Other than not being attractive, is the black mildew (I assume that's what it is) on the shingles (mostly in the shade) a problem?

I don't think it is anything more than an aesthetic problem. I think the mold stays on the surface of the shingle in this case. Sounds like these zinc strips will stop it from growing and when you reroof you should use a mildew resistant shingle.

some info

more info

Hope this helps

Mildew looks nasty and can also smell nasty.

Is it just on the shingles or is there an issue beneath?

Assuming that this is on the surface and just because the area is moist and in the shade there is no real concern about it. a Solution of 1 cup bleach to a gallon of water sprayed on with a spritzer and allowed to sit for about 15-30 minutes then rinsing should kill and remove the mildew.

Test on a small spot first to make certain that the shingle isn't discolored first.

If at all possible, lift up a bit of shingle and look underneath, if you see more of the same, then you may also want to go up to the attic in the same location and take a look see, see if there is any dampness or mildew growing inside.

Cheers

David

I have a tile roof here in SW Florida, and believe me, it gets moldy and mildewey...About every 3 years, I buy about a dozen bottles of bleach, put it into my pump-up sprayer, and spray the whole roof(make sure you are up-wind of the spraying)..
My next door neighbor has a regular singled roof, and it looked like hell...he did the same thing and you would'nt know it from a new rood job...

Here we have moss. It grows on the north side of shingled homes, and besides being unsightly it holds water for long periods of time so you get roof leaks. Nobody will finance a house that has moss on the roof. After you get done doing all that work, make sure you remove any tree branches that are drooping over your roof because if it rains they will be dripping all over your roof.

Catty

Thanks for the info folks. I've got no trees of any sort in close proximity, but there are some that cast shadows (especially in the AM). The discoloration is worse on the north side which makes sense, too. I'll pick up a couple containers of bleach and try them on the back side (north) to see if that'll make a difference. I didn't think it was anything other than unsightly, but knew our experts would either confirm or dispel that Thanks again!