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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

Roof discoloration & stain diagnosis, cure, prevention & advice for trimming trees away from buildings.

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.

This article gives a photo-catalog of types of stains that show up on roof surfaces and lists their common causes. For each stain color and type and cause we link to articles giving additional details about cause and remedy for that roof stain problem.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

How to Distinguish Roof Stain by Type of Proximate 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.

By proximate cause of a roof stain we mean that it's worth noticing the location of a stain on a building and the location and nature and condition of nearby building and site features that may be the cause of the stain.

For example, if you see stains around or below a chimney, take a closer look at the chimney as the stain source. It's pretty easy, right?

[Click to enlarge any image]

The red stains on the roof shown at left are almost certainly from rusting metal, perhaps rusty chimney flashing. These stains, cause ,and cure 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 (Steve Bliss, J Wiley & Sons) :

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:

Proximate cause roof shingle stains: tree debris

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

If you see a roof stain that develops only in limited areas of a single roof slope (photo above) 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).

See details at DEBRIS STAINING on ROOFS.

Our photo below shows brown stains below a metal roof chimney.

Roof stain caused by chimney(C) Daniel Friedman

Probably more important than the stains is that this chimney has lost its cap, risking damage to the flue, possibly damage to heating equipment, risking heater malfunction that could be unsafe.

See details at SOOT STAINING (chimneys) and also see

STAINS on/near CHIMNEYS

Catalog of 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.

Debris accumulated on this roof at its skylights should be removed (C) Daniel Friedman

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).

Roofing material defect / environmentally-caused roof shingle stains

Extractive bleeding roof stains (C) Daniel Friedman

Our stained asphalt roof shingle photo above show what looks like (but is probably not) black extractive bleeding stains on roof shingles. Even on darker shingles these dark streaky stains that seem to emanate from a single point may occur.

Most black stains on roof shingles are due to algae or sooty molds, but a few black stain shingle problems are traced to defective product.

Really? Well extractive bleeding does occur - it's a shingle product defect, though not a common one, perhaps lack of adquate binder, but in the photo above notice that there is no staining below that dormer window?

I [DF] suspect that metal oxides washing down off of the dormer flashing explain the absence of the black staining below that area. In turn, that suggests that these stains could be organic in nature: a black or sooty fungus or mold.

See BLACK BLEED-THROUGH SHINGLE STAINS or Extractive Bleeding Black Stains on Asphalt Shingles for details of shingle product defects that show up as black streaky areas on shingles.

The roof below may also have another roof product defect - thermal splitting. 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.

Extractive bleeding roof stains (C) Daniel Friedman

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.z

See CRACKS & THERMAL SPLITTING 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).

So Is the Black Staining a Sooty Mold?

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.

 

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 DUST / MOLD TEST KIT INSTRUCTIONS.

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 © D Friedman at InspectApedia.com 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:

  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 - 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.

Below I illustrate cutting away the branches of evergreen trees that were rubbing both the underside of the soffit and the roof surface of this home in Two Harbors Minnesota.

Tree limbs rubbing on a roof surface, Two Harbors MN (C) Daniel Friedman

In my photo above you can see the branches in contact with the asphalt shingle surface. Even branches that are a foot or two above the roof, when weighted by snow, will rub on the roof surface.

Below: as well as rubbing on the underside of the soffit - an engraved invitation to squirrels or other animals to invade the roof cavity by providing them an easy highway into the building.

Trim off this branch to help keep pests out of the roof cavity (C) Daniel Friedman

In my third tree trimming photo below you'll see that we cut away branches that were too close to the roof surface as well as too close to the soffit.

Tree limbs have been cut back properly away from the building (C) Daniel Friedman

I cut surrounding tree limbs back far enough that when swaying in the wind the tree will still not reach nor rub against the building.

Notice that trimming away branches left a squirrel's nest: good evidence that branches were providing a welcome passage into the soffit of this home. (Not that northern Minnesota squirrels can't jump, but at least let's not leave the door open for them.)

Roof Stain Research

 




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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Question: 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?

Reply:

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 ?

Reply:

Hodge,

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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