSIS on ROOFS

and see ALGAE, FUNGUS, LICHENS, MOSS on SHINGLES where we describe not only moss and lichens but black fungal stains on asphalt shingles and on other building surfaces.

For identifying and diagnosing stains on indoor building surfaces see STAIN DIAGNOSIS on BUILDING INTERIORS.

See STAINS on STONE, STUCCO DIAGNOSE & CURE for additional examples of diagnosing, cleaning, and preventing stains on building materials and artifacts.

Pollen Stains on Building Surfaces

Yellow pollen stains  on exterior stucco from nearby Hibiscus flowers  (C) InspectApedia GC2016/08/13 Colin Frankland said:
I have hard yellow orange stains running from under my flat roofed garage which are almost impossible to remove any ideas how to shift them

2016/08/16 Anonymous said:

1st time ever I have orange stain on my stucco exterior wherever I have hibiscus plants.

The stain follows the shape of the plant beside it, so I know it must be from the pollen. Any idea how to remove it? Thanks - Anonymous by comment & by private email

[Click to enlarge any image]

Reply: Tips from removing Hibiscus or other flower pollen stains from walls:

For space we have moved this discussion to a separate article.


Catalog of Types of Stains on Building Surfaces

In the following guide we list types of stains by stain color & appearance, by building location or material, and by stain cause. We distinguish among the following stuff that may stain or be found growing building roofs, walls, or other surfaces, with extra focus on asphalt shingle roofs as well as other roofing materials such as wood shingles, wood shakes, roll roofing, and even slate or tile roofs.

Some of these types of roof stains or discoloration are only cosmetic in nature, while others may indicate growths that are likely to reduce the roof covering life. A more detailed, illustrated version of the list below is given at STAIN DIAGNOSIS on BUILDING EXTERIORS.


Continue reading at STAINS on & in BUILDINGS, CAUSES & CURES - home, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see STAIN DIAGNOSIS on BUILDING EXTERIORS FAQs - questions & answers posted originally on this page.

Or see this

Article Series Stains on Building Exteriors

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STAIN DIAGNOSIS on BUILDING EXTERIORS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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