Moss is very common in this area. Moss and algea grow mostly in areas of low light and high moisture. Low light and high moisture pretty well describes most of the western Pacific Northwest and the wintertime. Some of the most common ways to treat and maintain your roof are listed below.
One of the areas that moss is a concern on homes in this area our roofs. Depending on the roofs exposure moss can grow most of the year. If sections of your home’s roof are shaded throughout the day and stay moist these are likely areas to grow moss.
Over time moss can damage your shingles if left un-checked. As the moss develops into a larger and larger colony more and more moisture is held against your roof. The colonies will also develop root systems that will dig in to the surface fibers on your shingles. As the colonies grow larger, they can actually lift the edges of the shingles. This can leave the shingles vulnerable to wind damage.
There are lots of ways to kill moss. Most of the good techniques involve some sort of the heavy metal application usually copper or zinc. Some really bad ideas involve laundry detergent and or power washers…..
In general the more trees you have around your house in the steeper your roof the more applications of moss killer you’ll need.
1. The best way to control moss is with an annual or biannual application of a powdered or liquid name brand moss killer designed for roofs. For steep roofs I have found a hose end attached shrub and tree sprayer to be a handy tool.
2. Another option for continuous moss control are some new
shingles that are actually impregnated with copper
granules. I have only seen the shingles used on two
different roofs and the major issue with these is the fact that
the ridge shingles were not impregnated in a still need to be
treated for moss/algae growth.
3. Mechanically removing the moss is also an option.
This option is really only for the very worst conditions.
It envolves a paint scraper, screwdriver, putty knife or
something similar and trecking across your roof slope and very
carefully removing the moss growth.
This technique is very prone to damage of the asphalt
composition shingles and should be used as a last resort.
4. Zinc or Copper strips- These may be ok for preventing
algea growth but moss looks at the little strips and laughs.
You may have noticed some homes around town that have
clear sections of shingles under metal roof vents.
What is going
on here is the
zinc in the
galvanized steel
is leaching on to
the roof every
time it rains.
This has lead to
people thinking that they could install little strips or even
sections of wire to kill moss and algea, but this is not
usually an effective technique.The difference is all
about-surface area.The roof vents have a rather large
amount of exposed surface and therefore a good amount
of rain hits the vents. Compare this situation to a 2 inch
wide strip of zinc and you
can see that there will be far less leaching occurring off
of the little strips.The strips usually are effective for 2 to 3 feet,
and I have seen the strips added every 2 or 3 feet down
an entire roof slope. This installation appeared to be
effective but I have only seen this once.
5. Power washing, scraping with brooms, or laundry detergent. Unfortunantly I see the aftermath of these steps to control roof moss on far too many Salem area home inspections. If you are reading this post you probably have educated yourself enough to know that blasting or scraping the surface off of your aspahlt composite roof is a bad idea. The folks that commit these heinous crimes are usually the people that already know-it-all and they maintain their homes the way they see fit.
-There may be some contractors who use power washers to clean roofs, but these individuals are licensed and bonded and have the experience to know which nozzle to use and how far to hold it away from the roof surface. Power washing should be strictly limited to the driveway and walkways surfaces around your home.
-Brooms and other mechanical abrasion are also techniques that should either be left on the ground or for qualified professional contractors. The removal of the surface granules on the asphalt shingle also removes the ultraviolet resistance and of the shingles.
-Laundry detergent, although will kill moss on your roof, is full of degreasers. An asphalt based composite shingle is a petroleum based grease compound. Do not put degreasers on your greasy roof. (Thanks Joe Ocilia for educating me on this fact!)
Salemoregonhomeinspector's Blog 
Do you have any evidence to back up the theory that laundry soap damages asphalt roofs? People have been doing this for decades and only recently have I heard that it is a problem.
Yes I do Don. I have seen large chunks deteriorate from the surface of comp shingles after laundry detergent was applied. Most of the time the soap doesn’t sit on the surface very long and the damage that can be noticed is subtle. If you accelerate the deterioration the surface of your 30-year roof by 4-5 years you usually won’t be able to notice this until the roofing is at the end of its life.