Guide to Stains on Building Roof Surfaces, Cause, Cure, Prevention InspectAPedia® -
Online guide to black, brown, green, and other stains on building roof surfaces
Diagnostic Guide to Types of Stains and Staining Materials Found on Roof Surfaces
Causes, cure, and prevention of black stains on roof shingles
How to trim trees and shrubs away from a building roof or walls
Questions & Answers about roof stains & tree trimming
This document tells readers how to identify & explain the most-common causes of black, brown, red, gray, green,
or white stains appearing on roof shingles and on other building surfaces. Roof shingle stains are often caused by black algae, bleed-through or extractive bleeding
of asphalt, dirt, soot, or organic debris.
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There are many sources of roof staining and they vary in significance, from cosmetic to harmful to the roof. Here we give an index of all sorts of roof stains and what causes them.
The remedies for and means of preventing stains
on roof shingles are discussed as well. In the photo shown here, the black stains on the lower roofs are bleed-through or
extractive bleeding. The upper roof, shaded by the large tree in the photo, was also moss covered. Notice the
streak of "clean" roof shingles that lack the stains on the upper roof near its left edge? Those shingles were
installed down-roof from an aluminum attic vent fan cover. The aluminum salts washing off of the fan cover
prevented moss, lichens, and algae from growing on those shingles.
Black streaks on shingles caused by
algae or fungal growth used to be limited to warm, humid
climates, but now this can be seen on houses as far north
as Canada. Some experts attribute the spread to the
increased use of crushed limestone as a filler material in
asphalt shingles.
Limestone is economical and makes a
durable shingle, but the calcium carbonate in the limestone
supports algae growth. In algae-resistant (AR) shingles,
zinc or copper granules are mixed in with the colored stone
topping. When the shingles get wet, the zinc or copper is
released, inhibiting algae growth.
Warranties for algae
resistance are usually for less than 10 years since the protection
ends when the mineral washes away. Some shingles
have longer lasting protection than others due to a
higher percentage of AR granules.
For distinguishing between an algae growing on shingles and a fungus growing on roof shingles, building or environmentally-caused roof staining, or other causes, here are several approaches:
Visual Roof Inspection & Roof Stain Area Context Help Diagnose Cause of Roof Stains
Proximate cause roof shingle stains
If you see a roof stain that develops only in limited areas of a single roof slope look for a relation between the stained area and a proximate cause such as a sooty chimney top or a tree that shades that section of the roof or drops organic debris onto it.
These include tree shade caused moss, tree-shade caused lichens growth on a roof, or dark stains on roof coverings caused by organic debris such as leaves and sticks that fall onto and collect on the roof surface (photo at left). Our photo at above right shows brown stains below a metal roof chimney. Probably more important, this chimney has lost its cap, risking damage to and unsafe conditions at the building heating equipment.
Roofing material defect / environmentally-caused roof shingle stains.
Our stained asphalt roof shingle photo at above lefty show black extractive bleeding stains on roof shingles colored other than white. Even on darker shingles these effects may occur. See Black Bleed-Through or Extractive Bleeding Black Stains on Asphalt Shingles. Click to enlarge our stained shingles photo at above left and you may also notice vertical cracks running up through shingles to the left of the dormer.
This roof may also have a defective roof product causing splitting shingles (see CRACKS in FIBERGLASS SHINGLES). We're not certain about the cause of the staining at above right. It is probably extractive bleeding but a second candidate might be roof algae. We need a closer look.
If the stain pattern on a roof surface occurs across the entire field of the roof and independent of proximate causes like chimneys or trees, it is more likely due either to the roofing material itself (possibly extractive bleeding), the overall environment (downwind from a chemical plant or incinerator or factory), or due to the slope' orientation (North slope, cooler, East or West roof slope -more sunlight). See ASPHALT SHINGLE LIFE / WEAR FACTORS for a discussion of sunlight effect on roof life.
While we have not confirmed its presence, some black stains on roofs and roof gutters might be due to black molds or sooty molds, for which the USDA has published How to Recognize & Control Sooty Molds. Black mold growth on leaves, on roof debris that has fallen from a tree overhead, or on an aluminum roof gutter, is not a predictor
that any of the species discussed in the USDA article will appear as black "sooty mold" growth on a roof shingle.
The chemistry of roof shingles, their granule coverings, and substrates is quite different from other organic substances
that are home to many molds. Further, many newer shingle products include chemicals to retard black algae growth that
may also retard mold growth. Specific mold genera/species like to grow on particular surfaces - it's their food, and while
some molds are more choosy than others (for example mildews grow only on living plants), you'll need to look carefully
at a roof and the conditions around it (such as trees, and areas of sun or shade) and perhaps even sample the black
debris to determine if it is mold and if so what is its species.
The roof cleaning methods to remove black algae will probably work well for black mold growth on a roof as well.
Roofing material defect / installation / structurally-caused roof shingle stains
If you see roof staining or defects that occur on all roof slopes regardless of slope orientation or presence/absence of proximate causes (chimneys, trees) then the condition may be due to roofing material, its installation, or the building itself (inadequate under roof venting).
Forensic sampling of Roof Stain Material
A simple collection of the
stained roof surface material using clear adhesive tape followed by a laboratory analysis by an experienced microscopist is conclusive. We don't recommend lab sampling of shingle stains
in most cases but if you want to sample a stain material for analysis in our forensic laboratory, instructions are
at Six Easy Steps to Get and Mail a Mold Test Kit.
Don't bother worrying about mildew on your roof. Mildew is an obligate parasite
which grows only on living plants. Unless your roof is covered with grapes it is not likely to have much mildew there.
How to trim trees and shrubs away from a building roof or walls
While most home inspectors will agree that we ought to keep shrubs at least 24" off of building walls, and that trees within five feet of a building wall risk damaging the structure by root pressure or impace, we don't think that there is an arbitrary correct distance for trimming trees back around a roof.
Our photo (left) shows the dramatic measures that a building owner followed to avoid removing a tree that was really too close to the building - in this case in Tupper Lake, NY.
For tree trimming advice, it is more useful to explain the objects of trimming trees away from a building so that the homeowner or maintenance worker can determine what is needed to meet those objectives at a particlar site. Here are some objectives to meet when trimming trees that may be close to a building:
Remove or trim large or tall sick trees: We don't want a big tree in poor health to fall or be blown onto the building, say in a storm, so it's important to cut off dead branches entirely and to have an tree expert assess the health of large trees near a building. Watch out: some "tree experts" just love to cut everything down. Consult experienced tree service companies, or an arborist is a real tree expert who knows not only trimming procedures but how to assess tree health. Also trees growing within five feet of a building are more likely to send out roots that can damage the structure - with the risk varying by tree species.
Remove dead branches: We don't want tree branches falling onto a roof where they may cause damage and leaks - see (1) above.
Prevent on-roof debris accumulation: We don't want tree leaves and debris accumulating on a roof because of both cosmetic staining and long term wear and leaks below the debris.
Reduce roof gutter clogging: We sometimes also trim brack branches whose pine needles or leaves are falling into and rapidly clogging gutters.
So the amount of trimming and safe distance depend on the home, roof slope, roof drainage details, and the height and health and distance of trees near the building.
Reduce moss, algae, and perhaps fungal growth on some roof surfaces by reducing shade on problem areas.
Watch out: you should hesitate to remove beautiful, large, old trees at a property. Consult with a tree expert and think twice before drastic cutting or removal of trees.
Online Guide to Types of Stains on Building Roofs, Surfaces, Walls
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 (roof debris), and see Proximate cause roof shingle stains (types of roof stain by color, location, probable cause), and see Black Bleed-Through (black bleeding stains from asphalt shingles - extractive bleeding).
Black or other colored chimney soot 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.
Questions & Answers regarding this article
Questions & Answers about types and colors of roof stains, stain diagnosis, removal, prevention, and related tree trimming away from roofs and walls
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Additional technical contributors & reference sources for this article are listed below.
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Roger Hankey is principal of Hankey and Brown home inspectors, Eden Prairie, MN. Mr. Hankey is a past chairman of the ASHI Technical Committee and the ASHI Standards Committee. Mr. Hankey serves as co-chairman of ASHI legislative committee, and has served in other ASHI professional and leadership roles. Contact: 952 829-0044 - hankeybrown@comcast.net
How to Recognize & Control Sooty Molds, USDA publication on the recognition and control of black sooty molds, including on buildings. This publication is also available in printed form from the U.S. Government Printing Office, 1992 657-152 HT-69 1992. The original article was authored by Kenneth K.Kessler, Jr., Principal Plant Pathologist, U.S. Forest Service, in the Department of Agriculture of the United States. Copies are also available from North Central Distribution Center, Forest Products Laboratory, One Gifford Pinchot Dr., Madison WI 53705-2398.
NRCA - National Roofing Contractors Association - http://www.nrca.net/, 10255 W. Higgins Road, Suite 600,
Rosemont, IL 60018-5607, Tel: (847) 299-9070 Fax: (847) 299-1183
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.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. 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.
Architectural elements: the technological revolution: Galvanized iron roof plates and corrugated sheets; cast iron facades, columns, door and window caps, ... (American historical catalog collection), Diana S Waite, available used out of Amazon.
Green Roof Plants: A Resource and Planting Guide, Edmund C. Snodgrass, Lucie L. Snodgrass, Timber Press, Incorporated, 2006, ISBN-10: 0881927872, ISBN-13: 978-0881927870. The text covers moisture needs, heat tolerance, hardiness, bloom color, foliage characteristics, and height of 350 species and cultivars.
Green Roof Construction and Maintenance, Kelley Luckett, McGraw-Hill Professional, 2009, ISBN-10: 007160880X, ISBN-13: 978-0071608800, quoting: Key questions to ask at each stage of the green building process Tested tips and techniques for successful structural design
Construction methods for new and existing buildings
Information on insulation, drainage, detailing, irrigation, and plant selection
Details on optimal soil formulation
Illustrations featuring various stages of construction
Best practices for green roof maintenance
A survey of environmental benefits, including evapo-transpiration, storm-water management, habitat restoration, and improvement of air quality
Tips on the LEED design and certification process
Considerations for assessing return on investment
Color photographs of successfully installed green roofs
Useful checklists, tables, and charts
Problems in Roofing Design, B. Harrison McCampbell, Butterworth Heineman, 1991 ISBN 0-7506-9162-X (available used)
Roofing The Right Way, Steven Bolt, McGraw-Hill Professional; 3rd Ed (1996), ISBN-10: 0070066507, ISBN-13: 978-0070066502
Slate Roofs, National Slate Association, 1926, reprinted 1977
by Vermont Structural Slate Co., Inc., Fair Haven, VT 05743, 802-265-4933/34. (We recommend this book if you can find it. It
has gone in and out of print on occasion.)
Roof Tiling & Slating, a Practical Guide, Kevin Taylor, Crowood Press (2008), ISBN 978-1847970237, If you have never fixed a roof tile or slate before but have wondered how to go about repairing or replacing them, then this is the book for you. Many of the technical books about roof tiling and slating are rather vague and conveniently ignore some of the trickier problems and how they can be resolved. In Roof Tiling and Slating, the author rejects this cautious approach. Kevin Taylor uses both his extensive knowledge of the trade and his ability to explain the subject in easily understandable terms, to demonstrate how to carry out the work safely to a high standard, using tried and tested methods.
This clay roof tile guide considers the various types of tiles, slates, and roofing materials on the market as well as their uses, how to estimate the required quantities, and where to buy them. It also discusses how to check and assess a roof and how to identify and rectify problems; describes how to efficiently "set out" roofs from small, simple jobs to larger and more complicated projects, thus making the work quicker, simpler, and neater; examines the correct and the incorrect ways of installing background materials such as underlay, battens, and valley liners; explains how to install interlocking tiles, plain tiles, and artificial and natural slates; covers both modern and traditional methods and skills, including cutting materials by hand without the assistance of power tools; and provides invaluable guidance on repairs and maintenance issues, and highlights common mistakes and how they can be avoided.
The author, Kevin Taylor, works for the National Federation of Roofing Contractors as a technical manager presenting technical advice and providing education and training for young roofers.
The Slate Roof Bible, Joseph Jenkins, www.jenkinsslate.com,
143 Forest Lane, PO Box 607, Grove City, PA 16127 - 866-641-7141 (We recommend this book).
Solar heating, radiative cooling and thermal movement: Their effects on built-up roofing (United States. National Bureau of Standards. Technical note), William C Cullen, Superintendent of Documents, U.S. Govt. Print. Off (1963), ASIN: B0007FTV2Q
"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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