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FLAT ROOF MOISTURE & CONDENSATION
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HEAT TAPES & CABLES for ROOF ICE DAMS
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MASONITE WOODRUF FIBERBOARD ROOFING
NOISE CONTROL for ROOFS
PLASTIC ROOFING TYPES
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ROOF JOB PROBLEMS, RESOLVING
ROOF LEAK DIAGNOSIS & REPAIR
ROOF NOISE TRANSMISSION
ROOF REPLACEMENT SNAFUs
ROOFING FELT UNDERLAYMENT REQUIREMENTS
ROOFING MATERIALS, Age, Types
ROOFING TILE SHAPES & PROFILES
ROOFING UNDERLAYMENT BEST PRACTICES
SADDLE CONSTRUCTION at CHIMNEYS
SNOW GUARDS & SNOW BRAKES
STANDARDS for ROOFING
STRESS SKIN INSULATED PANELS
TEST LABS - ROOF SHINGLE
TREES & SHRUBS, TRIM OFF BUILDING
TRUSSES, FLOOR & ROOF
UNDERLAYMENT REQUIREMENTS on ROOFS
VENTILATION in BUILDINGS
WALK-ON ROOF SURFACES
WARRANTIES for ROOF SHINGLES
WORKMANSHIP & ROOF DAMAGE
Moss on roofs or roof shingles:
Here we explain the causes of algae, moss, lichens, or fungal growth on roof shingles, the effects of these conditions, and how to cure or prevent them.
Just brushing or raking off moss won't prevent future moss growth, and if roof cleaning is not done with care you risk damaging the shingles and reducing the life of the roof.
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Causes of moss or lichens growth on roofs
Moss growing on any roof surface will be more severe on roof sections that area shaded and exposed to periodically damp cool weather conditions.
The presence or absence of much sunlight on a roof surface is often a determining factor in the amount of moss or lichens growth (more sun, less moss and lichens. When shade and cool moist conditions combine, moss or lichens growth on a roof surface is more likely. 
Moss and lichens are more than a cosmetic issue on many kinds of roofing materials - asphalt shingles, roll roofing, wood shingle roofs, wood shake roofs. By holding moisture against the roof surface lichens but more so moss speed the wear of the asphalt shingle surface in freezing climates by increasing frost damage to the mineral granule coating on the shingles.
Sometimes there is so much moss and crud on a roof that we're not sure what roofing material was used. The photo at right is actually of a wood shingle roof, though it may make more sense to call this a moss roof in Rhinebeck, New York.
The differences among algae, lichens, and moss on roofs are described at ALGAE, FUNGUS, LICHENS, MOSS COMPARED where we also show what moss looks like under the microscope. And at ALGAE STAINS on ROOFS we show what roof shingle algae looks like under the microscope.
Also see BLACK STAIN REMOVAL & PREVENTION and see STAINS on ROOF SHINGLES. This website tells readers how to identify & explain the most-common asphalt roof shingle failures and how to obtain asphalt roofing shingle failure claims assistance. We also discuss green algae and moss on building and artifact surfaces at Green Stains on Stone.
What are the effects of moss growth on roofs?
Because moss or lichens growing on a roof surface will hold moisture on the roof longer than other areas, these growths can reduce the life of the roof covering.
Particularly where the roofing materials are asphalt shingles or wood shingles, holding water on the roof surface by any means (leaves, debris, moss, or lichens) speeds up wear on these shingles. In freezing climates there may be faster frost damage, cracking, and wear of the shingles under the moss or lichens. Even in non-freezing climates, the roots or growth structures of moss or lichens eventually penetrate and separate the roof shingle materials, speeding their demise.
Is lichens on a roof as much of a problem as moss?
Because lichens growing on a roof surface does not have as much thickness of body as moss, it will hold less water on the roof surface and is less of a wear factor than moss.
Therefore if we have only lichens growing on a roof surface we would be less quick to try to clean it off since lichens not only has a tighter "grip" on the roof surface but the cleaning process for lichens risks doing more harm than good to the roof surface.
At ALGAE, FUNGUS, LICHENS, MOSS COMPARED we show photographs of lichens damage to asphalt shingle roofs.
Is green or black algae on roof shingles as much of a problem as moss or lichens?
Algae on a roof surface appears as a thin green coating which is mostly a cosmetic concern. The presence of algae on the roof is an indicator that this roof area is in a shaded spot where you may want to be alert for development of moss or other roof problems, but the level of damage from algae is probably low, even less than that caused by lichens. Avoid any aggressive cleaning methods that might damage the roof surface.
Black algae stains on asphalt singles
Some black stains on asphalt roof shingles are caused by a black algae (sometimes misnamed as a black fungus or even a "mildew") such as that shown in the photograph here. Black algae stains may be mistaken for but are not "extractive bleeding" - a product cosmetic defect.
When the staining or bleeding appears to run down the roof from individual small points or "spots" we think this is extractive bleeding or "bleed through" on shingles. (See ALGAE STAINS on ROOFS and also STAINS on ROOF SHINGLES for a more detailed discussion of bleed through or extractive bleeding as well as black algae stains on roof shingles.
When black stains on the roof surface are more uniform and cover a wide 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.
As we said about green algae on roofs, the presence of these black fungal or algal stains on the roof is an indicator that this roof area is in a shaded spot where you may want to be alert for development of moss or other roof problems, but the level of damage from the black fungus or algae is probably low, even less than that caused by lichens and certainly less than that caused by moss. Avoid any aggressive cleaning methods that might damage the roof surface.
Do not try to clean a roof like the one shown in this photograph. The shingles are worn out and fragile. It will be impossible to clean the roof without damaging it. A new roof is needed.
In our opinion, any roof surface brushing or raking should be done with great care to minimize damage to the shingles themselves. If there is any doubt about the condition of the shingles underneath the moss, work gently by hand on a small area first to see the condition of the shingles below.
If the roof shingles are fragile, brittle, cracking, curled, the risk of serious damage to the roof during moss removal is much more likely. The roof may look cleaner, but its remaining life may be reduced by aggressive cleaning.
Power washing or brushing: it is possible to remove moss from a roof surface by gentle cleaning using a soft brush or a power washer.
But be careful: power washing or even brushing or sweeping an asphalt shingle roof (or a wood shingle roof in old, worn, fragile condition) is itself a process that can damage the roof by breaking shingles or by loosening the protective mineral granules from the shingle surface. As stated at Power Washing Roofs we do not recommend power-washing asphalt shingles nor asbestos-cement roof shingles nor any other roofing product that can be damaged by high pressure spraying.
Raking off roof moss: using a flexible leaf rake to remove moss, providing the raking is done gently, may be easier than brushing. Debris on the roof after raking off moss can make it more difficult to see the actual condition of the shingles. Also, raking leaves more loose debris on the roof that may need to be gently washed off with a garden hose. Otherwise the debris will wash down and clog roof gutters
Chemicals that "kill" the moss or lichens risk also damaging the shingles or contaminating the environment, and have the further disadvantage that they still leave the moss or lichens in place where it holds water on the roof surface.
How to prevent moss or lichens growth on roof surfaces
On an asphalt or possibly some (treated) wood shingle roofs, shingle chemistry is also involved in moss, lichens and algae growth or resistance to it. Some roof shingle materials contain algaecides or fungicides which will also retard moss growth on the roof surface. You'll also notice that moss, lichens, and usually algae and fungus will be conspicuously absent from a roof surface down-roof from areas where galvanized metal, copper, or even aluminum flashing are installed. Mineral salts washing off of these metals will retard moss or algae growth on the shingles.
Use algae-resistant shingles when re-roofing. Chemically treated roof shingles are available from several asphalt roofing manufacturers who offer these products which are resistant to moss, lichens, or algae growth on roofs. Discuss this option with your roofer when it's time to replace the roof.
New asphalt roof shingles are available with an inclusion of chemicals that resist moss, lichens, algae, and even fungal growth. Resist does not mean moss-proof or lichens proof however for shingles subject to difficult conditions such as extensive shade under trees and lots of organic debris left on the roof surface.
At A Brief History of Algae-resistant AR-Shingles and What They Are Made-Of we give details about how shingles are made algae resistant - a property that appears to also retard moss growth by using zinc or copper treated mineral granules or flashings.
Metal Flashings can Retard Moss & Lichens Growth on Roofs
We have observed that the chemical or mineral salt wash-off from some building materials like aluminum flashing and copper flashing and even some paints which appear to kill of moss, lichens, algae, and fungus, as their extracts are washed over the roof surface. It's particularly easy to spot this effect by noticing where there are moss-free areas on an otherwise mossy roof surface.
One of our most obvious photos of rain wash off of copper flashing keeping moss off of a roof happens to be on a wood shingle roof, But we see this effect below copper flashing (and often aluminum flashing) on asphalt shingle roofs as well.
Notice the clear area below the turret on top of this wood-shingled pyramid shaped roof located in Key West Florida.
Chemicals from the metal cap atop the turret and from flashing at the turret base appear to be washing down the wood shingles in a path which prevents or even kills off moss, algae, and lichens on this roof (which we viewed from the Key West lighthouse museum.)
Installing copper or other metal strips along the ridge of an existing roof will slowly kill off moss or lichens as rainwater washes over the metal and down the roof surface. This method is suitable for both prevention of future or further moss or lichens growth on the roof and for gently treating an otherwise fragile old roof.
Here is another photograph demonstrating the effect of copper flashing on algae, moss or lichens on a roof.
Give the roof surface more sunlight: Trim back trees whose branches overshadow the roof surface. Keep the roof clean of organic debris like leaves or pine needles which may collect in valleys or at other roof locations.
Steps to prevent or retard moss growth on roof shingles also work about as well for preventing lichens growth. For advice on diagnosing, cleaning, and preventing algae stains on roofing, see Black Stain Removal & Prevention
Trim back trees too close to the building: see our tree trimming advice at TREES & SHRUBS, TRIM OFF BUILDING or select a topic from the More Reading links shown below.
Continue reading at BLACK or GREEN ALGAE STAINS on ROOFS or select a topic from the More Reading links shown below.
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Frequently Asked Questions (FAQs)
Question: What about using bleach to clean off roof moss?
I recently bought a home that has green moss on parts of the roof. The roof is only 12 years old and original, but it looks more like 18 years old where the moss is present. I was hoping to get a few more years out of it, before having to replace the shingles.
From everything I've read and/or been told that a mixture of hydrogen peroxide and water will do the job. I understand there are commercial products available like "Spray and Forget" that uses hydrogen peroxide as the main ingredient and is effective, environmentally friendly and won't damage trim, siding or decks. However, they are not readily available in Canada. My next door neighbor uses a bleach/water mix, this may do more harm than good!
Can you suggest a mix ratio for the HP/Water? If you have any other suggestions, they would be appreciated. - G.F.
Reply: Go Easy on Mossy Roofs or Risk Short Remaining Roof Life
Above at How to clean off mossy or lichens-covered roofs we discuss the pros and cons of even trying to remove moss from asphalt shingle roofing. I'm worried that with the heavy roof moss growth that you describe, you will pull off so much of the protective granule shingle coating along with the moss that your roof will be rather short-lived if it does not leak immediately, no matter how you remove the moss. Therefore, gentler is better.
At BLACK STAIN REMOVAL & PREVENTION we discuss using bleach, laundry detergent, and TSP-substitute cleaners to try to remove black algae stains from roof shingles, and some rough formulas are given. You could try those solutions on a mossy roof, but I wouldn't.
Algae stained shingles are not the same as mossy shingles. Most often on an algae-stained roof the shingle granules remain pretty much intact, so the risk of converting from a stained roof to a leaky roof is lower than when converting from a mossy roof to a "cleaned" roof.
Therefore in your case I'd either leave things alone entirely or I'd consider installing a zinc or copper ridge strip to slow down further moss growth on the roof.
And in any case I'd be saving up for a new roof. When you re-roof, see How to Prevent Moss, Lichens & Algae Growth on Roofs so that your roof moss problem is less severe in the future.
Question: how to treat or kill roof lichens before removing it
(Apr 22, 2011) david said:
(this lichens)no help for knowing what i can treat it with to kill it before i remove it?
RE: Question on how to treat or kill lichens before removing it:
For lichens already on a roof (as opposed to trying to prevent future lichens growth) the installation of the metal strips or flashings we described above in this article will slowly, over time, kill off lichens, moss, or algae on shingles.
But it is not necessary to kill lichens before removing it.
More important: removing lichens by any mechanical means like scrubbing or scraping is very likely to damage the shingles. Take a look at our page left link titled: ALGAE, FUNGUS, LICHENS, MOSS COMPARED and you will see a closeup photo of lichens on a roof as well as the hole left in the shingle if the lichens is physically pulled off of the shingle.
For this reason, in the case of lichens in particular, I do not recommend physically scrubbing it off. Either rely on the slow-death by metal flashing wash-down I describe above, or let it lie until you are re-roofing, then use algae-resistant shingles.
Question: marine paint for roof lichens?
(Oct 16, 2011) bob long said:
how effective would marine copper bottom paint be can one by six plank be sufficiently coated to leach out enough copper to kill moss mold lichens etc and recoated once or twice a year..might this be cost effective in light of price for copper sheeting. thank you for your comments
Great question, Bob, I don't know. I suspect that the marine copper boat bottom paint would be effective at controlling moss and lichens growth on a roof because the paint is indeed designed and intended to act as a sacrificial coating. But the paint is not intended for above ground sun exposure so I'm not sure how it will perform.
Finally, keep in mind that a lot of walking on a roof can be a source of wear, damage, or injury.
All that said, it's certainly worth the experiment. Please send along some photos (use the CONTACT link on our pages) and keep us posted - what you find will help other readers.
(Oct 24, 2011) Lisa McD said:
Hi, I have a 25 year architectural shingle roof in black that is 8 years old. I have a down spout just above the porch roof (okay, poor design - but, I didn't realize). This down spout pours water onto the porch roof leaving a white membrane/algae on the roof - but just in that area. The home faces east and is in full sun without trees. I would appreciate some suggestions. I can have the roof cleaned (but, there is no other algae showing) by a professional.... but, I don't want to be taken advantage of and am not knowledgeable in this area. Have been reading and thought, if I had the roof cleaned professionally, can I apply a piece of copper or zinc flashing on the downspoint exit to help prevent future mold or does this zinc/copper strip need to be placed at the top of the roof before the rain pours down... I do not know why the zinc/copper strip helps prevent the algae.
(Feb 26, 2012) L.Cooper said:
I have a grey looking type of mould that recently has appeared on my roof tiles at the back of my house. What could be the cause of this?
Question: orange coloured stain on shingle
(Mar 29, 2012) Shannon said:
Hi I have orange showing up on my shingles.
Shannon see RUST STAINS on ROOFS
Question: ok to roof over lichens on existing shingles?
(Apr 23, 2012) ken said:
I have one layer of shingle, there are some lichens, can i go over them?
Ken, I would think it's ok to roof over lichens on shingles as long as there are not thick growths that would make the new layer of shingles refuse to lie flat. As you are about to roof-over, you can also brush off any thick growths that might be a problem.
If your new shingles are AR (algae resistant) they may also resist lichens re-growth. Send me some photos and I can comment further ( use the CONTACT US link found at page top or bottom)
Question: power wash a flat roof?
(June 5, 2012) DAVID HARMAN said:
I have a rolled roof on the flat part of our cottage that is heavily covered with Lichens.I was going to 1st go after it with a power washer.(good idea or bad idea)?
David, IMHO using a power washer on asphalt roofing materials is a bad ideas - the risk is that you blow off the protective mineral granules and shorten the life of the roof. In my tests it is almost impossible to remove lichens without carrying away the granules and exposing the roofing substrate. I'd leave it alone until re-roof time, then use an algae/moss/lichens resistant material.
On the other hand, you should GENTLY sweep off loose debris such as leaves that have fallen onto the surface.
Question: likes using a power washer on algae, lichens, moss on roofs
(Aug 9, 2012) rjh said:
I've cleaned algae stains, moss and lichens off three roofs with a pressure washer. I did not see much granular loss. The lichens were eating holes in the shingles. The shingles were approx. 15 years old. I think the roof would deteriorate more rapidly from not pressure washing. I do not agree that the pressure washing should be avoided.
Thanks for the comment - respectful argument among conscientious readers & researchers is helpful for everyone.
We should distinguish between
algae stains - dark stains on roof shingles without moss or lichens - something that can be reduced by gentle washing or treatments (one reader used laundry detergent) - but a condition that has not caused significant granule loss
moss or lichens whose roots grip and pull granules from the shingle surface, hold water onto the roof surface accelerating wear especially in freezing climates, and whose removal immediately leave bald spots.
I imagine that working carefully, not spraying too closely or directly, a pressure washer might not always immediately destroy a roof but unfortunately more often we see the opposite: as soon as we try to remove the moss or lichens we remove so much material that the raw shingle asphalt-impregnated paper or fiberglass base is left exposed - which means the roof is pretty much shot. It would be fair to argue maybe it was shot already. Certainly we have received complaints and photographs from readers showing huge amounts of mineral granules on the ground and bald roof areas after pressure washing. It is not a recommended procedure. Niether is raking off moss as we observed in a newspaper article.
We agree with your own observation that when moss (or lichens) are removed, they carry along granules that previously were at least on, if poorly-attached to, the shingle substrate.
As soon as we leave bald substrate exposed to sunlight, the roof wear accelerates and the remaining roof life plummets.
The roof looks better, sure, but its life has been reduced still further.
Question: increasing frequency of staining on roofs?
(Oct 17, 2012) DW, Pennsylvania said:
Why has staining become such a problem over the recent years? Growing up and living in different homes as well as owning several homes over the years it appears that this has become an epidemic problem. It very noticeable even after only several years on the roofs of new homes, especially, the side that has a northern exposure. Is it primarily due to the cheap fillers that manufacturers are using in asphalt shingles that is causing the problem? Does anyone know of any class action suits involving the maufacturers of ashphalt shingles? It's a love shame for homeowners who take care of there homes/property, but then have to deal with roofs that look really shabby.
I've purchased and used a prouct that is a mixture of sodium hydroxide, surfactants, & wetting agents, which is dissolved in water and then applied with as a spray from a garden spray tank - I usually use a very soft bristle brush(used on cars) and then rinse it off with a stream of water from a garden hose. It works very well and after a couple of good rains, it's looks a bit better. It works but it's a pain in the butt. So far, I've managed to not fall off the roof. I appeciate hearing back/readers experiences and what they have heard re the cause and if they too feel that the problem seems to be getting worse/more common.
To answer your interesting question we need to look at
- shingle product properties (not all are algae or fungal resistant
- local air pollutants
- site features: nearby trees, for example
(Mar 9, 2013) biology report said:
how can i right a bibliography for this?
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(Mar 18, 2014) Mrs.Ward said:
Could you please tell me the reason for white marks on rough stone patio slabs that have appeared more since the rainy weather.
Question: use a roof sealer after cleaning shingles?
(June 13, 2014) Bill. D said:
After cleaning the shingles, is it worth spraying on a water based clear sealer to help protect them.
Bill, while the idea makes sense, my experience is that coating a weather-exposed silicone-based exterio clear sealer is effective for about a year.
Question: algae causes roof leaks
(Oct 15, 2014) Anonymous said:
I have architectial shingles I was told by a roofer that the reason my roof is leaking is because of algae build-up between the shingles. My roof is only about 6 yrs old. he said all of the shingles where the algae is needs to be replaced. does this sound right?
Question: roof cleaner likes to clean roofs rather than prevent algae, moss, lichens growth
Roof Cleaner said:
I would not recommend that anyone install zinc or copper strips on their roof, I have cleaned thousands of roofs where these were installed. I would also not just let the stain or moss go, it needs to be taken care of, The roofing material is just like any other part of your home and it needs maintenance also. With the cost of replacement well into the thousands of dollars in makes sense to clean the roof and get the full life span out of it.
Thanks for your opinion Roof Cleaner. But with respect, your advice does not acknowledge that physically removing lichens or moss leaves roofs damaged, pulling off still more granules from asphalt surfaces, and thus the "cleaned" roof is left with still shorter remaining life and that step does not prevent future moss or lichens growth.
The alternatives for slow-kill-off of algae, lichens, moss, as well as roofing with algae/resistant shingles are ones that work to give better roof life..
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