Guide to Red or Brown Rust Stains on Building Roofs & Chimneys

RUST STAINS on ROOFS - 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
CHIMNEY STAINS & LEAKS - separate article
STAINS on ROOFS - separate article
STAIN DIAGNOSIS on BUILDING EXTERIORS - separate article
Questions & answers about the causes of red or brown stains on roof surfaces or shingles
References

Click to Show or Hide Related Topics

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

This article explains red or brown rust stains that occur on building roofs. This article series 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.

RUST STAINING - Red or Brown Rust Stains on Asphalt Roof Shingles or on Metal Roof Surfaces

Photograph of reddish brown plumbing flashing rust stains on an asphalt shingle roof (C) Daniel Friedman

See Causes of Roof Stains and Catalog of All Roof Stains for the causes and types of stains that occur on building roofs. There are other sources of roof staining, from cosmetic to harmful to the roof. For the diagnosis and cure of other building stains, see STAIN DIAGNOSIS on BUILDING EXTERIORS and STAIN DIAGNOSIS on BUILDING INTERIORS. Also see Red or Other Colors on Stone. See STAIN DIAGNOSIS on STONE for additional examples of diagnosing, cleaning, and preventing stains on building materials and artifacts.

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 building interior.

Otherwise this is a cosmetic defect which will be most noticeable on light colored shingles.

Red Rust Stains in Concentrated Locations Traced to Chimney Caps or Top Flashings

The dark red stain on the side of the chimney shown in the photograph of a New York home chimney at below-left has its origin in the rusted galvanized metal chimney cap just above. Click to enlarge the photo and you'll observe a surprising detail: a comparatively small area of rust on the chimney cap can make a rather intense stain on surfaces below. We figure that's because of the concentration of iron oxide dripping onto a specific and concentrated location.

Red rust stain at chimney traced to rusting cap (C) Daniel Friedman

In our second chimney top and cap rust stain photo (above right) you can see another red rust stain spread pattern in the center of about five courses of brick below the very chimney top of this Maine home. Why does the stain appear about a foot below the metal chimney top cap rather than immediately below it? Possible explanations have to do with the combination of the bent drip edge on the bottom of the chimney top flashing, wind direction, and even variations in brick porosity.

Red Stains over Roof Surface Traced to Chimney but is it Rust, Unsafe Chimney Flue, or Heater Operating Problems?

In a different case, the six photographs below show extensive red rust-colored roof staining covering a much wider area than the cases we discussed before. These pictures were sent to us by a chimney professional in Connecticut who asked:

I having a problem with a customer's chimney on Long Island, NY. We installed a oil burning boiler in 2006 with a stainless steel chimney liner. He has orange color staining on his stone chimney and roof shingles. The top of the stone chimney is capped with stainless steel that seems to be rusting and it seems to be bleeding from there? Is it possible that the exhaust from our chimney is the cause? - W.R. Connecticut 2/20/2013

Red roof stains traced to chimney (C) InspectApedia

Note the broad area of rust-colored reddish-brown stains on the asphalt shingle roof (above left) and the extensive red-brown staining in the roof valley (above right). All of these stains are down-roof from the chimney. Discussing possible stain causes with Alan Carson, a home inspection professional & educator with Carson Dunlop Associates in Toronto we thought that the most likely explanation of the roof staining is rust from the chimney cap or top pan flashing shown at below left.

At above left notice the rusty edge of the drip collar just below the stainless steel chimney cap. Note the rust stains that extend from the chimney top pan flashing below this collar and how the stains run to the top flashing edge (arrows). I think we can follow the rust down 1-2-3 to the edge of the pan flashing. As with the earlier cases of red and rust stains on roofs it is surprising how much red iron oxide, sufficient to stain a wide area, can be generated by a relatively small area of rusting metal. Above right is another view of this area.

Follow the Rust Stains Up or Down the Roof between Suspected Source & Stain Locations

We also note in the photo at below left that red oxide is concentrated at the intersection of the base of the chimney and the rooftop where it appears that a roll-roofing or modified bitumen-covered cricket is installed. {We will eschew worrying about why we don't see any flashing, just caulking, at this location. Remember that water (and iron-oxide bearing water) flows downhill. Usually. At below left our red arrows continue to follow the iron oxide down the chimney and down the cricket to the rusty area of the shingles below.

Check the Flue for Combustion or Heater Stain Particle Sources

Our photo at above illustrates the chimney cap - a stainless steel component. We didn't see anything red coming out of the flue (though we address that question in a moment) though we did see traces of soot or creosote (brown arrow) and what looks like some funny loose duct tape where we wouldn't expect to find that material (orange arrow).

This combination of visual evidence, and absent other details that might be disclosed from an onsite inspection, Al and I didn't think the red stains are coming up out of the flue (see discussion below) but rather are originating at the galvanized steel collar and possibly receiving a contribution from a rusting galvanized chimney top pan flashing. Without an up-close inspection of the chimney pan flashing we don't know if the red there is just a deposit from above or if that component is rusting as well.

Follow-up comments and suggestions made another effort to rule out safety or mechanical system issues as follows. We considered these explanations for the red stains on the roof:

DJF Remarks:

First while there's no doubt about the metal chimney and its cap itself, I'm assuming you are certain that the chimney top crown/cover is stainless not galvanized (though even if it were poor quality SS or galvanized that would not explain the very large amount of red staining). Next: I have on occasion seen an oil burner producing red debris in an exhaust if

the system is misadjusted and running quite hot (certainly we're seeing so little soot that this could be the case, though the next guess is the most interesting one)
if this is a hydronic system (hot water boiler) or a steam boiler too if there is a water leak inside the boiler. Water dripping into the combustion chamber and cooking away in the exhaust can produce quite a bit of red debris.

Take a look into the combustion chamber of the boiler - look up at the underside of the heat exchanger for white crud, mineral deposits, stains, and also look at the combustion chamber liner and floor for signs of water or chamber liner damage. Let me know what you see at and inside the boiler.

It would make sense, just to assure the safety of the occupants, that you have the entire flue interior inspected using a ChimScan camera; you will assure the flue is safe, and further we might see deposits that shed more light on the red stains.

Al Carson adds:

Rust seems logical. Products of combustion from an oil burner condensing? Possible but not usually without added circumstances like the water leak in the boiler you suggested.
Is the chimney unusually tall or cold? Does it have 3 outside walls?
Is there a contaminant in the combustion air? Chemicals in the basement? Anything corrosive in the environment of the boiler itself?

Our conclusion, supported by the close-up cropped from one of WR's photos [a good reason to take sharp high resolution photographs when diagnosing a building problem], is that the galvanized steel collar and chimney top pan are rusting.

Watch out: Of course, weaseling a bit, an on-site inspection is always orders of magnitude more reliable than sending photos by email. When you send photos to someone they are very helpful, even key. But the remote "expert" can only examine and comment on what you saw and photographed, not the other information, clues, or defects to which you never aimed eye nor camera.

That said, W.R. provided some excellent photos. Reader comments are welcome.

As we discuss at Power Washing Roofs we do not recommend roofing.

Reader Follow-Up: forensic lab test results of stained shingle area confirms FeO₂ Iron Oxide

We sent a small piece of shingle and steel from that customer's roof and chimney cap to a lab in Georgia and this is the results of their findings.

Proving that the red stain is rust. Now we just have to find the source or sources of all this rust.

Yes some is coming from the stainless steel top but for all the rust that's running down his roof you would think that there would be nothing left of that top.

[Click the chart above or any InspectApedia illustration to see an enlarged, detailed version.]

Reply:

Thanks for the report, it confirmed what we all thought. (Though there are some red algae that sometimes form stains on buildings, this report puts that worry to bed).

I started thinking at the same point I infer from your comment, that the amount of staining was just too huge to be blamed on that rusty stuff we see at the chimney top.

But having reviewed my library of rust stains on roofs I found other cases where there was dramatic staining traced to a relatively small (in square inches) area of rusting steel. I speculate that the number of molecules of FeO₂ (iron oxide) needed to stick to and noticeably discolor a shingle or other surface, especially one that is good at capturing the FeO₂ , is small - that is, a small amount of rusty water falling off of a chimney or flashing can produce a big area of staining. Take a look again at the photos in the article we've done on this - in the article above. Some small areas of rust can produce a large stained area.

In sum, a small volume of rusting iron or steel can produce a large volume of water carrying staining iron oxide molecules that will adhere to different surfacesd at rates varying by the surface properties.

The second leg (I barely have one to stand on, I admit) is the tracing of the stain patterns uphill back to the probable source. Stains generally follow the water down the roof.

If there were a third leg (don't go there) it would be that previously in the same location as area of rust stain origin there had been other more badly rusted components at or around the chimney top that were replaced before you began. While you're at it, take another close look at that very orange-looking caulk I see in one of your photos at the base of the chimney. The reason I do not suspect it is confirmed by your lab test - the test found iron oxide - we've then got to be looking for a source of FeO₂ - back to flashings, current or prior. If you can find out prior history and if any parts were changed at the chimney top that would help.

If you take a look (or have someone else expert do so) at the heating appliance and its combustion chamber to finish closing the door on that (now looking unlikely) possibility that there was a heating system snafu, that'd leave us with just what's at the chimney top. I'd like to see you remove all of the galvanized at the chimney top, replace it all with stainless.

The next question is how much cleaning should be done on the shingles. Power washing shortens roof life; I've seen some try treatment with iron removers (IronOut) used on boilers and on plumbing fixtures - but watch out: if the cleaner runs down a building wall and is not washed off immediately we've created a new problem.

...

Frequently Asked Questions (FAQs) about red stains or rust stains on roofs

...

Ask a Question or Search InspectApedia

Questions & answers about the causes of red or brown stains on roof surfaces or shingles.

Ask a Question or Enter Search Terms in the InspectApedia search box just below.

Technical Reviewers & References

Related Topics, found near the top of this page suggest articles closely related to this one.

Roger Hankey is principal of Hankey and Brown home inspectors, Eden Prairie, MN. Mr. Hankey is a past chairman of the ASHI Standards Committee. Mr. Hankey has served in other ASHI professional and leadership roles. Contact Roger Hankey at: 952 829-0044 - rhankey@hankeyandbrown.com. Mr. Hankey is a frequent contributor to InspectAPedia.com.
Best Practices Guide to Residential Construction, by Steven Bliss. John Wiley & Sons, 2006. ISBN-10: 0471648361, ISBN-13: 978-0471648369, Hardcover: 320 pages, available from Amazon.com and also Wiley.com. See our book review of this publication.
Asphalt Roofing Residential Manual,
Building Pathology, Deterioration, Diagnostics, and Intervention, Samuel Y. Harris, P.E., AIA, Esq., ISBN 0-471-33172-4, John Wiley & Sons, 2001 [General building science-DF] ISBN-10: 0471331724 ISBN-13: 978-0471331728
Building Pathology: Principles and Practice, David Watt, Wiley-Blackwell; 2 edition (March 7, 2008) ISBN-10: 1405161035 ISBN-13: 978-1405161039
Concrete Folded Plate Roofs, C. Wilby PhD BSc CEng FICE FIStructE (Author), Butterworth-Heinemann, 1998, ISBN-10: 0340662662, ISBN-13: 978-0340662663
Concrete Shell Roofs, C. Wilby PhD BSc CEng FICE FIStructE (Author),
Concrete Dome Roofs (Longman Concrete Design and Construction Series),
Historic Slate Roofs : With How-to Info and Specifications, Tina Skinner (Ed), Schiffer Publishing, 2008, ISBN-10: 0764330012 , ISBN-13: 978-0764330018
Low Slope Roofing, Manual of, 4th Ed., C.W. Griffin, Richard Fricklas, McGraw-Hill Professional; 4 edition, 2006, ISBN-10: 007145828X, ISBN-13: 978-0071458283
- Roof failure causes in depth (and specific methods for avoiding them)
- Roof design fundamentals and flourishes, based on voluminous industry research and experience
- New technologies and materials -- using them safely and correctly
- Comprehensive coverage of all major roofing systems pecifications, inspection, and maintenance tools for roofing work
Metal Roofing, an Illustrated Guide, R.A. Knowlton , [metal shingle roofs],
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.)
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).
Slate Roofing in Canada (Studi4es in archaeology, architecture, and history),
"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
...

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, 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.

Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.


	InspectAPedia^®	InspectAPedia
	Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair	Ask a Question or Search InspectAPedia

Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair

Ask a Question or Search InspectAPedia