Prevalent in Northern Virginia, and to the south, is a roof staining problem called "black algae." It looks like a dripping, running black stain, usually on a shaded roof, or on the side facing away from direct sunlight. Incorrectly called mildew and fungus, the staining is an algae, typically Gloeocapsa magma. I see it all the time.
The black algae is usually distinguished as a general stain.
It runs down the roof as it is carried by water.
It grows where it obtains food in the roofing materials.
I do not know how prevalent it is around the country, but suspect it is common.
Algaes can grow on all types of roof surfaces - I have seen it growing on asphalt shingles, clay, concrete asbestos and even slate.
It seems to me that it has become more common in the last 15 years or so. One theory as to why is that manufacturers began then to change the composition of shingles, adding more limestone to lend more weight. Black algae LOVES a lime buffet!
Sometimes it is hard to distinguish from a roofing problem.
Asphalt shingles can sometimes be damaged on installation and a pin prick spot opens up and becomes a problem. Over time the asphalt can begin dripping out. The single point of such a black discharge may be what you see in the lower right corner of this roof, and under the window.
But that could be black algae also!
The drippy black on the top right of the roof is likely black algae.
I have been told by more than one roofer that this algae is one way to date shingles.
It seems the black algae begins taking hold and manifesting on shingles at about 8 or 10 years of age.
That time frame has proved true on my inspections.
So, what can a homeowner do?
Well, you already know NOT TO PRESSURE WASH IT!
Be sure your shingles are not old and fragile. If so, you may as well replace them. Then, test your cleaning on a small area to see if it works.
The algae must be killed for any cleaning to be effective. The oxygen cleansers (sodium percarbonate) will clean the roof, but not necessarily kill the algae.
One formula I found is this:
Tri-sodium Phosphate (the substitute, not the original which is environmentally unsafe) can be diluted and sprayed onto the shingles.
THE RECIPE:
4 Gallons Water mixed with 1 Gallon Bleach. Stir in 1 Cup of TSP (phosphate free) until diluted. Tri-sodium Phosphate can be obtained at any hardware or paint store. Remember to use the safer phosphate-free mix.
Spray on the roof, let it sit for 15 or 20 minutes and rinse off with a hose.
This black algae is aggressive and hard to get rid of. So good luck!
My recommendation: when you see what is likely black algae on a roof, it is ugly but does not eat the roof very quickly. And it might be an indicator as to shingle age! It can be cleaned! All three of the roofs above are 12 years old and in the same neighborhood.
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