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ROOFING INSPECTION & REPAIR
AMERICAN CEMWOOD ROOFING
BEST ROOFING PRACTICES
BUILT UP ROOFS
CATHEDRAL CEILING VENTILATION
CERTIFICATIONS for ROOFING CONTRACTORS
CHIMNEY FLASHING Mistakes & Leaks
COLD WEATHER ROOF TROUBLE
DECKS, ROOFTOP CONSTRUCTION
EPDM, RUBBER, PVC ROOFING
EXTRACTIVE BLEEDING on SHINGLES
FIRE RETARDANT PLYWOOD
FLASHING on BUILDINGS
FLASHING, ASPHALT SHINGLE VALLEYS
FLASHING, CHIMNEY Mistakes & Leaks
FLASHING, CLAY TILE ROOFS
FLASHING MEMBRANES PEEL & STICK
FLASHING for METAL ROOFS
FLASHING ROOF WALL DETAILS
FLASHING ROOF-WALL SNAFU
FLASHING SIDING DETAILS
FLASHING WALL DETAILS
FLASHING WINDOW DETAILS
FLASHING WOOD ROOF DETAILS
FLAT ROOF MOISTURE & CONDENSATION
Green House or Solarium Roof Leaks
HEAT TAPES & CABLES on Roofs for Ice Dams
ICE DAM PREVENTION
MASONITE WOODRUF FIBERBOARD ROOFING
NOISE CONTROL for ROOFS
PLASTIC ROOFING TYPES
PVC, EPDM, RUBBER ROOFING
ROOF ARCHITECTURAL STYLES - PHOTO GUIDE
ROOF CLEANING RECOMMENDATIONS
ROOF COLOR RECOMMENDATIONS
ROOF DORMER TYPES - PHOTO GUIDE
ROOF INSPECTION SAFETY & LIMITS
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
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. We cite and discuss roofing manufacturer shingle warranty exclusions or inclusions for algae and algae-related roof stains at ALGAE STAINS ON ASPHALT ROOF SHINGLES.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
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 ASPHALT ROOF SHINGLES 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?
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
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.
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
Frequently Asked Questions (FAQs) about about cleaning off moss from building roofs
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.
Questions & answers or comments about how to remove moss from building roofs.
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