Slimy Green Algae on Portland, Oregon Roofs
Green Slimy Algae on My Roof in Portland Oregon
Green roof algae is that green slimy stuff on your roof. It grows where there are enough sunlight, nutrients and water. It looks unattractive, like swamp muck, but doesn't really "stain" your roof, at least not permanently. Algae is an indicator that you have dirty gutters/roof above and poor gutter and/or roof drainage. Algae or roofs can reveal inadequate building practices.
Difference Between Green Algae and Green Moss
On inspecting the picture below, you can see that slimy algae grows across the face of the shingles in a drainage patter below a downspout. The moss is the clumpy stuff in straight lines across the bottom edge of the shingles. Besides the growth pattern, algae is flat up to 1/8" thick of slime, while moss is slightly fluffy (growing up to 2 inches tall).
Where it grows
Green roof algae grows where the nutrients are. You'll often see concentrations of it on older roofs where upper level downspouts empty onto lower roofs. The dirt and accumulation in the gutters washes out on to the roof below. This creates a great place for the algae to grow.
Algae grows in the concentration of nutrients, sunlight and water.
Getting It Off Your Roof
On a wet day, if you can safely get on your roof (at your own risk, mind you) it can be hosed off with a brass jet that pinpoints the hose water into a stream. Just don't angle it up under the shingles, just downslope only and maybe sideways if you're careful. Other than that, algae can be washed off your roof with an algae roof cleaner. There are products specifically formulated to wash roofs without removing the oils in the composite shingles. This leaves the roof clean and algae-free. See also the service I provide of washing your roof. Please don't throw some soap or detergent up there. It can remove the oils from your shingles and damage them. I really, really don't recommend pressure washing off the algae. This is major overkill. That woudl be like using a sand-blaster to wash your car.
Preventing Green Roof Algae
Roof Algae can be prevented by gently washing the roof regularly and keeping the gutters cleaned. Washing off the dirt off the bottom edge of the shingles is a good place to start. Just use gentle hose pressure to do this. This way there are fewer nutrients on the roof for the algae to feed on. Also, upper level downspouts can be extended over the roof to drain directly into the lower gutter. This keeps the nutrients off the roof and in the gutters where the algae is not unsightly. Also, water from upper gutters should go through a downspout and not over the roof. If it goes over the roof, then it wears on the shingles and removes granules.
Why It Should Be Removed
I don't know of any damage that algae does to roof except for hurting its appearance. Asthetics are the chief reason for cleaning algae from roofs. I see a lot of slimy algae even on newer construction because of dumping upper gutter water on the roofs below. Since Oregon is so damp and wet for longer stretches, our gutter water needs to be kept in downspouts and drains to prevent building damage and unsightly algae.