Guide to Stains on Building Roof Surfaces, Cause, Cure, Prevention
Home page for diagnosing, removing, preventing stains on roofs
STAINS on ROOFS - CONTENTS: 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
Stains on roofs have a variety of causes, but noticing the stain color, its location, and the relation of he roof stain color and location to roof components such as chimneys, flashings, and the stain relation to roof conditions (pitch, shading, nearby trees, debris, roofing materials) will usually lead to a quick reliable diagnosis of the cause of the staining.
Knowing the cause guides us to knowing the cure for unsightly black, brown, green, red, rust, or other colored stains on building roofs. 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.
How to Distinguish Roof Stain by Type of Cause, Color, Source
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.
Dark black or brown roof shingle stains are often caused by black algae, bleed-through or extractive bleeding
of asphalt, dirt, soot, or organic debris. But there are other roof stain colors and causes. The remedies for and means of preventing stains
on roof shingles are discussed as well.
In the photo shown at page top, the black stains on the lower roofs are bleed-through or
extractive bleeding. The upper roof, shaded by the large tree in the page top photo, was also moss covered.
streak of "clean" roof shingles that lack the stains on the [page top photo] 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
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. 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.
See CRACKS in FIBERGLASS SHINGLES
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).
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
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 impact, 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 particular 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 back 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.
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.
Algae-causedblack or green stains on roof or other building surfaces - Some photos of black algae stains are
at BLACK ALGAE STAINS on Asphalt Shingles.
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.
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.
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.
Red or Rust Stains on building chimneys, roofs, walls & swarf stains on metal roofs: These red, brown or even black roof stains are often caused by corrosion of on-rooof or above roof metal components but may also be the result of improper combustion of wood, coal, or oil fired, possibly also gas fired heating appliances vented above the roof. Rust stains on metal roofs, including particularly steel roofs, may be occur at nicks or gouges in the protective coating or they may be swarf stains caused by leaving the products of roof cutting (shards, shanks, nails, rivets, or any discarded steel object) on the roof surface. Metal roof stains also may be caused by walking on roof surfaces with rough-soled shoes or walking on debris left on the roof surface during or after installation.
Because Swarf Staining is unfamiliar to many homeownes we include these excerpts from the New Zealand Steel Installers Guide:
Swarf is the term given to the steel debris arising from cutting or piercing operations when using
friction saws, drills etc., on steel roofing and cladding products. The use of abrasive discs is not
recommended. Whilst comprising mostly fine steel particles, in this context swarf may also be taken
to include any other discarded steel objects such as rivet shanks, nails, screws and nuts, which may
come into contact with coated products; ie: COLORSTEEL® prepainted steel, ZINCALUME® coated
steel and galvanised steel.
Swarf particles, if left on the surface, will corrode and cause rust stains which will detract from the
finished appearance of a project. These stains are often mistaken for early deterioration of the
roofing and cladding itself.
Prevention of swarf staining is the responsibility of the installer and it is strongly suggested that the
recommendations contained in this brochure be followed. ... Fresh swarf stains are characterised by small red-brown coloured areas with a central dark spot
(the remains of the steel particles). The surface will feel like sandpaper, and the particle may be
lifted with a fingernail.
An old swarf stain will appear as a localised red-brown stain, the steel particle having corroded
away, and the surface will be smoother.
... The effect of swarf staining itself on COLORSTEEL® prepainted steel products, is generally
aesthetic and may not be detrimental to the performance of the product. The product life will,
however, be severely affected where attached swarf particles have penetrated the prefinished film
and are in contact with the protective metallic coating, although this only occurs in severe cases.
(New Zealand Steel Installers Guide, NZ Steel Co. 2005) - see REFERENCES to obtain a copy of this very helpful document.
See RUST STAINING - Red or Brown Rust Stains (on shingles) discussing rust stains on asphalt roof surfaces.
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
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 roofs may also be a calcium salt or similar mineral deposit left by roof-mounted evaporative coolers or swamp coolers that drain or leak onto the roof surface.
See EVAPORATIVE COOLING SYSTEMS.
Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
white stains on roof
(Aug 15, 2014) Deborah said:
I have a new roof with architectural shingles that has a white discoloration on it for the moment it was installed. The roofer said it was back surfacing mineral and would wash away. It has been months now. It has faded, but not completely. It is most noticeable when the sun goes behind the house in the afternoon. In the morning, ( house faces east) it is barely noticeable. He said it will weather off. Is this true?
I don't know, Deborah as I don't know what's on the shingles. Please take a look at WHITE STAINS on ROOFS
Question: roof wear or stains above windows
9/1/14 Hodge said:
We have a 4 yr old roof in great shape except for sign of early wear above two windows. The areas of wear are the same width of the windows and extend up and gradually fade . Any ideas ?
If the windows are a source of drafts, temperature variations in the roof above, or moisture sources, or if construction details obstructed roof venting over those areas those might explain variation in roof shingle wear. Use our CONTACT link email to send me some photos and I can comment more usefully.
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"New Zealand Steel Installers Guide, COLORSTEEL®, ZINCALUME® steel and GALVSTEEL™ Installer's Guide", [PDF], New Zealand Steel Limited, Private Bag 92 121, Auckland, New Zealand. Tel: 0800 100 523, or 0-9-375 8999 Fax 0-9-375 8213, Website: http://www.nzsteel.co.nz/, Email: email@example.com - retrieved 2015/12/07, original source: http://www.nzsteel.co.nz/content/media/ 77012/New%20Zealand%20Steel%20Installers%20Guide%20-%20November%202005.pdf - regarding Swarf Staining
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 - firstname.lastname@example.org
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.
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
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).
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: email@example.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, 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. Field inspection worksheets are included at the back of the volume.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
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