Algae growth on stone walls at the entry to the dungeon, Goodrich Castle, Ross on Wye, Herefordshire, England UK (C) Daniel Friedman Stone Stain Diagnosis
How to Identify, Diagnose, Clean & Prevent Algae, Fungus, Lichens, Moss on Stone Tombstones, Graves, Roofs, Walls, Sidewalks, Sandstone & Other Building Surfaces

  • STAINS on STONE, DIAGNOSE & CURE - CONTENTS: Algae, fungus, lichens, moss growth on various outdoor surfaces: problem diagnosis & cure - How to clean graves, tombstones, stone walls, sidewalks
  • POST a QUESTION or READ FAQs on the cause, cure, & prevention of algae, fungus, moss or lichens stains & growth on exterior surfaces such as stone monuments, buildings, roofs, & other site features.

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Algae, Moss, Lichens on stone:

How to diagnose, remove & prevent stains due to algae, fungal growth, or moss on stones, monuments, & on surfaces of building exteriors or roofs: this article describes and provide photographs and advice on identifying, cleaning, and preventing algae, moss, lichens, or fungal growth that occurs on stone surfaces such as buildings, gravestones, sidewalks, stone walls, and in nature.

We include links to references useful in the identification of algae, moss, lichens, and mold. Our page top photograph algae growing on the stone wall at the entry to the dungeon of Goodrich Castle, Ross on Wye, Herefordshire, England U.K.

Green links show where you are. © Copyright 2015 InspectApedia.com, All Rights Reserved.

Appearance of Algae, Fungus, Lichens, or Moss on Stone Surfaces

Yellow orange lichens, Portland MaineArticle Contents

Colors & Types of Stains, algae, lichens, moss, fungae, or other substances found on Stone Surfaces

Black brown roof algae stains (C) Daniel FriedmanOnce we include lichens (such as our photo at above left, from Portland Maine), the range of colored growths or stains on stone surfaces is quite large. But the most common stain colors are

  • BLACK or DARK BROWN - most often due to algae, bacteria, dirt or soot, fungus. Fungus may also on occasion produce green or white films on stonework.

    [A Photo of black-brown roof algae on antique metal roof shingles is shown at left, illustrating that on building exteriors these organisms can invade various materials besides stone]
  • BROWN or RED STAINS - occur on masonry usually due to combination of water and metal components
  • GREEN STAINS - most often due to algae or moss
  • LIGHT GREEN, YELLOW, RED, GRAY, WHITE STAINS - most often due to lichens.

Below we provide example photographs of different colors and sources of stain or contaminants on stone or masonry surfaces, followed by advice on cleaning and stain prevention.


Black Stains on Stone Surfaces due to Algae, Fungus, or Dirt, Soot, Debris:

Black fungal stains Patzcuaro Mexico (C) Daniel Friedman

The presence of algae or fungus (mold) on a stone surface most often appears as a function of both weather exposure and the location of the surface in a shaded spot where you may want to be alert for development of moss as well.

Our photo (left) shows black staining, probably fungal in origin, on historic structures in Patzcuaro, Mexico.

The level of damage to a stone surface from black algae or any colored fungus is likely to be less than from moss and less than that caused by lichens.

But on stone or masonry building surfaces or on works of art or artifacts, Mandrioli et als. point out, and as we list in their Catalog of Biological Substances that Alter Stone or Other Inorganic Materials, there can be serious aesthetic and conservation issues.

Black Staining & Stone Damage from Cyanobacteria - Gloeocapsa sp. and fungi

Black stains on stone are quite often caused by a cyanobacteria (see Catalog of Substances that Alter Stone, Glass, Steel) Gloeocapsa sp. that not only stain the stone black, but also increase water absorption by penetrating veins in the stone (or marble, for example) leading to honeycomb weathering damage to the stonework.

Wet stone exposed to either freeze-thaw cycles or heating by bright sun can be spalled or cracked by these forces. For an example that received media attention, the New York Times reported ("Microbes Eating Away at Pieces of History", 24 June 2008) extensive damage to Angkor Wat, a twelfth century Hindu temple in Cambodia. Sixty to seventy percent of the Angkor Wat temple is black and deteriorating.

Significantly for building diagnosticians, the Times article reported that originally people believed that the black on this stone was due to "weathering". The Times continued that only recently have experts begun to realize the important role of both bacteria and fungi in the deterioration of cultural sites around the world. The article also noted that air pollution can increase the effects of biodeterioration.

Black stains on stone wall  (Daniel Friedman) Stone roof tile delamination (C) DanieL Friedman\

Our photo (above left) shows black staining on the stone surface of a mausoleum in Buenos Aires. Without closer examination we could not be sure if this was dirt and debris from city and traffic air, a dark fungus, or algae.

Our second photo of black growth on a stone roof (above right) was taken in Molde, Norway. Also

Algae and sometimes mold often appear on organic and even inorganic surfaces of buildings and in nature such as on asphalt roof shingles shown
at ALGAE STAINS on ROOFS. It is possible that on these metal roof shingles (Justin Morrill Smith historic home in Strafford VT) mold or algae is hosted by a painted coating on the roof.

Green Stains on Stone Surfaces due to Algae or Moss

Green algae on the Mills Mansion (C) Daniel Friedman

Green stains due to algae: green stains also appear on buildings including on shingles, siding, and even on masonry walls, sidewalks, planters, and retaining walls: stone, concrete block, and concrete.

Our photo (left) shows green algae on a fallen decorative brownstone artifact at the Mills Mansion, in Stattsburgh, New York.

If you see flat green stain on a building exterior and that is not producing any plant-like raised growth it is likely to be an algae. Our photo (below-left) shows green algae on concrete at a sidewalk and basement window grate surround.

Algae under the microscope: Our second photo (below right) shows what algae growth looks like under the microscope.

Algae stains on concrete (C) Daniel Friedman Algae under the microscope (C) Daniel Friedman

Watch out: on walks and decks algae makes for a dangerously slippery surface when it is wet.

More photos of algae, lichens, and moss, including both on building or stone surfaces and under the microscope are

Green stains due to moss: unlike the algae staining above, thick green growth on a stone or masonry surface is more likely to be moss such as the mossy concrete entry platform at the New York home shown at below left. The moss on this surface holds water, is a slip, trip, or fall safety hazard, and depending on the hardness of the concrete, it may also lead to spalling or frost damage to the concrete itself.

Mossy entry platform at a home (C) Daniel Friedman Moss and Lichens in the Quetico (C) Daniel Friedman

Reindeer moss: Our second moss and lichens photo (above right, Quetico boundary canoe area, Canada) demonstrates that moss and lichens can coexist happily on the same surface, and even intermixed on a surface where we show two kinds of moss. In our photo, the taller pale growth among the green moss is itself "Reindeer Moss, Caribou Moss" or in other references "Antler Moss".

Our photograph above shows Cladonia rangeferina or perhaps Cladonia sp. along with both moss and other lichens on the same stone. Reindeer moss is itself actually a lichen, and is an important food source for reindeer and caribou). Reindeer moss is quite fragile and slow growing, found in both hot and cold climates and in alpine tundra - don't trample it in the wild.

Moss under the microscope (C) Daniel Friedman

Moss under the microscope. As our photo (at left) shows, it's easy to see the plant-like structure of moss fragments, as our photograph (left) demonstrates. This moss sample was collected from a basement wall: we suspect that the damp conditions that produced this moss growth on the building wall also increased the risk of a mold problem in the same structure.

But you shouldn't need to use a microscope to identify moss. For a species/genera identification guide to mosses we include some moss information resources below.

What does Lichens look like on Stone Surfaces

Lichens on grave, Poughkeepsie Cemetary, Vassar (C) Daniel FriedmanSee LICHENS on STONE SURFACES for our complete article on this topic. Excerpts follow.

Also see Lichens on Roofs for a discussion of lichen damage to shingles and how to prevent lichens growth on roofs.

Lichens, one of the most hardy growth organisms found in nature, can grow in harsh conditions. Moss and lichens are both more than a cosmetic issue on most materials.

By holding moisture against the stone or other material surface lichens but more so moss speed the wear of the surface in freezing climates by increasing frost damage to the surface and by action of the organism's "roots" that penetrate the material surface.

Our photo (left) shows green lichens growth on a gravestone near Vassar College, in Poughkeepsie, NY. Notice that the lichens appears first on the more roughened stone surfaces inside the carved gravestone lettering, and on the rough-cut north face of the stone.

In addition to stone surfaces and mineral-covered roof surfaces, lichens also grows just fine on clay tile roofing and on wood (as those photo links show).

Lichens on Roofs demonstrate the mechanical damage that can be caused by lichens on surfaces.

See LICHENS on ROOFS for details about the causes, effects, and prevention of lichens growth on roofing surfaces. Lichens is often found growing on roof shingles, especially asphalt and wood shingles as we show here where we demonstrate the damage to the surface when lichens is removed. Our photos (below) illustrate how serious lichens damage can be to a mineral surface.

Photograph of lichens growth on asphalt shingles (C) Daniel Friedman Photograph of lichens growth on asphalt shingles (C) Daniel Friedman

Does Lichens Always Need to be Cleaned Off of Stone Surfaces?

Lichens on a stone wall (C) Daniel Friedman

No, of course not. The lichens growing on this stone wall in Poughkeepsie, NY (constructed by and found at Adams Fairacre Farms) is not going to do any harm, and in fact adds to the attractiveness of the wall.

Red or Other Color Stains on Stone Surfaces

Red stains on stone or masonry surfaces may be due to lichens, or on occasion bleed-down from rusting iron as we demonstrate just below.

Red stains on the surface of and as much as 1.5mm into Carrara and Condoglia Marble used in Italian monuments (the Certosa of Pavia facade, the Orvieto cathedral, the fountain statues of Villa Litta near Milan) have been reported by E. Zanardini et. als in Art, Biology, and Conservation: Biodeterioration in Works of Art, Robert J. Koestler et als. Eds., Metropolitan Museum of Art, 2003, ISBN 1-58839-107-8.

Apparently there is an association between a lead source, lead found in and on these works and lead-resistant microorganisms. The authors investigated this phenomenon and concluded that although red-pigmented microorganisms were present in the stains, "... their presence does not appear to be related either to the red coloration or to the presence of lead;" The conclusion was that the red stains on the Italian marble studied were due to the presence of lead.

Brown or Reddish-Brown Rust Stains on Stone, Concrete, Brick, Masonry Surfaces

Brown stains on stone, concrete, or brick are often caused by bleed-out from rusting components.

Our photo of the concrete tower in Buenos Aires (below left) shows rust stains on concrete at the catwalk brackets (click to enlarge).

In our rust-on-stone photo (below right) the deep brown stains in the bottom center of this photo (79th St. Boat Basin, New York City) are probably caused by the combination of salt from the roadway above, water leaks into and through the structure, and steel reinforcing rod or wire.

Brown rust stains on stone (C) Daniel Friedman Brown rust stains on stone (C) Daniel Friedman

Biological Substances that Alter Stone or Other Inorganic Materials

Please see Catalog of Substances that Alter Stone, Glass, Steel where we provide an extensive table of biodeterioration agents that damage stone, glass or steel,

excerpted from Cultural Heritage and Aerobiology, Methods and Measurement Techniques for Biodeterioration Monitoring, Paolo Mandrioli et als.,

expanded with three additional table lines for Actinobacteria and Gram-Bacteria listed in Art, Biology, and Conservation: Biodeterioration in Works of Art, Robert J. Koestler et als. Eds., Metropolitan Museum of Art, 2003, ISBN 1-58839-107-8, and finally, with minor additional editing and commentary by Daniel Friedman.

For conservators, the latter text, (Art, Biology ...) includes important studies on treatment and prevention of artifact deterioration, including studies of the use of biocides, anoxic eradication of fungi, and the use of other biocides or preparations for use in defense of cultural artifacts against microbial and environmental agents.

Methods for Cleaning & Preventing Future Organic Growth (Algae, Fungus, Lichens, Moss) from Stone, Brick, or Concrete Surfaces

Please see our complete article on this topic at Methods for Cleaning Stone Surfaces. Excerpts follow. Especially if you are restoring artifacts, art works, tombstones, or graves, avoid any aggressive cleaning methods that might damage the roof surface.

Mechanical Cleaning of Brick, Stone or Concrete Surfaces

The gravestone that was shown at the top of this page and appears below in "before and after" photos was successfully cleaned using simply a soft bristle brush and plain water. Here is what Patrick Walsh said about the cleaning procedure:

I only used regular cold tap water, no soap, and a soft bristled dish-brush. You should wet down and rub in water on the stone by hand (the growths were oily, and repelling water!). Then brush in circular pattern to break down the lichens, moss, or algae, using plenty of water for lubrication. Hand wipe the gravestone down before it dries again.

Start cleaning the tombstone at the top and work down. That way you won't have to re-scrub the run-off. This was a 7-foot high grave stone including the cross. All told, (minus the cross) it took me about 45 minutes (I started on the back, so I would be inspired to do the rest and in case I found a problem. Then I cleaned the gravestone's sides and top. The front surface was cleaned last.

Photos of Australian tombstone before and after cleaning Photos of Australian tombstone before and after cleaning

Whatever cleaning is done on a stone or masonry surface, make sure the process does not cause more harm than good to the artifact and to the environment around it.

Question: How to clean off & prevent moss or green algae/fungal growth in a car wash

Car wash moss and algae or mold (C) D FriedmanHi, I own a car wash. I've been having a moss/algae problem that's becoming progressively worse. The problem is in the car wash tunnel. It began growing on the cloth used to clean the cars and now has spread to some of the steel components and the concrete floor as well.

The Area is very damp, abundant in sunlight, and poorly ventilated overnight when the garage doors are closed.

Originally I had thought my water might be contaminated but I was told that because I have many windows in my tunnel that the sunlight is causing this, while another professional told me it was more likely due to poor ventilation.

I've been in the business for a long time and have had other locations, but never had a problem like this. I'm wondering if you might have any thoughts on what's causing this aside from the dampness factor which I can't help. Any input would be greatly appreciated. I've attached some photos. thank you, E.L. 6/12/2012


Car wash moss and algae or mold (C) D FriedmanA competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem with moss, algae, or fungal growth on building surfaces and materials, though I expect you are far more an expert on car wash operation and maintenance than most outside "consultants" you might find.  That said, here are some things to consider:

Some of your photos indeed look to me like moss - I agree, especially along the steel guide track.

But others, on the cloths, may be algae or even a fungus. I couldn't tell for sure from your photo. We could have you take samples and send them to me for analysis to ID for sure what you've got, but because I suspect that the causes and remedies are going to be the same, I'm not sure that step is worth the trouble.

I'll do what I can to assist, pro-bono as long as we can keep the time and writing from becoming too consuming - what we learn will be useful to both of us.

Diagnosis of moss or algae growth in a car wash

Moss, algae, and fungal spores are airborne and are, in a practical sense, everywhere, all the time. So when one or more of these becomes a problem in a car wash it is helpful to ask why here, now and not in other car wash operations? The answer to cause is important in deciding on the cure. Otherwise, you could go to the expense and trouble of a clean-up only to have the problem occur all over again.

Let me pose some questions and perhaps when you see my line of thinking you, knowing the operation well, will think of similar questions to ask or discuss.

Are there significant differences at this car wash from other units you know about in regards to:

1. Water chemistry or water recycling and treatment before re-use

2. Ventilation design and operation. Are your vent fans working, ducts unobstructed, that is, not simply that they run, but do they work?

3. Sun exposure, including near by shade trees, forest, wet areas around your property. It would be lack of sunlight, not presence of sunlight, that would encourage the growth of these substances in the car wash.

4. Cleaning fabric chemistry: the age, duty cycle, fabric used, manufacturer or source or even batch number of the cleaning cloths that show a green growth (algae or moss or fungus or even a combination of all of these?)

5. Rinse water: the cleanliness, temperature, chemistry of rinse water

6. Maintenance: car wash facility cleaning & maintenance schedule or procedures

7. Age: How long has the green growth problem been evident? To my eye, moss like shown in two of your photos does not appear over night or even in a month.

Cleanup & prevention of moss & algae in a car wash facility

I'm not satisfied to just clean the problem surfaces without having a guess at the underlying cause, but nevertheless we can discuss some beginning options.

Steel surfaces with thick mossy growth may need to be hand-scraped as a starting cleanup step.

Use a power washer (with a deck cleaner solution or you may have your own detergent solution) to clean the contaminated surfaces. If you want to salvage and continue using the suspended cleaning cloths, that's a synthetic fabric that should tolerate being removed from the spinner drum, placed on a flat concrete surface, and spray-washed with the power sprayer.

That cleaning won't remove 100% of the moss or algae but it should remove most of it and leave the materials looking clean.

I am doubtful that using bleach or other sanitizers is necessary and also doubtful that it would be long term effective on cloths and on unpainted steel surfaces. If there are painted surfaces nearby that also suffer these growths you might want to use a fungicidal sealant paint the next time those areas are cleaned, dried thoroughly, and then re-painted.

There are also algaecide products intended to be added to pools to prevent algae growth (Utikem and other ammonium chloride based products) but again I'm not sure.

Cause & Cure for Black Stains on Re-Constituted Sandstone & Building Exterior Masonry Surfaces

Black stains on re-constituted sandstone on  a building exterior (C) InspectAPedia C&D Black stains on building exterior stonework (C) InspectAPedia C&D

[Click to enlarge any image]

Reader Question: Having looked on your website I was wondering if you could tell me if you think these black stains on this re constituted sandstone could be bacterial and what damage could this be doing and the best way to treat it. C&D 9/23/2014

Black stains on re-constituted sandstone on  a building exterior (C) InspectAPedia C&D Black stains on building exterior stonework (C) InspectAPedia C&D


Black stains on building exterior stonework (C) InspectAPedia C&DWhile bacteria may be a factor in black stains on masonry-like building exteriors, more often the colour is due to an algae or a fungus. That's what I see in your first three photos of black stains on building masonry exteriors.

At above right I see what looks like moss or algae or both on a brick wall inside corner where one would infer the presence of roof runoff spillage that might be better controlled or routed. But shade is also a factor in moss and algae growth on buildings.

And in our photo at left we see a heavy moss growth on the building balcony edge suggesting a long-standing combination of watr and possibly partial shade in this location.


Sandstone & Other Building Masonry Cleaning Advice for Black Stains, Moss, Algae

We discuss the cleaning/restoration of the stone grave marker shown at page top as well as other stone surface cleanign methods in more detail

Watch out: Grimmer (1992) warns about using abrasives to clean stone exteriors, and further quoting another expert on cleaning sandstone:

Avoid the use of nonproprietary acids on sandstone, which can damage the material irreversibly.

Proprietary cleaning materials fall into two categories—general purpose and cleaners for metallic-sensitive masonry. Only cleaners for metallic-sensitive surfaces should be considered.

Always use a test panel and evaluate the re- sults for at least two weeks be- fore determining acceptability. -  Schierhorn _ret. 2014)

In addition to reviewing the article above, you'll want to take a look at STONE CLEANING METHODS [live link is given below]

Also see the citations we offer below, particularly Grimmer (1992) and Christopher (1990)

Research on Re-Constituted Sandstone constituents, installation, troubleshooting, cleaning, repair

  • Alvarado, Giovanny, Neville Lui, and Matthew R. Coop. "Effect of fabric on the behaviour of reservoir sandstones." Canadian Geotechnical Journal 49, no. 9 (2012): 1036-1051.
  • ASTM C 119-94a, “Standard Terminology Relating to Dimension Stone,” ASTM, American Society for Testing and Materials, 100 Barr Harbor Dr., West Conshahocken, PA 19428
  • Boornazian, Glenn, and Norman R. Weiss. "SANDSTONE: HISTORY OF USE AND PRESERVATION." Structural repair and maintenance of historical buildings (1989): 469.
  • Clifton, James R. "Laboratory evaluation of stone consolidants." Studies in Conservation 29, no. Supplement-1 (1984): 151-155.
  • Christopher, D. "Exterior Cleaning of Sandstone Buildings in Edinburgh: Technical and Aesthetic Considerations: Submitted for the Degree of Master of Science [Architectural Conservation]." PhD diss., School of Architecture, Edinburgh College of Art/Heriot-Watt University, Edinburgh, 1990.
  • Dajnowski, A. "Laser cleaning of the Nickerson Mansion: The first building in the US entirely cleaned using laser ablation." In Lasers in the Conservation of Artworks: Proceedings of the International Conference Lacona VII, Madrid, Spain, 17-21 September 2007, vol. 3, p. 209. CRC Press, 2008.
  • Dapples, E. C. "Some concepts of cementation and lithification of sandstones." AAPG Bulletin 56, no. 1 (1972): 3-25.
  • Dimension Stone Design Manual IV , 1991, Marble Institute of America, 33505 State St., Farmington, MI 48335
  • Grimmer, Anne E. Keeping it clean: removing exterior dirt, paint, stains and graffiti from historic masonry buildings. DIANE Publishing, 1992.
  • Grimmer, Anne E. Dangers of Abrasive Cleaning to Historic Buildings. [Department of the Interior], Heritage Conservation and Recreation Service,[Office of Archeology and Historic Preservation], Technical Preservation Services Division, 1979.
  • Heidelmann, Hendrik. "Voruntersuchungen zum Zustand der Architekturteile und des plastischen Zierats des Sandsteinbaus der Kunstakademie Dresden." Arbeitsblätter für Restauratoren. Gruppe 6. Stein 27, no. 2, Gruppe 6 (1994): 313-318.
  • Herget, Frederick A., and Robert W. Crooks. "Deterioration and Stabilization of Berea Sandstone on the Hamilton County Courthouse." ASTM SPECIAL TECHNICAL PUBLICATION 1180 (1993): 412-412.
  • Hyslop, Ewan. The performance of replacement sandstone in the new town of Edinburgh: evidence from grant-aid repair schemes of the Edinburgh new town conservation committee. Historic Scotland, 2004.
  • Leitch, E. C., V. J. Morand, C. L. Fergusson, R. A. Henderson, and P. F. Carr. "Accretion and post‐accretion metamorphism in subduction complex terranes of the New England fold belt, eastern Australia." Journal of Metamorphic Geology 11, no. 3 (1993): 309-318.
  • Preston, John. "The surface restoration of buildings–An investment in the present as well as in the future." Structural Survey 7, no. 4 (1989): 450-460.
  • Sargent, Karen. "Exterior sandstone restoration of Alexander Hall." In Preservation and restoration of cultural heritage: proceedings of the 1995 LCP Congress, Montreux, 24-29 September 1995= Conservation et restauration des biens culturels: actes du congresLCP 1995, Montreux, 24-29 septembre 1995, pp. 231-234. Laboratoire de conservation de la pierre, 1996.
  • Schierhorn, Carolyn. "Sandstone." [PDF] retrived 9/23/2014, original source: http://www.masonryconstruction.com/Images/


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