How to clean building stains & algae, fungus, lichens & moss from exterior surfaces:
This article describes cleaning, and preventing future algae, moss, lichens, or fungal growth that occurs on stone, brick, and other surfaces such as buildings, gravestones, sidewalks, stone walls, walks, roadways and in nature.
We include links to references & researh useful in the identification & control of algae, moss, lichens, and mold.
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Especially if you are restoring artifacts, art works, tombstones, or graves, avoid any aggressive cleaning methods that might damage the roof surface.
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.
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.
Watch out: don't try any cleaners or preservative treatments on a valuable cultural artifact or art work without first discussing options with a professional art conservator. If you are proceeding to use any cleaning procedure or chemical, try it first on a small, unobtrusive area of your artifact or surface.
A comprehensive survey prepared by Caltrans ( 2011) examined studies by various U.S. states concerned with the control of moss growth on pavement. Those experts examined a wide range of methods used to remove moss growth as well as to control or prevent its recurrence, yielding advice on the effectiveness and costs as well as safety of various approaches from which we include excerpts here.
The Caltrans information is followed by advice from the USDA on controlling sooty molds appearing on exterior surfaces.
Treatment Options for Algae, Fungal, or Moss Growth on Stone & Masonry Surfaces
These moss cleaning & control methods studied for application on roadways in the U.S. may be applicable in whole or part to treatment & control of algae, lichens, moss and even fungi on exterior surfaces such as masonry or stone walls, walks, and monuments outdoors
|Moss Treatment Method||Description||Comments
|Synthetic moss growth control products||Wet & Forget. This commercial product can be used in residential and commercial applications. Marketed as noncaustic, nonacidic and safe for all outdoor surfaces, the product’s active ingredient is alkyl dimethyl benzyl ammonium chloride .||Environmental concerns;|
|Glyphosate. Two applications per year of this herbicide should be adequate to kill the moss. Glyphosate is the active ingredient in commercial herbicides such as Roundup®||May not be suitable for areas of animal or human occupancy or direct-contact; Environmental concerns;|
|Natural-based moss growth control products||
Acids: acetic acid, citric acid, pelargonic acid, cloveoil, corn glugen meal were cited by Caltrans as under study, generally less effective and more costly than the synthetic alternatives.
|Baking soda, vinegar, biodegradable fatty acids. Certain kinds of soap and borax are also recommended.|
Electrolyzed water: Identified as a cost - effective and environmentally friendly alternative that can be used as a contact fungicide on plant surfaces and for general sanitation in greenhouses.
A cell electrolyzes water, resulting in two types of water possessing different properties:
An electrolyzed basic solution that has strong reducing potential and can be used as a cleaning solution.
An electrolyzed acid solution with a strong oxidation potential that can be used as an alternative to chlorine - based compounds for killing bacteria.
|Recent procedure, uses water and saturated salt (sodium chloride) to create an electrolyzed acid used as a contact fungicide on plant surfaces|
|Mechanical moss removal methods||High - pressure, low - volume power washer. A small - scale test of this mechanical method involves the use of a portable power washer to determine how much pressure is needed to remove moss when moving along at a reasonable rate of speed (that is, 10 mph).||
Watch out: as we also cite at STAINS on STONE, STUCCO DIAGNOSE & CURE, Grimmer (1992), - Schierhorn ret. 2014) and other experts warn about using abrasives to clean stone exteriors of buildings or stone cultural artifacts. .
|Brooming - sweeping has been used on roadways.|
|Moss cutting machine. This machine, used by an irrigation district in Oregon to remove moss from canals, applies similar technology|
|Light & Heat to kill, remove or prevent moss growth||Infrared heaters. An infrared heater is used to direct a propane flame at a ceramic or metal plate, which radiates heat onto the moss. Other applications use water and intense heat in an enclosed space|
|UV light: shielded high-intensity ultraviolet light has been used to treat moss growth on roadways.||
Widely used in water treatment & in forensic investigation.
|Coatings to retard moss or algae growth on surfaces||
Fog line paint: Caltrans observed that the paint used to provide white fog lines along roadways appeared to be a moss growth retardant in areas where otherwise moss appeared along roadway sides and suggested the paint might be moss-retarding.
In our [DF] opinion an error in this consideration was the omission of the moisture-absorption effects of the fog line painted along roadways.
By coating the roadway surface the pavement will surely absorb less moisture and perhaps fewer spores as well, acting thus as a moss growth retardant in an effect beyond the biocidal effect of the paint itself.
This observation suggests that where other conditions permit, periodic coating of some stone surfaces with moisture repellants such as Thompson's water seal may retard moss, algae, lichens ane even fungal growth on those areas.
However in our personal experience those silicone-based coatings are not durable and may need annual re-application, making them not cost-effective for some cases. And on hard surfaces such as walkways there may be slip hazards as well.
Paints including fungicides may use elemental zinc or other metals or chemicals.
Not suited for artifacts, most building exteriors, most walkways, masonry stairs, etc
|Laser Cleaning of Sandstone or Stone Surfaces||
Building & stone artifact conservators have researched and used lasers as a method for cleaning some stone surfaces. See
See additional sandstone cleaning citations at
Adapted to include excerpts from
References on building or monument moisture control, moss, historic preservation
Not all roadway moss control measures in the original source are included in the table above. For example we exclude "Pavement mix selection" as a means of moss control as it pertains more narrowly to roadways not to other outdoor surfaces such as stone walls and brick structures.
Using UV light in a wide range of applications inculding the control of bacteria, mold, and algae or moss growth is also discussed at
The USDA has published How to Recognize & Control Sooty Molds. That publication includes suggestions for removing sooty black molds (the author considers Cladosporium sp. and Aureobasidium sp. as sooty black molds though they may in fact be brown or black and may not easily wipe off as "sooty" molds). Excerpting & commenting on the USDA advice for controlling sooty molds on surfaces: :
Sooty molds can be indirectly controlled by reducing populations of sucking insects that excrete honeydew. Outdoor furniture can be hosed down with water during periods of honeydew excretion, particularly during drought. The growth of sooty mold fungi is inhibited by preservatives used in treated wood in rustic outdoor furniture. Here is the recipe for a good cleaning solution for removing sooty molds from plastic or painted surfaces:
USDA Stone Cleaning Agent Formula
|Powdered household detergent||1/3 cup|
|Household liquid bleach||1 quart|
|Trisodium phosphate||2/3 cup [note: for environmental reasons TSP is no longer used, try a TSP substitute]|
Watch out: Be sure to wear rubber gloves when cleaning with this solution. [Note that trisodium phosphate is no longer used as a cleaning agent because it is an environmental contaminant.
However substitute products may be available. Be careful not to add a cleaning agent to bleach that can cause production of toxic chlorine gas.
Power washing, using plain water or perhaps a deck cleaner can be very effective for cleaning off stone, concrete, or masonry sidewalks and some building surfaces where valuable artifacts are not involved. First be sure that the surface to be cleaned is sound and secure - both to avoid causing serious damage and to avoid rocketing a piece of stone or brick into someone's eye during power-washing.
We have had some trouble with this method, however, when power-washing dry-laid brick and stone walks. The power washer wanted to also remove sand between the bricks or even to lift them out of place. Work carefully, and at a high angle to the surface to be cleaned if you have this problem.
Watch out: don't power-wash antique brick without first testing your procedure. If you sand-blast or use high pressure washing on soft brick or other masonry surfaces you may cause irreparable damage. In freezing climates, blasting the original glaze off of a brick surface means that it will absorb more water, grow more moss or lichens faster, and suffer more freeze damage than ever.
Also be careful when power-washing not to blast water into building wall or roof cavities where you risk cosmetic or even costly mold damage.
In greater proportions, reader Norman reports using 1 cup of trisodium phosphate, one gallon bleach and 4 gallons water on his own roof and ... "it has worked just fine, spraying it on lightly, then rinsing thoroughly with water about 15 minutes later."
Don't use the original TSP (tri-sodium phosphate) cleaner - it is an environmental contaminant. TSP substitutes such as the product shown (at left) are available at building supply stores and hardware stores.
Some deck cleaning sprays such as StainhandleR roof and deck cleaner are also sold for use as cleaning solutions for stained roofs and might work on other surfaces - check with the manufacturer. We discuss removing stains from roofs separately
at Black Stain Removal & Prevention.
Procedures for cleaning marble surfaces in or on buildings are
at MARBLE CLEANING METHODS
Trim back trees whose branches overshadow the stone surface if it is possible to do so without compromising the aesthetics of the site. Keep the flat or low-slope stone surfaces clean of organic debris like leaves or pine needles which may collect in valleys or at other roof locations.
Chemical sealants or treatments may be permitted for some stone surfaces outdoors, though where we have tried clear silicone water-resistant coatings outdoors they were not very long-lasting.
The New York Times reported that for the Angkor Wat historic Hindu temple, Dr. Warscheid devised a biocide "melange d'Angkor" that was to be used to whiten parts of the temple and to retard the blackening effect of the cyanobacteria or fungi causing the blackening of that stonework.
Watch out: Don't put slippery sealants or coatings on a walking surface.
OPINION-DF: for some artifacts it may be possible to reduce, prevent, and even "kill off" existing algae, fungal growth, or lichens on stone surfaces by installing a zinc or copper strip or cap in a position that sends dissolved copper salts down the surface in wet weather.
We have seen the success of this approach on building roofs where copper flashing was installed (photo of copper-flashed chimney on an otherwise mossy wood shingle roof at left), or where anti-fungal/moss/lichens metal strips were added.
Watch out: don't try any cleaners or preservative treatments on a valuable cultural artifact or art work without first discussing options with a professional art conservator.
Also, as the Times reported in 2008, there was no point in coating the entire Hindu Temple of Angkor Wat with biocide, since within a decade experts anticipated that the fungi or cyanobacteria present would simply adapt to the chemistry of the biocide treatment. Rather, routine cleaning and maintenance were called-for.
This article series tells readers how to identify, evaluate, remove or prevent stains on building surfaces.
I hope you may be able to help me.... I've been speaking to a client who has a very old historic building and they lit the tunnels beneath the structure a few years ago (not by me). They are experiencing growth on the stone walls (sample photo attached) which is causing them concern and we think is related to the lighting, which is LED.
It could be also down to the fact that there is now a lot of human traffic passing through the tunnels (thus creating moisture I guess). I'm not a plant expert (or anything even remotely like one!) but I understand that while plant growth can be stimulated using a combination of far red and UV lighting - hence the High Pressure Sodium lighting used for the purpose, that UV can be utilised to kill algae (or prevent growth).
The chap on site says it's more like a moss than an algae but I think moss is different, so really I'm wondering if you know of any ways of preventing moss growth using artificial lighting. It's a Listed Building so it can't be treated with chemicals, we just need to prevent any additional growth if possible and ideally, kill whatever is there. orry, I realise this might take a combination of my knowledge of lighting and yours on horticulture but my Googling hasn't come up with any answers! Thanks very much in advance. Best regards - L.P., West Yorkshire, England 3/28/14
At STAINS on STONE, STUCCO DIAGNOSE & CURE we discuss moss and algae on stone as well as other "stains" such as from fungi and in this article above STONE CLEANING METHODS we discuss methods that can be used to clean and remove such materials while respecting the surface or artifact being cleaned.
I don't agree that because a building is historic that no steps whatsoever can be taken to address moss growth on its surfaces; conservators have dealt with these problems for a very long time and have advice on least-invasive methods that still clean and protect the surface.
In this article I'm adding some helpful citations found by my research in an effort to answer your question, from which I excerpt the following:
Of several surface treatment methods used to control moss growth there are these that you might consider
1. Wet & Forget. This commercial product can be used in residential and commercial applications. Marketed as noncaustic, non-acidic, and safe for all outdoor surfaces, the product'sactive ingredient is akyl dimethyl benzyl ammonium chloride
2. Natural based products including products that contain citric, acetic or pelargonic acid; clove oil; and corn gluten meal have been included in trials conducted by other agencies. Results, where available, indicate that these products are generally less effective than synthetic alternatives and tend to be more costly to apply.
3. Electrolyzed water. This relatively new process uses water and a saturated sodium chloride solution to create an electrolyzed acid solution that can be used as a contact fungicide on plant surfaces
Also listed but not something I'd try indoors or in a traffic area is Glyphosate. Two applications per year of this herbicide should be adequate to kill the moss. Glyphosate is the active ingredient in commercial herbicides such as Roundup
Two mechanical methods are listed for removing and controlling future moss growth on surfaces such as the stone tunnel that you describe, two of which might work in the case you mention:
1. power washing or use of a "moss cutting machine" (not useful for your case)
2. Shielded high - intensity ultraviolet light. This light source might burn off moss or delay its growth by triggering a negative phototrophic response. This method is used to treat recirculating water in greenhouses to remove algae and fungi.
Heat is also listed as a moss control measure:
1. Infrared heater. An infrared heater is used to direct a propane flame at a ceramic or metal plate, which radiates heat onto the moss. Other applications use water and intense heat in an enclosed space.
It seems to me that an initial effort to clean and treat the surfaces combined with UV light might be the approach of choice.
If it were possible to add heat and reduce the moisture level in the tunnel I'm reasonably confident that the moss problem will be controlled.
If we are not able to make the tunnel more dry, that is if it stays cool and wet, I suspect that a combination of UV lighting and some environmentally-friendly topical treatment may be needed. Some of the "environmentally-friendly" methods discussed by Caltrans were not economically feasible for highway roadside moss control is needed but might be economically acceptable in a smaller more controlled area such as the tunnel you describe.
Keep us posted, as what we learn will assist others. DF
Please see BRICK SURFACE PAINT REMOVAL as those methods pertain also to stone surfaces.
Continue reading at LICHENS on STONE SURFACES or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
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(Sept 16, 2011) Marge said:
How can we clean cloth awnings of algae, lichens, fungi, muld. It is snall, yellowish, circular tufts, similar to daisy or aster centers. Thanks for your help.
Typically we clean awnings using a combination of a soft scrub brush and a sprayer, or a power washer.
Stay away from bleach lest you discolour the awning fabric.
You may increase the resistance of the cloth awnings to further algae and mold growth by treating the dry fabric with a water repellant.
Judy Frisch said:
So are you saying the TSP PF mixture is OK for using on granite stones to clean off the mold and fungus?
Judy, yes TSP-PF, a TSP substitute, is used on some stone surfaces. The orignal TSP is no longer used because it's an environmental contaminant. Depending on what the stone is and what sort of staining is discolouring it, you might want to review the advice at MARBLE CLEANING METHODS
(Oct 17, 2012) Shawn said:
What should I use to remove mold that has developed due to a leak and the mold has formed on my used brick wall in my entry way in my home.
Shawn we give extensive mold cleaning procedures and advice
at MOLD CLEANUP GUIDE- HOW TO GET RID OF MOLD
Watch out: often on masonry people mistake leak or moisture related white mineral salts for mold.
(Nov 13, 2012) Ewan said:
My concrete roof tiles were repainted 4 years ago and due to trees in the area the tiles have developed lichen on some surfaces. How can I remove the lichen without damaging the repainted surface?
(June 23, 2014) corker2012 said:
I have a heavy lichen buildup on outdoor patio furniture (not wood or metal). Some type of fabric. It's getting out of control and I don't want to replace.
Chances are you cannot mechanically remove lichens growth without damaging the surface on which it is adhered. But use of metallic strips whose wash-down will over time kill off lichens or moss on a roof can be helpful.
See the advice at ALGAE, FUNGUS, LICHENS, MOSS on SHINGLES.
Lichens on fabrics is a bit unusual, are you sure it's not an algae? Try a non-sudsing detergent cleaner and scrub brush.
Metal salts and some fungicidal sprays will deter lichens growh, as you can see on many roofs where the area of wash-down below metal flashing is often lichens and moss free.
the bleach method you describe may be perfect for patio furniture is not suitable for some building surfaces such as roofs or siding as it can can cause run-down stains and discoloration; also aggressive removal of lichens from some surfaces, particularly asphalt shingles or roll roofing, causes additional mechanical damage. Better solutions for lichens in particular are
at LICHENS on STONE SURFACES
at ALGAE, FUNGUS, LICHENS, MOSS on SHINGLES
(July 16, 2014) Anonymous said:
algae and fungai in water tank suggest how to clean
You will need to drain and disinfect the water storage tank. See these two procedures that will help you out
inspectapedia.com/water/Well_Chlorination_Shocking_Procedure.php - shocking the well or sanitizing the well
inspectapedia.com/septic/Water_Softener_Cleaning.htm - sanitizing a water softener - a similar problem to yours
(Nov 21, 2014) Anonymous said:
I am new here, can you please tell me how to clean a white marble?
and plese tell me which chemical is best for use...
sure anon, see MARBLE CLEANING METHODS
(Nov 30, 2014) Charlie Chisppe said:
Black stains have developed on the facade of my house which is made from breeze block
How can I clean?
In the article above see the section titled
Do-it-yourself Cleanup for Algae or Fungal Stains on Stone, Brick, Concrete Surfaces
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