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Photograph of mossy growth on a worn out asphalt shingle roof Remove or Prevent Roof Moss, FAQs
Q&A on Removing, Preventing Algae, Fungus, Lichens, Moss on Roofs

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FAQs for getting rid of or preventing lichens or moss problems on roofs or roof shingles:

These roof moss questions and answers address the causes of algae, moss, lichens, or fungal growth on roof shingles, the effects of these conditions, and how to cure or prevent them.



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FAQs for Algae, Fungus, Lichens, Moss on Roofs

Photograph of lichens growth on asphalt shinglesQuestion: What about using bleach to clean off roof moss or lichens?

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.

[Click to enlarge any image]

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?

Reply:

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.

- Editor

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

Reply:

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.

Question:

(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?

Reply:

x

Question: orange coloured stain on shingle

(Mar 29, 2012) Shannon said:

Hi I have orange showing up on my shingles.

Reply:

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?

Reply:

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)?

Reply:

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.

Reply:

RJH

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

and

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.

Reply:

DW

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

Question:

(Mar 9, 2013) biology report said:

how can i right a bibliography for this?

Reply:

Biology:

If you are asking how you can write a citation for your report that cites this article, take a look at our citation example at our "Website Article Citation Guide"

Just use the page top or bottom search box to search for "Website Article Citation Guide" to read the article.

Question:

(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.

Reply:

See EFFLORESCENCE SALTS & WHITE DEPOSITS

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.

Reply:

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?

Reply:

No.

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.

Reply:

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.

Question: roof cleaning company advises against using other methods to keep moss or lichens off of roofs

(Dec 18, 2014) 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.

Reply:

Thanks for your opinion Roof Cleaner. But your advice does not acknowledge that physically removing lichens or moss leaves roofs damaged and with shorter remaining life and that step does not prevent future moss or lichens growth.

The alternatives work.

Question: which roof slopes get more moss?

(Mar 10, 2015) JERRY said:
What side of a wooden roof is moss generally more prevalent on? North, South, East or West?

Reply:

Great question. I have to wave my arms and make up a waffling answer:

It depends ...

Typically we'd say "north" - after all moss and lichens grow on the north side of trees in the forest

all other things being equal.

But if another roof slope is overhung by trees, shaded, damp, it may be the moss's most attractive spot.

Question: moss and corrosion resistance of plastic coated metal roofs

(Apr 5, 2015) Hugh ( on the ground) said:
what about those plastic-coated metal roofs.

My neighbour is complaining that the green fungus from an adjacent sycamore tree has cause pinholes to appear in the coating, and may, he says, corrode the underlying metal sheet and let rainwater in.

(I have not yet climbed up to inspect the described damage)

Reply:

Hugh,

Whose roof is in danger, yours or your neighbours? What view is available, and how closely has the roof been inspected.

While we don't like to see algae or fungal or other crud forming on roofs for cosmetic reasons, I agree that there could be wear factors as well, even on a metal roof.

There are PVC "plastic" (polyviny chloride) coated metal roofing products sold in the U.S. including products from China. Some of these are on a galvalume steel base.

I agree that there may be a failure risk, and I'd consider whether you can trim the tree to improve sunlight and reduce algae (or possibly fungal) growth on the roof surface; the roof may warrant cleaning.

I suspect (but don't know) that actually the wear rate of a PVC coated metal roof may be affected by weather exposure and sunlight (photooxidation of the plastic coating) as much as by the concerns that you raise.

Research on corrosion in plastic coated metal roofing suggests that while the present risk to you may be low, it's worth taking a closer look. See these discussions of plastic roof coating system failures:

Roof Algae Lichens Moss or Stain Articles

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Or see BLACK or GREEN ALGAE STAINS on ROOFS

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