Online 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
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Here we describe and provide photographs and microscopic photographs of algae, moss, lichens, or fungal growth that occurs on buildings and in nature. 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. Readers should see STAIN DIAGNOSIS on ROOFS and see ALGAE, FUNGUS, LICHENS, MOSS on SHINGLES where we discuss 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 Indoor Surfaces. See STAIN DIAGNOSIS on STONE for additional examples of diagnosing, cleaning, and preventing stains on building materials and artifacts.
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.
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.
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.
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.
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.
Online Guide to 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.
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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