What are those long black soot-like stains (steaks) that appear on my roof shingles on the north side?

Question

We live in N.Y. and on the north side of my roof, long black, soot like streaks have appeared. I notice it on some of my neighbors roofs too ... any ideas???? Also, does anyone know how to get rid of clumps of moss that are growing on my roof --- btw, near those black stains.

Answer 1

I live in Tennessee and had a light colored shingle before replacing my roof and we had streaks develop that were a fungus.  I thought at first it was the shingle material washing off in places due to heavy rain, etc but it was a type of fungus that was growing.  I would check it closely, especially if you have trees near the house.  There is a spray you can use if it is a fungus.

Answer 2

Common Clorox bleach will kill the moss instantly. Mix 1/2 gallon of bleach to 1/2 gallon water, pour over the moss and it will kill it. This mixture will not harm vegetation below that might come in contact.

Answer 3

I did some research on this subject because my own roof, my dad's, and practically everyone's in the neighborhood (suburban Philly) are showing signs of this.

I once asked a roofer about it, and he claimed it was a defect in the material that was allowing the black asphalt to seep thru and run down the roof.  I rather suspected that as balderdash, it just doesn't seem likely that a water-insoluble substance like tar would spread and run down a roof, and only on the north side?

Doing some more web research I did find that as an explanation, however the vast majority of opinions say it is a form of a 'black algae' that for some reason has become much more prevalent in recent years.

It favors certain roof orientations, generally north side as you've seen, but it still seems to need adequate sun exposure to survive (so you may not see it on a totally shaded roof, although that may experience other growths, like moss).

Fixes seem to range all over the map - there are various services that will come out and clean it up, and there's a number of solutions I've seen touted.  I see Home Depot sells a roof cleaning product, but it is like $40 for a bottle, and I don't know how much it would cover or how effective it is against this algae.

One almost universal caution I've seen is do NOT use a pressure washer, unless very gently, you don't want to do anything to loosen your granules.

I tried simple chlorine bleach, found I had to use it at pretty much full strength to make a difference.  In the source section, I've included a link to an article from the Asphalt Roofing Manufacturers Association, who amongst most anyone I would consider authoritative.  They recommend a mixture of bleach, TSP, and water.

Finally, most opinions I've read say it is not particularly destructive to your roof, albeit unsightly.