Online Photo Guide to Diagnosing Stains on Building Exteriors InspectAPedia® -
Online guide to diagnosing, removing, or preventing stains on building exterior surfaces: roofs, walls, siding, trim, walks, drive, stone, wood stains, moss, algae, mold, etc.
Photographs of Algae, lichens, moss, mold help identify these substances in nature and on buildings
Questions & answers about how to identify types of stains on building exterior surfaces
Building exterior stain cause diagnosis: this article describes and provide photographs and microscopic photographs of algae, moss, lichens, or fungal growth that occurs on buildings and in nature.
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We include links to references useful in the identification of algae, moss, lichens, and mold. Our photo at page top shows moss on an asphalt shingle roof. Our page top photo shows algae growing on the shaded side of a vinyl-clad building.
A Detailed Online Guide to Types of Stains or Marks on Building Exterior Surfaces: roofs, walls, decks, siding, trim
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.
Animal stains in buildings may be caused by animals brushing against walls or trim of the inside or outside of a building, or may be due to more odious causes such as pet urine staining.
Our photo shows an indoor black stain on walls caused by a dog who slept against this part of the room.
See Black stains from animals for details about pet stains on building floors (urine) and for black or other pet stains on walls see Pet Stains on Walls for diagnosing stains such as the black marks in our photo at left.
Algae causes black stains on roof surfaces and green stains on other building exterior surfaces such as concrete (photo above left), decks, and building siding (above right).
Algae can produce black stains on asphalt roofing products. Algae very often appears as a flat green coating or stain on building siding and even on sidewalks and concrete (photo at left). See ALGAE, FUNGUS, LICHENS, MOSS COMPARED for additional photos (including algae in the microscope) and details.
Algae or Mold Stains on Building Siding may be 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 (left) 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 are the same: moisture, shade, and from the stain pattern, possibly the absence of insulation and a moisture barrier in the wall structure.
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 stains on asphalt roofing products such as asphalt shingles or roll roofing, caused by extractive bleeding (product defects) shown at above left. - see Bleed-Through and see Roofing material defect / environmentally-caused roof shingle stains.
Black stains on vinyl or painted building exterior wall siding may also be mold, as our photo shows at above right.
Black, brown, or gray stains on roofing products due to debris left on the roof surface, such as piles of leaves and organic waste from nearby trees. See Debris Staining and see Proximate cause roof shingle stains. Black or other colored stains on roof surfaces may be caused by soot from chimneys, both wood-fired and oil-fired devices. Black or other colored stains may also appear on roof surfaces from nearby industrial activities.
Black or other colored stains on roof surfaces may be caused by soot from chimneys, both wood-fired and oil-fired devices. Black or other colored stains may also appear on roof surfaces from nearby industrial activities. See Soot Staining.
Black mold in buildings: often is mistaken as being the only or most severe mold problem - be sure to look for other molds that are perhaps equally or more toxic but are harder to spot. Our photo (left) shows cosmetic black mold on building framing. See MOLD APPEARANCE - WHAT MOLD LOOKS LIKE for complete details, and see Black Mold for specifics about black mold growth in buildings.
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.
Brown or red stains on roof surfaces are often caused by rust from wearing steel flashing or other on-roof metal products. See Rust Stains on Shingles.
Debris stains on roof shingles - see Debris Staining - Black Debris Stains on Asphalt Shingle Roofs. Debris stains on building walls, including stains from animals may appear as black smudges that may be mistaken for mold.
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.
Sealers and stain-blocking primers help to minimize
this problem but do not always eliminate it. If staining
occurs, the first step is to eliminate the moisture problem.
Then, if the extractive bleeding is mild, remove the
stains with a mild detergent and water. More severe cases
will require cleaning with an oxalic acid solution.
Carefully follow the manufacturer’s instructions when
using oxalic acid, as the bleaching solution will harm
plants and may bleach existing finishes on siding, trim,
and other woodwork.
After washing, the oxalic acid must
be thoroughly rinsed with clean water and the wood dried
before finishing or refinishing. If the extractive bleeding
has been allowed to bake in the sun, it may have hardened
and be difficult to remove. In this case, you will need to
apply a stain-blocking primer before refinishing.
Our clapboard staining photo below left shows staining in an inside corner below a roof valley where we suspect there was gutter spillage or leakage down the wall, exacerbating siding staining. Look at the edge of the stains and you'll see some extractive bleeding. Our stained clapboard siding photo at below right is a closeup of stains found on the home shown in our photograph just above.
How to Diagnose & Prevent "Mildewed" or Moldy Building Exteriors
Mildew? ... Really? Speaking accurately, which would be refreshing in politics as well as forensic building diagnosis, there is no mildew found on building surfaces.
Mildew, which is comprised of two groups of fungi within the larger class of molds, grows only on 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 mildew on a building exterior, unless the exterior is made of living plants like grapes, 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 see MILDEW in buildings ?. Also see our photos of mold that is often mistaken for mildew at MILDEW ERRORS - MOLD PHOTOS and finally, see MILDEW REMOVAL & PREVENTION.
Algae or Mold Stains on Building Siding may be 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, left) 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 are the same: moisture, shade, and from the stain pattern, possibly the absence of insulation and a moisture barrier in the wall structure. Also see VAPOR BARRIERS & CONDENSATION in buildings.
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.
Moss on roof surfaces - see ALGAE, FUNGUS, LICHENS, MOSS on SHINGLES. Green or other colored mosses are a bit easier to remove than lichens on roofing surfaces, but both can carry away mineral granules and damage the roof surface. 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.
Rust Stains on buildings - see Rust Staining - Red or Brown Rust Stains (on shingles)
Soot or Creosote black or brown stains on roof surfaces near or below chimneys - see SOOT STAINING - Black Soot Stains on Roof Shingles Around Chimneys and see Proximate cause roof shingle stains
Stains on stone or masonry surfaces - see STONE CLEANING METHODS for cause, cure, and prevention
Thermal tracking stains or marks on buildings: Thermal tracking stains will not normally be visible on a building exterior, but we might see similar tracking of wall studs or roof rafters (photo, above left) where temperature differences in a building or points of heat conductivity and heat loss cause variations in snow-melt (on roofs) or moisture condensation (on walls).
Black stains on building interior surfaces may follow a similar pattern (photo, above right) due to deposits of house dust on cool or damp building surfaces often follow building wall framing or ceiling framing studs or joists. See THERMAL TRACKING & THERMAL BRIDGING for details.
What Causes White Blotches or Blooms in Paint on Exterior Stucco?
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.
White stains on roof surfaces may be from wash-down of pigments in paint on surfaces above the roof such as a gable end or dormer wall; white or less-black roof surfaces may also be found below copper or aluminum flashing whose salts run down the roof surface in wet weather, preventing algae or moss growth and leaving a white area on an otherwise stained roof surface.
White stains on masonry walls (powdery or crystalline marks on the wall in our photo at above-left) and on some plaster walls (white-yellow bubbly) are usually mineral salts - efflorescence left by evaporating water and are a sign of wet or damp conditions. See Efflorescence & white or brown deposits
Yellow stains in buildings - crystalline or round shiny stains on wood framing may be sap that exuded from building framing as it was heated, such as on an attic rafter.
Yellow stains indoors may also be due to mold or pets.
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.
Black stains on asphalt roofing products such as asphalt shingles or roll roofing, caused by extractive bleeding (product defects) - see Bleed-Through and see Roofing material defect / environmentally-caused roof shingle stains.
Black, brown, or gray stains on roofing products due to debris left on the roof surface, such as piles of leaves and organic waste from nearby trees. See Debris Staining and see Proximate cause roof shingle stains
Black or other colored stains on roof surfaces may be caused by soot from chimneys, both wood-fired and oil-fired devices. Black or other colored stains may also appear on roof surfaces from nearby industrial activities. See Soot Staining.
Brown or red stains on roof surfaces are often caused by rust from wearing steel flashing or other on-roof metal products. See Rust Stains on Shingles.
Green roof stains on roof shingles, clay tiles, cement tiles, and asbestos cement roof shingles or newer fiber cement roof shingles are often caused by algae. For details see ALGAE STAINS ON ASPHALT ROOF SHINGLES. Moss on roofs is also often green and should be unmistakable in appearance. See ALGAE, FUNGUS, LICHENS, MOSS on SHINGLES.
Moss on roofs and other building surfaces: see ALGAE, FUNGUS, LICHENS, MOSS on SHINGLES, and Green Stains on Stone. Green or other colored mosses are a bit easier to remove than lichens on roofing surfaces, but both can carry away mineral granules and damage the roof surface. 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.
Rust Stains on buildings - see Rust Staining - Red or Brown Rust Stains (on shingles)
Soot or Creosote black or brown stains on roof surfaces near or below chimneys - see SOOT STAINING - Black Soot Stains on Roof Shingles Around Chimneys and see Proximate cause roof shingle stains
Stains on stone or masonry surfaces - see STONE CLEANING METHODS for cause, cure, and prevention
White stains on roof surfaces may be from wash-down of pigments in paint on surfaces above the roof such as a gable end or dormer wall; white or less-black roof surfaces may also be found below copper or aluminum flashing whose salts run down the roof surface in wet weather, preventing algae or moss growth and leaving a white area on an otherwise stained roof surface. Our article at ALGAE, FUNGUS, LICHENS, MOSS on SHINGLES includes a photograph of this condition on a wood shingle roof.
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Additional technical contributors & reference sources for this article are listed below.
Guides to identifying mosses: Graphic Guide Ontario Mosses (some of which appear widely dispersed by climate and geographic area, not just in Ontario) which offers a graphic guide to mosses. worldofmosses.com/ggom/index.html
Also see the sources listed at worldofmosses.com/ggom/ggomBibliography.html
Also see the Journal Folia Geobotanica, Springer, Netherlands ISSN1211-9520 (Print) 1874-9348 (Online) Issue Volume 11, Number 2 / June, 1976 DOI10.1007/BF02854759 Pages217-224
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Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Analysis of Modern Paints, Thomas J.S. Learner, Research in Conservation, 2004 ISBN 0-89236-779-2 [Chemistry of modern paints, overview of analytical methods, pyrolysis-gas chromatography signatures of basic modern paints and their constituents, Fourier transform infrared spectroscopy for paint analysis, direct temperature-resolved mass spectrometry, and analysis in practice - technical reference useful for forensic paint science, focused on art works -DF]
Understanding Ventilation, John Bower, The Healthy House Institute, ISBN 0-9637156-5-8, 1995 [General building science-DF - ** Particularly useful text. Mr. Bower has retired from the field but his book continues to be important]
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop, and from the InspectAPedia bookstore. The 2010 edition of the Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
Certainteed Weatherboard fiber cement siding and trim products - see certainteed.com/ or see certainteed.com/resources/sidingandtrimspecsheet.pdf
"Moisture Control in buildings: Putting Building Science in Green Building," Alex Wilson, Environmental Building News, Vol. 12. No. 5. [Good tutorial, "Moisture 101" outlining the physics of moisture movement in buildings and a good but incomplete list of general suggestions for moisture control - inadequate attention given to exterior conditions such as roof and surface drainage defects which are among the most-common sources of building moisture and water entry.--DJF]
Paint Handbook: testing, selection, application, troubleshooting, surface preparation, etc., Guy E. Weismantel, Ed., McGraw Hill Book Company, 1981, ISBN-10: 0070690618, ISBN-13: 978-0070690615, [Excellent but a bit obsolete paint theory and practice, also a bit light on field investigation methods, out of print, available used-DF] How to select and apply the right paint or coating for any surface. The first major reference to help you choose the correct paint or other finish to do the job best on a particular surface exposed to a particular environment. Experts in the field give full advice on testing surface preparation, application, corrosion prevention, and troubleshooting. The handbook covers wood, metal, composites, and masonry, as well as marine applications and roof coatings. A ``must'' working tool for contractors, architects, engineers, specification writers, and paint dealers.
Paint and Surface Coatings, Theory and Practice, R. Lambourne & T.A. Strivens, Ed., Woodhead Publishing Ltd., William Andrew Publishing, 1999 ISBN 1-85573-348 X & 1-884207-73-1 [This is perhaps the leading reference on modern paints and coatings, but is a difficult text to obtain, and is a bit short on field investigation methods - DF] Provides a comprehensive reference source for all those in the paint industry, paint manufacturers and raw materials suppliers, undergraduate and postgraduate students, and industrial paint users. R. Lambourne was in the Research Department at ICI Paints Division and the Industrial Colloid Advisory Group, Birstol University, UK.
Seeing Through Paintings, Physical Examination in Art Historical Studies, Andrea Kirsh, Rustin S. Levenson, Materials in Fine Arts, 2000 ISBN 99-051835 [ forensic science, technical reference, focused on art works - DF]
Sealants, Durability of Building Sealants (RILEM Proceedings), J.C. Beech, A.T. Wolf, Spon Press; illustrated edition (1995), ISBN-10: 0419210709, ISBN-13: 978-0419210702 This book presents the papers given at the RILEM Seminar held at the Building Research Establishment, Garston, UK in October 1994. The book provides an opportunity for researchers to review up-to-date progress towards the achievement of the objectives of the standardisation of laboratory techniques of sealants in the variety of service conditions to which they are exposed.
Soiling and Cleaning of Building Facades (RILEM Report), L.G.W. Verhoef (Editor), Routledge; 1 edition (November 3, 1988), ISBN-10: 0412306700, USBN-13: 978-0412306709 The report of a comprehensive investigation by RILEM which examines all aspects of the cleaning of facades, subject to soiling by both biological and non-biological agencies. The contributors are international authorities working in this field giving essential advice to all those who need to know how to approach the problems connected with the soiling and cleaning of building facades.
Staining, Prevention of Premature Staining in New buildings, Phil Parnham, Taylor & Francis; 1996, ISBN-10: 0419171304, ISBN-13: 978-0419171300 The appearance of ugly staining early in a buildings life, ruins an otherwise pleasing appearance, tarnishes the image of the owners and gives rise to costly refurbishment works. In this book Phil Parnham raises a number of questions that should be considered whenever a new building is being designed or built. These are: * why has staining become so prominent; * what causes premature staining; which parts of new buildings are likely to be affected; * how can it be avoided? By using a number of highly illustrated case studies, the author answers these questions and ends by suggesting measures that should be taken by all design and construction professionals to prevent premature staining.
"Weather-Resistive Barriers [copy on file as /interiors/Weather_Resistant_Barriers_DOE.pdf ] - ", how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
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