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Photograph of  black stains on this asphalt shingle roofGuide 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
  • POST a QUESTION or READ FAQs about roof stains & tree trimming
  • REFERENCES
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Roof discoloration & stain diagnosis, cure, prevention:

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.



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How to Distinguish Roof Stain by Type of Cause, Color, Source

Red roof stains traced to chimney (C) InspectApediaThere 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.

[Click to enlarge any image]

The red stains on the roof shown at left are discussed and diagnosed
at RUST STAINS on ROOFS.

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.

Notice the 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.

As stated in Best Practices Guide to Residential Construction:

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

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

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.

See Debris Staining (trees),

see ALGAE STAINS on ROOFS (algae)

and Soot Staining(chimneys).

Roofing material defect / environmentally-caused roof shingle stains

Extractive bleeding roof stains (C) Daniel Friedman Extractive bleeding roof stains (C) Daniel Friedman

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).

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.

Also see TEST LABS - ROOF SHINGLE for chemical and durability tests on roofing materials.

How to trim trees and shrubs away from a building roof or walls

Tree through roof chimneyTupper Lake NY (C) D FriedmanWhile 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:

  1. 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.
  2. Remove dead branches: We don't want tree branches falling onto a roof where they may cause damage and leaks - see (1) above.
  3. 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.
  4. 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.
  5. Reduce moss, algae, and perhaps fungal growth on some roof surfaces by reducing shade on problem areas.
  6. Remove vines from chimneys and from building walls
    see VINES & SHRUBS on BUILDING WALLS, CHIMNEYS

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.

For the diagnosis and cure of other building stains,
see STAIN DIAGNOSIS on BUILDING EXTERIORS

and STAIN DIAGNOSIS on BUILDING INTERIORS.

For roofing material testing services and shingle testing
see TEST LABS - ROOF SHINGLE.

As we discuss at Power Washing Roofs we do not recommend power-washing asbestos-cement nor any other roofing. See Black Stain Removal & Prevention for advice on diagnosing, cleaning, and preventing stains on roofing.

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

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.

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Continue reading at ALGAE STAINS on ROOFS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see TREES & SHRUBS, TRIM OFF BUILDING

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STAINS on ROOFS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

INDEX to RELATED ARTICLES: ARTICLE INDEX to BUILDING ROOFING

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