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

  • STAINS on STONE, STUCCO DIAGNOSE & CURE - CONTENTS: Algae, fungus, lichens, moss growth on various outdoor surfaces: problem diagnosis & cure - How to clean graves, stucco surfaces, stones, stone walls, sidewalks, tombstones, monuments, carvings, artifacts & artwork outside.
  • 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 or other stains on stone & stucco surfaces:

Catalog of colors & types of stains or discoloration found on stone & stucco or othe routdoor masonry surfaces including monuments & tombstones: causes & cures.

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 series 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 2017, All Rights Reserved.

Appearance of Algae, Fungus, Lichens, or Moss on Stone, Stucco, Masonry 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 Friedman

Once 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

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, Stucco, or other Masonry 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.


Thought this photo of a house we are considering would be a nice addition to your library. Is this algae from excessive gutter runoff? - Anonymous by private email 2016/08/03

- Photograph temporarily OFF pending reader approval.


Thanks for the photo, .... It doesn't show enough information for a reliable guess, but the dark pattern I see on the left edge of what looks like a stone wall or a stone veneer wall could be from roof drainage or gutter spillage; wetting the area can indeed encourage black staining that is often dominated by algae or the cyanobacteria we discuss here at STAINS on STONE, STUCCO DIAGNOSE & CURE.

Black Algae Stains on Stucco Wall Surfaces

Black algae stains below roof drain on stucco wall in Pozos, Guanajuato, Mexico (C) Daniel Friedman

Above: black algae stains mark the painted stucco wall of this home in Pozos, in Guanajuato, Mexico. Water from the roof drain is the most-likely cause of these stains.

Rain splash up  wets lower wall leading to dark stains (C)

\Above: black stains on the stucco wall of this Long Beach California home in the U.S. Notice the drip line in the center of the sidewalk; I'd bet that there is a clogged or overflowing gutter at a roof eave above this location.

The reader who offered this photo commented:

This started about two weeks ago and I noticed today. I live in long beach ,ca and this is the right side of house which gets no sun. This side is lined by trees from my neighbor's house . I bought the house in May 2016 and this wasn't there. I also recently started running the Hvac system for the house.

I hope it is just mold or algae and not something caused inside walls. The water heater ,kitchen and master bath are on this side wall too but it's so uniformed in pattern. - Anonymous by private email 2016/07/12

Comment from moderator:

Looks to me as if there is a roof edge that drains into this area - see the dripline marks on the concrete - and that rain-splash is soaking the exterior wall near the ground. That wetness combined with shade is probably inviting algae or possibly mold or both to grow on the surface.

We often are convinced that some building condition occurred immediately before we noticed it - sometimes that's so, but not always. Sometimes a building condition such as a stain has actually been present for longer than we realized. It was less severe, or it had simply not been called to our attention.

Reader reply:

Looks to me as if there is a roof edge that drains into this area - see the dripline marks on the concrete - and that rain-splash is soaking or has previously-soaked the exterior wall near the ground. That wetness combined with shade is probably inviting algae or possibly mold or both to grow on the surface.

If I didn't see that drip line along the sidewalk I'd still think rain-splash wetting of the wall, though any water source that wet the wall uniformly might be expected, such as use of a sprinkler hose in some situations.

Moderator reply: you'll want to get those gutters working.

You'll want to get those gutters working.

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. Details are

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 lichens on decorative stone, South Island, New Zealand (C) Daniel Friedman

Above: red lichens (and also some green moss) on decorative stone around a building on South Island in New Zealand. [Click to enlarge any image].

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.

Yellow orange lichens, Portland Maine Yellow orange lichens, Portland Maine

Above: red-yellow-brown lichens on rock along the seacoast at Portland, Maine.

Question: what about red stains on buildings from Serratia marcescens

2017/04/11 Kai said:

I'm surprised you didn't mention Kai said:

I'm surprised you didn't mention Serratia marcescens.

Reply: Serratia marcescens is an important human pathgen but not something that likely to be found as large area red stains on building surfaces

Thanks for the suggestion, Kai.

Serratia marcescens is an enterobacteria and is a human pathogen and is an important cause of nosocomial infections. On host surfaces such as bread it can appear bright red. Wikipedia's entry on this bacteria includes this commentary:

It is commonly found in the respiratory and urinary tracts of hospitalized adults and in the gastrointestinal system of children.

Due to its abundant presence in the environment, and its preference for damp conditions, S. marcescens is commonly found growing in bathrooms (especially on tile grout, shower corners, toilet water line, and basin), where it manifests as a pink, pink-orange, or orange discoloration and slimy film feeding off phosphorus-containing materials or fatty substances such as soap and shampoo residue.

Once established, complete eradication of the organism is often difficult, but can be accomplished by application of a bleach-based disinfectant. Rinsing and drying surfaces after use can also prevent the establishment of the bacterium by removing its food source and making the environment less hospitable.

Although small (in total area) colonies of serratia marcescens might appear on some indoor surfaces as described, e.g. in bath tile grout, the total area is likely to be trivial and can be addressed by cleaning and disinfecting.

Serratia marcescens has not been reported (by scholarly research) as a common or widespread stain on stone surfaces of buildings.

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

Red rust stains on stucco & stone, Grace Church, Millbrook NY (C) Daniel Friedman

Brown stains on stone, concrete, or brick are often caused by bleed-out from rusting components. But on occasion we find orange rust stains on both stones and on building surfaces around included stone components that originate at iron inclusions in the stone itself. That seems to be the case in our photo of red rust stains appearing on both stone components and on nearby stucco exterior walls on the Grace Church in Millbrook New York (above, and in a closer-look just below).

Red iron stains on stone and stucco, Millbrook NY Grace Church (C) Daniel Friedman

Rust colored stains often appear on masonry structures due to water penetration into building surfaces where it finds and rusts steel reinforcement mesh or re-bar.

While it is possible to clean these stains from a building surface, if the stain source remains, the stain will recur.

You may be able to retard the reappearance of these rust stains emanating from iron inclusions in the stone itself by periodic use of a clear sealant but I don't recommend that step without testing the approach on an unobtrusive building surface first.

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

Brown rust stains on stone (C) Daniel Friedman

In our rust-on-stone photo (below) 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

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 STONE SURFACE CLEANING METHODS.

Especially if you are restoring artifacts, art works, tombstones, or graves, avoid any aggressive cleaning methods that might damage the roof surface.

Photos of Australian tombstone before and after cleaning .

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

We moved this information to a new article now found at STAINS on SANDSTONE, DIAGNOSE & CURE


Continue reading at STONE SURFACE CLEANING METHODS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


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