Re-roofing the house soon and have a question.

We get those ugly black mildew streaks on the north side of the roof, even though it still gets a fair amount of afternoon sun, as we aren't facing due north. I've been told there's not a lot that we can do to prevent it, just treat it when it gets bad enough to bother us. However, I believe one of the roofing guys that gave us an estimate said that a heavier weight felt paper underlayment would help to minimize it. Is this true?

If you do a quick search (I use Dogpile instead of google) you'll find answers like this :

If you live in a humid area of the country, you’ve probably seen unsightly dark streaks on asphalt shingle roofs. Though often attributed to an accumulation of dirt, defective shingles, mold, or mildew; the most common culprit is actually a blue-green algae known as Gloeocapsa Magma that is spread by airborne spores.

Prevention

To keep algae from coming back once your roof is clean, install a strip of copper or zinc coated sheet metal along each side of the roof just below the ridge. To be effective, 2" to 4" of metal should be visible on the roof.
While copper is more toxic to algae, galvanized sheet metal is much less expensive. Both copper and galvanized metal are available in rolls of various widths and thickness both online and from local metal suppliers. You can also purchase zinc strips specifically made to eliminate roof algae from Z-Stop.
Narrow strips of sheet metal can be attached directly to your roof using roofing nails or screws with a rubber washer. For wider pieces of sheet metal, loosen the self-sealing tabs on the top row of shingles with a putty knife, slip part of the sheet metal under them, and nail it in place under the shingles with roofing nails.
The methods used to eliminate and prevent algae on your roof can also be applied to moss

Excellent reply, BK....I have done a LOT of roofing, both new construction and replacement, but have never heard of using 30# rather than the standard 15# felt for that reason alone. I suspect the person who gave the OP that solution was simply trying to pad the estimate.

Thanks guys. Yes it is algae, not mildew. I do remember him saying that now. Also, he set out the 30lbs. felt as an option, so it wasn't actually in the quote, it would have been additional. I'd emailed the company today to tell them my shingle choice and had asked about this also. The rep said no the 30 pound paper would not help with the algae, it would give a little more protection in case of a bad storm, and boy we've had some doozies latley, but that's all. He did remind me that the quote is for algae resistant shingles, which I'd missed when looking through the paperwork.

Thanks JD.

I did find reference to shingles with the copper or galvanized material in the stone on the shingles (that would make them resistant). Knowing the problem exists in your area, I would make sure that the shingle lists that on the packaging and not just take the roofers word for it. Most are trustworthy and honest, but it does not hurt to double check BEFORE it is installed.
Don't forget the Ice Dam underlayment in places that might have that problem,