InspectAPedia ^TM

Black Stains on Asphalt Roof Shingles, Cause, Cure, Prevention RoofAPedia © Enter your search terms Submit search form   Search This Website - InspectAPedia.com

• Causes, cure, and prevention of black stains on roof shingles

This document tells readers how to identify & explain the most-common causes of black, brown, or gray stains appearing on asphalt roof shingles. These stains may be black algae, bleed-through or extractive bleeding of asphalt, dirt, soot, or organic debris. The causes and remedies of black stains on roof shingles are discussed. 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.

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

BLACK STAINS - Aesthetic or Cosmetic Roofing Issues - Black Stains on Asphalt Shingles

Some consumers have concerns with how their shingles look on the roof as much as with how long the roof will last. Roofing manufacturers offer a wide variety of products which give different "looks" and shadings. It's possible that in addition to site and installation conditions, variations in manufacturing process (granule adhesion, bleed-through) can affect how the roof looks from the ground. We've also investigated client concerns with shadows appearing in early morning or late afternoon which show variations in the roof surface. (Some shadows which are only of cosmetic nature may be caused by slight buckling or unevenness in the roof decking and may not indicate a structural or durability concern.) If you have particular concerns about roof appearance ask your roofer if s/he can direct you to a house where the product which interests you is already installed. Remember that site differences (orientation to sun, shade trees, height above ground, roof pitch, and probably other factors) may make shingles look a bit different on your house.

Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website. If you just "scroll down" you'll miss some important articles. See links at page left.

BLEED THROUGH - Black Bleed-Through or Extractive Bleeding Black Stains on Asphalt Shingles

Extractive bleeding or "bleed through" stains leaving black streaks running down an asphalt shingle roof is an indication of a defective roofing product. The chemistry of the shingle is permtting black pigment from the asphalt to leach to the shingle surface and run down the roof. Typically bleed-through on asphalt shingles appears as black streaks running down shingles. You'll see black streaks of varying length. Comparing bleed-through stains with black algae stains: Both bleed-through and algae stain sources on roof shingles will produce a black stain that "runs down" the roof slope in a widening pattern. Bleed-through asphalt shingle stains may be mistaken for but are not black algae or black "fungus", nor are they soot. When the staining or bleeding appears to run down the roof from individual small points or "spots" I think this is extractive bleeding or "bleed through" on shingles. Furthermore, bleed-through stains, since they are a feature of the asphalt shingle product itself, should occur on a roof regardless of the roof orientation, slope, or shading. We describe and contrast black algae staining in the next section, just below. Because of variations during the shingle manufacturing process, and possibly because of variations in site conditions (sun, shading, slope, moisture), different roof areas or slopes on the same building may display different amounts of black staining. This term is commonly used with wood shingles where use of steel or possibly even copper nails can also produce staining. (Try stainless steel nails when re-roofing with wood shingles or shakes). Remedies for bleed through staining on asphalt shingles: I wouldn't do much to a roof with this staining since I worry that power washing or chemical treatments may reduce the remaining roof life. At re-roof time I'd buy a better-grade replacement shingle.

Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website. If you just "scroll down" you'll miss some important articles. See links at page left.

BLACK ALGAE - Black Algae Stains on Asphalt Shingles

Mis-named as "mildew" or "fungus" by some writers, algae staining on asphalt shingles usually in shaded areas or on the more-shaded roof slopes- characterized by black staining fairly uniform over shingles, but appearing specifically in areas of the roof shaded by nearby trees. Black algae stains on asphalt shingles tend to be fairly uniform over the stained area, not streaky as with bleed-through stained shingles. When stains on the roof surface are more uniform in pattern and when they cover a wider area that does not originate at one or more single pinpoints in the shingles, this may be a black algae growing on the shingle surface. Since black roof algae is more likely to grow on a shaded, cooler, or more damp roof surface, it is less likely that you'll find black algae stains uniformly on all roof slopes on a building. Black algae staining on an asphalt shinlge roof is actually dead organic debris from an alga, (possibly Gloeocapsa magma).

How to distinguish roof algae from roof mold, soot, organic debris, or other substances

As for distinguishing between an algae growing on shingles and a fungus growing on roof shingles, a simple collection of the stained material using clear adhesive tape followed by a laboratory analysis by an experienced microscopist is conclusive. We don't recommend it 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.

Remedies for black algae staining on shingles

Black stains on asphalt shingles caused by algae is avoided by some of the new roof shingle products which include chemicals to retard algal growth. Cutting back overhanging tree branches for more sun can also help reduce this problem. Zinc or copper strips tacked along the ridge will produce (in rain) metal salts which will kill algae as they wash down the roof. New shingles offered by some manufacturers include algae resistant chemicals in their mineral granule coating.

Power washing to get rid of roof stains? Some folks recommend power washing to get reduce the staining - I'd be concerned that the power washing process might damage the shingles and significantly reduce the remaining shingle life. Whatever cleaning is done make sure the process does not cause more harm than good to the roof and to the environment around the building.

Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website. If you just "scroll down" you'll miss some important articles. See links at page left.

DEBRIS STAINING - Black Debris Stains on Asphalt Shingle Roofs

Debris staining on roofs produces fairly uniform black or brown staining in areas where due to low-slope or presence of overhanging trees, organic debris collects on the roof slope. Decaying debris may encourage fungal or agal growth (cited above).

Lichens or even thick moss may also grow on such roof areas. Debris, lichens, or moss on a roof will shorten its life by holding water on the roof surface, preventing drying, and (in cold climates) adding wear and tear on the roof surface by freeze-thaw cycling.

Remedies for black or brown or gray debris staining on asphalt shingles: As cited above, cleaning off debris (gentle sweeping or washing, watch out for damaging the roof during cleaning), or cutting unnecesary overhanging tree branches may help. Also see ALGAE, FUNGUS, LICHENS, MOSS on SHINGLES for details about these growths on buildings.

Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website. If you just "scroll down" you'll miss some important articles. See links at page left.

SOOT STAINING - Black Soot Stains on Roof Shingles Around Chimneys

Soot from fireplace flues washing down onto the roof - characterized by staining appearing below and in line with the chimney Similarly, soot from oil-fired heating flues washing down onto the roof, characterized by the same pattern of staining around and below the chimney; also probably an indicator of an operating problem with the heating system. That was clearly the case in the photograph shown here. Gas Flue Safety Warning: If this same sooting appears on and around a gas-fired appliance flue, there is a very unsafe condition present and risk of fatal carbon monoxide production inside the building. Immediate action is be needed. [Photograph of a soot-stained roof top courtesy of Roger Hankey a Minnesota home inspector.

Unlike other roof shingle stain patterns shown in this article, black stains caused by soot staining from a chimney will be prominent in a wide swath along the sides and below the chimney and will be little in evidence elsewhere on the roof.

RUST STAINING - Red or Brown Rust Stains on Roof Shingles

Metal roof flashings, metal chimneys, antenna mounts, or other metals that rust can stain roof surfaces a red or reddish brown color. Expect to see rust stains only around and below rusting metal components on a roof. If the rust staining is due to a deteriorated metal chimney, chimney cap, or roof flashing, further investigation is in order: make sure that the chimney is safe (no holes) and that any rusted roof flashings are not leaking or about to leak into the buildng interior. Otherwise this is a cosmetic defect which will be most noticeable on light colored shingles.

Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website. If you just "scroll down" you'll miss some important articles. See links at page left.

Technical Reviewers

Particular thanks are due to experts and also consumers who read these articles and suggest corrections, changes, and additions to the material. Content suggestions, technical corrections and content critique are invited for any of the content at our website.

• Daniel Friedman - principal author
• 7/07: thanks to Roger Hankey & Cheryll Brown, ASHI home inspectors in Minnesota, for the rooftop soot staining photograph. Mr. Hankey is a past chairman of the ASHI Technical Committee, serves as co-chairmain of ASHI legislative committee, and has served in other ASHI professional and leadership roles.
• Critique, contributions wanted: Contact Us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

More Information on Building Diagnostic Inspections and Repairs

• Roofing
• ASPHALT ROOF SHINGLES

InspectAPedia ^TM Home & Site Map - Building Inspection, Diagnosis, & Repair, Environmental Inspection & Testing - Research Website

The Mold Information Center: What to Do About Mold in Buildings, When and How to Inspect for Mold, Clean Up Mold, or Avoid Mold Problems

Home Inspection Construction Consulting Services & advice for home buyers

Contact Daniel Friedman for website content suggestions or for fee-paid consulting

	Enter your search termsSubmit search form
	Search This Website - InspectAPedia.com